trev 263
100
100
From the coherer to DSP
M. Lemme and R. Menicucci (Vatican Radio)
This article reviews the development
of electronic devices used over the
last century in wireless
communication. It looks at early
receiving devices such as the
coherer, the magnetic detector and
the cat’s whisker, progressing to the
thermionic valve, the semiconductor,
the microchip and digital signal
processing.
On the transmission side, the early
devices discussed include the
spark-gap generator, the voltaic-arc
generator and static frequency
multipliers. This is followed by a
brief description of more modern
power devices, including thermionic
valves and electron-velocity control
tubes.
1.
Introduction
The electromagnetic-wave generator that was
first conceived by Hertz, in 1887, consisted of a
copper rod 3 m long which had a 30 cm zinc
sphere attached to either end. The rod was
divided in the middle to include a spark gap of
0.75 cm between two small brass spheres. This
assembly – which we now call a Hertz dipole –
was connected across the secondary winding of
an induction coil. When the primary coil was con-
EBU Technical Review Spring 1995
Lemme & Menicucci
nected to a battery, a spark would appear across
the gap, generating an oscillating current. As a
consequence, damped electrical waves would
propagate across the room.
The wavelength produced by the Hertz resonator
depended on the inductance and the capacitance
of the circuit (the large zinc spheres could be
moved along the rod to vary the wavelength of
the oscillations). Hertz was aiming to reproduce
phenomena similar to light and the shortest
wavelength he managed to generate was about
30 cm, equivalent to an oscillation frequency of
1000 megacycles/second (1000 MHz in modern
parlance).
The energy radiated by the dipole was called
“Force Diffusion” by Hertz; it was Lord Kelvin
who later gave it the name electromagnetic wave.
Sir William Crookes was very surprised when he
noticed that electromagnetic waves could pass
through “the walls and the fog of London”. In
1892, he gave an explanation of this phenomenon
and was even able to foretell wireless telegraphy.
2.
2.1.
Early receiving devices
Hertz resonator
The first and simplest detector of electromagnetic
waves was the Hertz resonator. It consisted of an
open metal ring with a small metal sphere attached
at either end. A spark jumped across the small gap
between the two spheres, whenever a spark was
generated in the transmitting apparatus.
63
100
100
From the coherer to DSP
M. Lemme and R. Menicucci (Vatican Radio)
This article reviews the development
of electronic devices used over the
last century in wireless
communication. It looks at early
receiving devices such as the
coherer, the magnetic detector and
the cat’s whisker, progressing to the
thermionic valve, the semiconductor,
the microchip and digital signal
processing.
On the transmission side, the early
devices discussed include the
spark-gap generator, the voltaic-arc
generator and static frequency
multipliers. This is followed by a
brief description of more modern
power devices, including thermionic
valves and electron-velocity control
tubes.
1.
Introduction
The electromagnetic-wave generator that was
first conceived by Hertz, in 1887, consisted of a
copper rod 3 m long which had a 30 cm zinc
sphere attached to either end. The rod was
divided in the middle to include a spark gap of
0.75 cm between two small brass spheres. This
assembly – which we now call a Hertz dipole –
was connected across the secondary winding of
an induction coil. When the primary coil was con-
EBU Technical Review Spring 1995
Lemme & Menicucci
nected to a battery, a spark would appear across
the gap, generating an oscillating current. As a
consequence, damped electrical waves would
propagate across the room.
The wavelength produced by the Hertz resonator
depended on the inductance and the capacitance
of the circuit (the large zinc spheres could be
moved along the rod to vary the wavelength of
the oscillations). Hertz was aiming to reproduce
phenomena similar to light and the shortest
wavelength he managed to generate was about
30 cm, equivalent to an oscillation frequency of
1000 megacycles/second (1000 MHz in modern
parlance).
The energy radiated by the dipole was called
“Force Diffusion” by Hertz; it was Lord Kelvin
who later gave it the name electromagnetic wave.
Sir William Crookes was very surprised when he
noticed that electromagnetic waves could pass
through “the walls and the fog of London”. In
1892, he gave an explanation of this phenomenon
and was even able to foretell wireless telegraphy.
2.
2.1.
Early receiving devices
Hertz resonator
The first and simplest detector of electromagnetic
waves was the Hertz resonator. It consisted of an
open metal ring with a small metal sphere attached
at either end. A spark jumped across the small gap
between the two spheres, whenever a spark was
generated in the transmitting apparatus.
63
100
Figure 1
Circuit diagram of the
transmit-receive
system used during
Marconi’s Lavernock
Point trials in May
1897
(as published by the
General Post Office
in the Engineer-inChief’s report of
1903).
2.2.
Coherer
An improved detector, the coherer, was invented
by the Frenchman, Edouard Branly. It was based
on the discoveries of the Anglo-American, D.E.
Hughes, and the Italian, Calzecchi Onesti – which
had been further updated by the Englishman, Sir
Oliver Lodge, as well as the Russian, A.S. Popov.
Figure 2
Marconi’s system for
long-range radio
reception, as fitted to
the American liner
St. Paul in 1899
(Patent No. 7777).
The coherer consisted of a tube filled with fine
metal filings (Marconi used 95% nickel and 5%
silver). When RF energy was passed through the
filings, the particles cohered and the resistance
dropped. The coherer could be made responsive
to a change of state, i.e. to the presence or
absence of a signal. When a signal was present,
a battery-powered circuit attracted the moving
element of a relay and, consequently, the whole
assembly could be used to receive Morse code
signals from a remote transmitter. In order to
separate the particles inside the bulb, so that the
apparatus could be operated again, some form of
Coherer
Jigger
Jigger
“de-coherer” such as a solenoid tapper or a
mechanical shaker was necessary.
A coherer with an antenna and an earth connection
formed the receiving part of the equipment used by
Marconi in 1896/7 to carry out his experiments at
Pontecchio near Bologna, Italy, and at Lavernock
Point near Cardiff, Wales. A circuit diagram of the
latter installation is shown in Fig. 1.
The coherer was also used in the system Marconi
developed to communicate with ships at the turn
of the century. In this system, according to his
patent No. 7777 (Fig. 2), Marconi connected the
transmitting antenna to the secondary winding
(jigger) of a “Tesla transformer” (formerly the
antenna had been connected to ground via the
metal spheres of the exciter). The primary winding of this transformer was connected in series
with the exciter and a capacitor, formed by
Leyden jars. At the receiving station, the antenna was connected to ground through the primary
winding of a Tesla transformer, whose jigger was
connected to the coherer. The first apparatus of
this type was installed by Marconi in the American liner St. Paul at the end of 1899.
Although the coherer was used successfully for
several years, it soon showed its poor efficiency
and sensitivity. In December 1901, at St. John’s in
Newfoundland, Canada, Marconi replaced it with
a mercury drop detector.
2.3.
Transmitter
64
Receiver
Magnetic detector
The early detectors were rather unstable and
irregular in operation. These problems, along
EBU Technical Review Spring 1995
Lemme & Menicucci
100
Figure 1
Circuit diagram of the
transmit-receive
system used during
Marconi’s Lavernock
Point trials in May
1897
(as published by the
General Post Office
in the Engineer-inChief’s report of
1903).
2.2.
Coherer
An improved detector, the coherer, was invented
by the Frenchman, Edouard Branly. It was based
on the discoveries of the Anglo-American, D.E.
Hughes, and the Italian, Calzecchi Onesti – which
had been further updated by the Englishman, Sir
Oliver Lodge, as well as the Russian, A.S. Popov.
Figure 2
Marconi’s system for
long-range radio
reception, as fitted to
the American liner
St. Paul in 1899
(Patent No. 7777).
The coherer consisted of a tube filled with fine
metal filings (Marconi used 95% nickel and 5%
silver). When RF energy was passed through the
filings, the particles cohered and the resistance
dropped. The coherer could be made responsive
to a change of state, i.e. to the presence or
absence of a signal. When a signal was present,
a battery-powered circuit attracted the moving
element of a relay and, consequently, the whole
assembly could be used to receive Morse code
signals from a remote transmitter. In order to
separate the particles inside the bulb, so that the
apparatus could be operated again, some form of
Coherer
Jigger
Jigger
“de-coherer” such as a solenoid tapper or a
mechanical shaker was necessary.
A coherer with an antenna and an earth connection
formed the receiving part of the equipment used by
Marconi in 1896/7 to carry out his experiments at
Pontecchio near Bologna, Italy, and at Lavernock
Point near Cardiff, Wales. A circuit diagram of the
latter installation is shown in Fig. 1.
The coherer was also used in the system Marconi
developed to communicate with ships at the turn
of the century. In this system, according to his
patent No. 7777 (Fig. 2), Marconi connected the
transmitting antenna to the secondary winding
(jigger) of a “Tesla transformer” (formerly the
antenna had been connected to ground via the
metal spheres of the exciter). The primary winding of this transformer was connected in series
with the exciter and a capacitor, formed by
Leyden jars. At the receiving station, the antenna was connected to ground through the primary
winding of a Tesla transformer, whose jigger was
connected to the coherer. The first apparatus of
this type was installed by Marconi in the American liner St. Paul at the end of 1899.
Although the coherer was used successfully for
several years, it soon showed its poor efficiency
and sensitivity. In December 1901, at St. John’s in
Newfoundland, Canada, Marconi replaced it with
a mercury drop detector.
2.3.
Transmitter
64
Receiver
Magnetic detector
The early detectors were rather unstable and
irregular in operation. These problems, along
EBU Technical Review Spring 1995
Lemme & Menicucci
100
with some polemics, induced Marconi to aim his
research at a new type of detector. Inspired by a
paper published in 1896 by Prof. Rutherford,
Marconi developed the magnetic detector which
was a significant improvement over the coherer.
The magnetic detector employed clockwork
mechanical power to magnify the weak incoming
pulses from a spark transmitter so that they were
strong enough to drive a pair of headphones.
Basically, the magnetic detector operated as follows (Fig. 3). A loop of hard-drawn iron wires
(a) , wound together in the form of a “rope”, was
made to move endlessly through two coils, by
means of a clockwork drive. The inner coil (b)
– a single-layer winding – was connected between the aerial and the earth via a tuning filter.
The outer coil (c) was made from a large number
of turns of fine wire and was connected directly
to the headphones (T). Two permanent magnets
(d), with their “like-poles” together, produced a
quadropole field that caused the moving iron
rope to divide into two magnetic domains separated by a “wall” which was stationary relative to
the two coils.
Whenever a high-frequency “pulse” was received
from the transmitter, the resultant magnetic field
acted – via the primary coil – on the wire rope.
Incoming half-waves of one polarity would cause
the magnetic domain wall to “flick” backwards
and produce –via the secondary coil – a “click” on
the headphones, while incoming half-waves of the
other polarity had little effect on the wall. Thus,
the device could reproduce the dots and dashes
sent out from a spark transmitter, just like a crystal
detector – except that the magnetic detector also
offered considerable gain.
Marconi’s magnetic detector was patented in 1902
and was widely used for about 20 years.
Figure 3
Diagram from
Marconi’s Patent
No. 10245 of 1902
showing the
magnetic detector.
Looking at the pictures and circuit diagrams of
the time, an observer with today’s technical
knowledge could be excused for thinking that
such devices were rather simple and rudimentary. However, reading the journals of those
earlier experimenters, one can get a feeling of the
problems they had to overcome. Every circuit
component caused problems, the solution of
which needed months of work and repeated
Figure 4
A very early receiver
using a galena crystal
and cat’s whisker as
the detector.
EBU Technical Review Spring 1995
Lemme & Menicucci
65
100
with some polemics, induced Marconi to aim his
research at a new type of detector. Inspired by a
paper published in 1896 by Prof. Rutherford,
Marconi developed the magnetic detector which
was a significant improvement over the coherer.
The magnetic detector employed clockwork
mechanical power to magnify the weak incoming
pulses from a spark transmitter so that they were
strong enough to drive a pair of headphones.
Basically, the magnetic detector operated as follows (Fig. 3). A loop of hard-drawn iron wires
(a) , wound together in the form of a “rope”, was
made to move endlessly through two coils, by
means of a clockwork drive. The inner coil (b)
– a single-layer winding – was connected between the aerial and the earth via a tuning filter.
The outer coil (c) was made from a large number
of turns of fine wire and was connected directly
to the headphones (T). Two permanent magnets
(d), with their “like-poles” together, produced a
quadropole field that caused the moving iron
rope to divide into two magnetic domains separated by a “wall” which was stationary relative to
the two coils.
Whenever a high-frequency “pulse” was received
from the transmitter, the resultant magnetic field
acted – via the primary coil – on the wire rope.
Incoming half-waves of one polarity would cause
the magnetic domain wall to “flick” backwards
and produce –via the secondary coil – a “click” on
the headphones, while incoming half-waves of the
other polarity had little effect on the wall. Thus,
the device could reproduce the dots and dashes
sent out from a spark transmitter, just like a crystal
detector – except that the magnetic detector also
offered considerable gain.
Marconi’s magnetic detector was patented in 1902
and was widely used for about 20 years.
Figure 3
Diagram from
Marconi’s Patent
No. 10245 of 1902
showing the
magnetic detector.
Looking at the pictures and circuit diagrams of
the time, an observer with today’s technical
knowledge could be excused for thinking that
such devices were rather simple and rudimentary. However, reading the journals of those
earlier experimenters, one can get a feeling of the
problems they had to overcome. Every circuit
component caused problems, the solution of
which needed months of work and repeated
Figure 4
A very early receiver
using a galena crystal
and cat’s whisker as
the detector.
EBU Technical Review Spring 1995
Lemme & Menicucci
65
100
thin metal (the anode or plate). When the filament
was heated until white hot, the emitted electrons
reached the anode plate. If the latter was at a
positive potential with respect to the filament, it
attracted the electrons so that a current flowed in
the filament-plate circuit. If the plate became
negative, no current flowed through the valve.
Thus, by using a thermionic diode, very small HF
currents could be rectified into unidirectional
currents.
3.2.
Figure 5
An early thermionic
diode, circa 1904.
experiments. Only rough materials were used:
for example, the prototype of the magnetic
detector was enclosed in a Havana cigar box.
There were no suitable instruments to measure
RF currents, nor was there any well-grounded
theory to aid the design of these early wireless
installations.
2.4.
Cat’s whisker
Another simple detector around this time was the
famous cat’s whisker (Fig. 4). It consisted of a fine
wire electrode whose pointed tip was pressed
against the crystal of a detector (very often a
crystal of galena). When connected to a suitable
tuned circuit, an antenna, an earth point and a
headphone, this simple circuit formed a very
useful receiver for use with local stations.
3.
3.1.
Thermionic valves
Diodes
All the above detectors eventually gave way to
the thermionic valve (Fig. 5) which was conceived by Prof. Fleming in 1904. Marconi soon
realized the practical importance of this invention and told one of his collaborators at Clifden:
“the future of the radio will be based upon this
small lamp. Its use will require the help of special
devices. Many experiments will be needed to
improve it”.
In 1905, Fleming’s diode was installed in the
receivers of the Marconi company. The diode
consisted of a glass bulb containing a carbon filament (the cathode) surrounded by a cylinder of
66
Triodes
In 1906, Lee de Forest experimented with
Fleming’s valve and ingeniously conceived a third
electrode, the control grid, in order to control the
flow of electrons. When the grid was negative with
respect to the filament, the electrons were repelled
so that the anode current was reduced. When the
potential of the grid changed to positive with
respect to the filament, the anode current increased
so as to produce an amplifying effect. At first, the
triode valve proved to be a sensitive detector; it
was not until 1912 that Lee de Forest appreciated
and exploited its possibilities as an amplifier.
In the first triode receivers, the radiofrequency
signal was taken direct from the antenna to the
valve input via a Tesla transformer with a tuning
capacitor, thus providing a tuned-resonance
circuit. Rectification was achieved by means of
either a single crystal or diode, if only half-wave
rectification was required, or a pair of crystals or
diodes if full-wave rectification was required.
Around this time, it had also been “discovered”
that the triode valve could be used to generate
continuous radiofrequency oscillations and,
moreover, these oscillations could be modulated
at audio frequencies. The stage was now set for
rapid developments in the field of radiotelephony
and Marconi was just one of many to exploit this
new communications medium. In 1914, he used
valve oscillators to carry out regular radiotelephony transmissions for the Italian Navy.
Marconi was able to achieve distances of around
110 km by using devices based upon the
oscillating circuit of Captain Round (Fig. 6).
A triode oscillator works on the principle of feedback from the anode circuit to the grid circuit.
When an oscillating voltage of suitable amplitude
and phase is fed back from the anode plate to the
grid so as to increase the grid signal, the circuit
starts to oscillate with a frequency determined by
the circuit constants (inductance and capacitance)
of the anode and the grid.
EBU Technical Review Spring 1995
Lemme & Menicucci
100
thin metal (the anode or plate). When the filament
was heated until white hot, the emitted electrons
reached the anode plate. If the latter was at a
positive potential with respect to the filament, it
attracted the electrons so that a current flowed in
the filament-plate circuit. If the plate became
negative, no current flowed through the valve.
Thus, by using a thermionic diode, very small HF
currents could be rectified into unidirectional
currents.
3.2.
Figure 5
An early thermionic
diode, circa 1904.
experiments. Only rough materials were used:
for example, the prototype of the magnetic
detector was enclosed in a Havana cigar box.
There were no suitable instruments to measure
RF currents, nor was there any well-grounded
theory to aid the design of these early wireless
installations.
2.4.
Cat’s whisker
Another simple detector around this time was the
famous cat’s whisker (Fig. 4). It consisted of a fine
wire electrode whose pointed tip was pressed
against the crystal of a detector (very often a
crystal of galena). When connected to a suitable
tuned circuit, an antenna, an earth point and a
headphone, this simple circuit formed a very
useful receiver for use with local stations.
3.
3.1.
Thermionic valves
Diodes
All the above detectors eventually gave way to
the thermionic valve (Fig. 5) which was conceived by Prof. Fleming in 1904. Marconi soon
realized the practical importance of this invention and told one of his collaborators at Clifden:
“the future of the radio will be based upon this
small lamp. Its use will require the help of special
devices. Many experiments will be needed to
improve it”.
In 1905, Fleming’s diode was installed in the
receivers of the Marconi company. The diode
consisted of a glass bulb containing a carbon filament (the cathode) surrounded by a cylinder of
66
Triodes
In 1906, Lee de Forest experimented with
Fleming’s valve and ingeniously conceived a third
electrode, the control grid, in order to control the
flow of electrons. When the grid was negative with
respect to the filament, the electrons were repelled
so that the anode current was reduced. When the
potential of the grid changed to positive with
respect to the filament, the anode current increased
so as to produce an amplifying effect. At first, the
triode valve proved to be a sensitive detector; it
was not until 1912 that Lee de Forest appreciated
and exploited its possibilities as an amplifier.
In the first triode receivers, the radiofrequency
signal was taken direct from the antenna to the
valve input via a Tesla transformer with a tuning
capacitor, thus providing a tuned-resonance
circuit. Rectification was achieved by means of
either a single crystal or diode, if only half-wave
rectification was required, or a pair of crystals or
diodes if full-wave rectification was required.
Around this time, it had also been “discovered”
that the triode valve could be used to generate
continuous radiofrequency oscillations and,
moreover, these oscillations could be modulated
at audio frequencies. The stage was now set for
rapid developments in the field of radiotelephony
and Marconi was just one of many to exploit this
new communications medium. In 1914, he used
valve oscillators to carry out regular radiotelephony transmissions for the Italian Navy.
Marconi was able to achieve distances of around
110 km by using devices based upon the
oscillating circuit of Captain Round (Fig. 6).
A triode oscillator works on the principle of feedback from the anode circuit to the grid circuit.
When an oscillating voltage of suitable amplitude
and phase is fed back from the anode plate to the
grid so as to increase the grid signal, the circuit
starts to oscillate with a frequency determined by
the circuit constants (inductance and capacitance)
of the anode and the grid.
EBU Technical Review Spring 1995
Lemme & Menicucci
100
Marconi was right when he foretold that Fleming’s
valve would open up new horizons for radiotelephony. However, there were still many problems to be solved, first of all concerning the
vacuum in the valve bulbs. A small amount of gas
in the glass bulb of de Forest’s Audion seemed at
first to be advantageous for it to function. However, the operation of the de Forest and Fleming
valves was irregular due to the gas within them.
On the one hand, the amount of gas increased with
time; on the other hand, a variation of the bias
between the anode plate and the filament under
such conditions was sufficient to cause the breakdown of the valve, due to the increased bombardment of positive ions onto the filament.
Langmuir, an American, discovered that vacuum
reduction in the bulb was caused by gas atoms
contained in the molecules of the filament and in
the metal and glass parts of the valve. He constructed new pumps to create a hard vacuum in the
bulb. He also studied ways of expelling the gas
atoms from the metal and glass parts of the valve
by the so-called “clean-up effect”, and he devised
a method to seal the filaments inside the glass bulb
at a very high temperature. Lee de Forest was the
first to introduce metal filaments in place of the
carbon filaments used previously. Further experiments led to the metal filaments being coated with
alkaline oxides which improved the efficiency of
the thermionic triode even more.
3.3.
4.
Superheterodyne receivers
Thereafter, the superheterodyne (superhet)
receiver was developed. It had several important
advantages over earlier types of receivers. In the
superhet circuit, the incoming RF signal was
changed to a lower frequency known as the intermediate frequency (IF). The major part of the
amplification then took place at this lower frequency before detection occurred in the normal
manner. In this way, it was possible to avoid the
drawbacks of tuned radiofrequency (TRF) circuits, which were simple to design and construct
but which presented the risk of instability and
had a problem of the RF gain being dependent
upon frequency.
5.
5.1.
Early transmitting devices
The spark gap
Let us turn now to the first transmitters. Following
on from the early experiments of Marconi and
others, a lot of improvements were achieved in
both the circuits and the power supplies of transmitters, due to the cooperation of Fleming. One
such circuit is shown in Fig. 7. In order to avoid
too high a voltage at the secondary of the transformer, a dual transformation technique was used.
Here, a capacitor was first charged with a voltage
of 20 kV, then its oscillatory discharge was
Tetrodes and pentodes
The plate-grid capacity – the cause of many problems for technicians – was reduced to almost one
hundredth that of a triode by providing a second
grid (the so-called screen grid) to act as a screen
and prevent the control grid from being influenced
by the anode voltage changes. The screen grid
reduced the anode-grid feedback and, hence, it
prevented instability.
Later, the pentode (containing a third grid called
the suppression grid) and other types of valves
were developed, leading to further progress in the
design of receivers and transmitters. In addition to
the amplification and detection abilities of these
devices, they could also be used to generate controlled oscillations within the receiver. Thus, for
example, these more sophisticated valves allowed
Prof. Fessenden to construct the first heterodyne
receiver, which achieved signal detection by beating the incoming RF signal against one produced
by a local oscillator in the receiver. In this way, an
unmodulated RF carrier was made audible, as the
beat note was at an audible frequency.
EBU Technical Review Spring 1995
Lemme & Menicucci
M
Figure 6
The oscillating circuit
developed by Captain
H.J. Round.
67
100
Marconi was right when he foretold that Fleming’s
valve would open up new horizons for radiotelephony. However, there were still many problems to be solved, first of all concerning the
vacuum in the valve bulbs. A small amount of gas
in the glass bulb of de Forest’s Audion seemed at
first to be advantageous for it to function. However, the operation of the de Forest and Fleming
valves was irregular due to the gas within them.
On the one hand, the amount of gas increased with
time; on the other hand, a variation of the bias
between the anode plate and the filament under
such conditions was sufficient to cause the breakdown of the valve, due to the increased bombardment of positive ions onto the filament.
Langmuir, an American, discovered that vacuum
reduction in the bulb was caused by gas atoms
contained in the molecules of the filament and in
the metal and glass parts of the valve. He constructed new pumps to create a hard vacuum in the
bulb. He also studied ways of expelling the gas
atoms from the metal and glass parts of the valve
by the so-called “clean-up effect”, and he devised
a method to seal the filaments inside the glass bulb
at a very high temperature. Lee de Forest was the
first to introduce metal filaments in place of the
carbon filaments used previously. Further experiments led to the metal filaments being coated with
alkaline oxides which improved the efficiency of
the thermionic triode even more.
3.3.
4.
Superheterodyne receivers
Thereafter, the superheterodyne (superhet)
receiver was developed. It had several important
advantages over earlier types of receivers. In the
superhet circuit, the incoming RF signal was
changed to a lower frequency known as the intermediate frequency (IF). The major part of the
amplification then took place at this lower frequency before detection occurred in the normal
manner. In this way, it was possible to avoid the
drawbacks of tuned radiofrequency (TRF) circuits, which were simple to design and construct
but which presented the risk of instability and
had a problem of the RF gain being dependent
upon frequency.
5.
5.1.
Early transmitting devices
The spark gap
Let us turn now to the first transmitters. Following
on from the early experiments of Marconi and
others, a lot of improvements were achieved in
both the circuits and the power supplies of transmitters, due to the cooperation of Fleming. One
such circuit is shown in Fig. 7. In order to avoid
too high a voltage at the secondary of the transformer, a dual transformation technique was used.
Here, a capacitor was first charged with a voltage
of 20 kV, then its oscillatory discharge was
Tetrodes and pentodes
The plate-grid capacity – the cause of many problems for technicians – was reduced to almost one
hundredth that of a triode by providing a second
grid (the so-called screen grid) to act as a screen
and prevent the control grid from being influenced
by the anode voltage changes. The screen grid
reduced the anode-grid feedback and, hence, it
prevented instability.
Later, the pentode (containing a third grid called
the suppression grid) and other types of valves
were developed, leading to further progress in the
design of receivers and transmitters. In addition to
the amplification and detection abilities of these
devices, they could also be used to generate controlled oscillations within the receiver. Thus, for
example, these more sophisticated valves allowed
Prof. Fessenden to construct the first heterodyne
receiver, which achieved signal detection by beating the incoming RF signal against one produced
by a local oscillator in the receiver. In this way, an
unmodulated RF carrier was made audible, as the
beat note was at an audible frequency.
EBU Technical Review Spring 1995
Lemme & Menicucci
M
Figure 6
The oscillating circuit
developed by Captain
H.J. Round.
67
100
supplied through a Tesla transformer to a second
oscillating circuit provided with a spark gap. This
circuit was connected to the antenna via the
jigger (secondary winding) of a second Tesla
transformer.
The spark gap consisted of two aligned steel
disks, almost tangential to each other with a
narrow spark gap between their edges. The
lower portion of each disk was dipped in its own
separate vessel filled with mercury, to avoid the
overheating of the disks during electrical discharges. It was an installation such as this
which succeeded in repeatedly transmitting the
famous Morse code signal for the letter “S”
(dot-dot-dot) from Poldhu in Cornwall, England, to St. John’s in Newfoundland, Canada, in
December 1901.
This momentous event gave new impetus to the
researchers. The simple spark gap needed to be
improved. After arcing, it did not lose its conductivity immediately, because of ionization
effects, and this prevented the capacitor from
recharging quickly, resulting in a greater energy
consumption and the generation of fewer wavetrains. The simple spark gap was duly replaced
by the quenched spark gap and, afterwards, by
the rotary discharger (Marconi, 1905) in order
to cause the electrode to quench and the sparks
to be interrupted more quickly.
However, there was still the problem of damped
waves and low efficiency, which allowed only the
transmission of telegraphy signals; intermittent
operation was not adaptable to telephony modulation.
5.2.
Voltaic-arc generators
There was a need to go back to basic theory. In
1850, Sir W. Thompson had defined the relationship between the resistance, inductance and capacitance that was needed to provide an oscillatory
discharge from a circuit. Based on those ideas,
generators of undamped waves were eventually
developed, along with voltaic-arc generators
which used carbon electrodes.
The operating principle of the voltaic-arc
generator (Fig. 8) is based upon the negative
resistance of the arc. A series capacitor and inductor, connected in parallel with an arc, are charged
by a DC power supply. This causes the voltage
across the arc and the LC circuit to increase. However, the capacitor is charged to a greater voltage
than that of the arc, due to the effect of the inductor.
When the capacitor starts to discharge, the current
across the arc increases, thus causing the voltage to
decrease. When the voltage of the capacitor
equals that of the arc, the inductor maintains the
discharge current, causing the voltage across the
capacitor to become lower than that of the arc. At
the end of the discharge cycle, the capacitor is
charged again and the process is repeated.
Figure 7
Circuit diagram of a
Marconi transmitter
around the time of the
Poldhu - St. John’s
trials (1901).
Power
meter
Tesla
transformer
Tesla
transformer
ÉÉÉÉ
É
ÉÉÉÉ
É
ÉÉÉÉ
ÉÉÉÉ
68
EBU Technical Review Spring 1995
Lemme & Menicucci
100
supplied through a Tesla transformer to a second
oscillating circuit provided with a spark gap. This
circuit was connected to the antenna via the
jigger (secondary winding) of a second Tesla
transformer.
The spark gap consisted of two aligned steel
disks, almost tangential to each other with a
narrow spark gap between their edges. The
lower portion of each disk was dipped in its own
separate vessel filled with mercury, to avoid the
overheating of the disks during electrical discharges. It was an installation such as this
which succeeded in repeatedly transmitting the
famous Morse code signal for the letter “S”
(dot-dot-dot) from Poldhu in Cornwall, England, to St. John’s in Newfoundland, Canada, in
December 1901.
This momentous event gave new impetus to the
researchers. The simple spark gap needed to be
improved. After arcing, it did not lose its conductivity immediately, because of ionization
effects, and this prevented the capacitor from
recharging quickly, resulting in a greater energy
consumption and the generation of fewer wavetrains. The simple spark gap was duly replaced
by the quenched spark gap and, afterwards, by
the rotary discharger (Marconi, 1905) in order
to cause the electrode to quench and the sparks
to be interrupted more quickly.
However, there was still the problem of damped
waves and low efficiency, which allowed only the
transmission of telegraphy signals; intermittent
operation was not adaptable to telephony modulation.
5.2.
Voltaic-arc generators
There was a need to go back to basic theory. In
1850, Sir W. Thompson had defined the relationship between the resistance, inductance and capacitance that was needed to provide an oscillatory
discharge from a circuit. Based on those ideas,
generators of undamped waves were eventually
developed, along with voltaic-arc generators
which used carbon electrodes.
The operating principle of the voltaic-arc
generator (Fig. 8) is based upon the negative
resistance of the arc. A series capacitor and inductor, connected in parallel with an arc, are charged
by a DC power supply. This causes the voltage
across the arc and the LC circuit to increase. However, the capacitor is charged to a greater voltage
than that of the arc, due to the effect of the inductor.
When the capacitor starts to discharge, the current
across the arc increases, thus causing the voltage to
decrease. When the voltage of the capacitor
equals that of the arc, the inductor maintains the
discharge current, causing the voltage across the
capacitor to become lower than that of the arc. At
the end of the discharge cycle, the capacitor is
charged again and the process is repeated.
Figure 7
Circuit diagram of a
Marconi transmitter
around the time of the
Poldhu - St. John’s
trials (1901).
Power
meter
Tesla
transformer
Tesla
transformer
ÉÉÉÉ
É
ÉÉÉÉ
É
ÉÉÉÉ
ÉÉÉÉ
68
EBU Technical Review Spring 1995
Lemme & Menicucci
100
Poulsen then experimented with arcs dipped into a
large tank of hydrogen. This type of generator
produced quasi-continuous waves which could be
amplitude modulated for telephony applications.
The system offered the great advantage of simplicity and economy, although it generated harmonics and consequently, RF interference. In
spite of its drawbacks, many researchers deemed
that the Poulsen arc was the successor of the sparkgap system and devoted themselves to improve it
even further. Prof. Majorana – who experimented
with multiple-spark generators for radiotelephony
applications in 1903-1904 – used the Poulsen arc
to transmit telephony signals over a range of about
400 km (the maximum distance that could be
reached by radiotelephony at the time).
5.3.
DC power
supply
C
Carbon
rods
Oscillatory
circuit
L
Isolating
choke
Figure 8
Circuit diagram of a
voltaic-arc generator.
parallel. An array of a further 300 valves with their
anodes in parallel was driven by the above circuits
and supplied its output energy to the high antenna
of the Arlington station.
Static frequency multipliers
Special alternators were developed to generate
continuous RF waves, but they had many
mechanical problems due to the very high frequencies that had to be reached. One method of
overcoming such problems was to use a “static
frequency multiplier” – a magnetic-core device,
similar to a peaking transformer, which provided
harmonics by distorting a sinewave. The multiplier was supplied directly by the alternator and
operated as an oscillation exciter in a circuit
which was tuned to the desired frequency and
coupled to the antenna.
This type of system was limited to longwave
applications. The cost of the installation was
prohibitively high because, in addition to the
alternators, several 200 m high towers were
required to support the horizontal longwave
radiators. This prompted the researchers to develop a simpler, more economic, generator of RF
oscillations.
6.
Isolating
choke
Thermionic valve
generators
It was around this time that the first thermionicvalve generators appeared. A lot of time was
devoted to their development, before the advantages of their application could be enjoyed. At that
point, there were many clashes between the
proponents of valve generators and the supporters
of systems which used RF alternators.
In 1915, considerable progress was obtained by the
stations at Arlington and Honolulu. An oscillatory
valve was coupled to the grid circuit of a valve of
greater power. The plate current of the latter drove
in turn the grid circuits of a further 10 valves in
EBU Technical Review Spring 1995
Lemme & Menicucci
Marconi had already experimented with valve
transmitters for the navy in 1914. With his own
experience and with that of Arlington and Honolulu, he became convinced that a transmitter which
used alternators, a voltaic-arc generator or rotary
discharger could not provide a commercial service
more convenient than the wired service. Therefore, he decided to develop systems that would
require new valve designs which were capable of
delivering power levels of greater than 1500 W.
In order to achieve the required power levels,
Marconi connected 48 valves in parallel and
increased the anode voltage to 10 kV. The resultant current in the aerial was then 330 A. A new
problem to be overcome was that of keeping the
frequency constant.
In 1921, Franklin devised the “Master Oscillator” consisting of an oscillating valve circuit
which generated signals of constant frequency,
independent of temperature changes. It was connected to a number of amplifier stages in order to
reach the desired power for supplying the aerial.
Thereafter, as it was easier to stabilize an oscillator that operated at a low frequency, the “drive
oscillator” was arranged to generate a frequency
which was a submultiple of the frequency of the
transmitter. The drive oscillator was followed
by several frequency doublers and triplers which
also operated as amplifiers. Very stable oscillators with “quartz crystals” (based upon piezoelectrical phenomena) were later developed: a
suitably-cut quartz plate is a resonator of excellent quality, especially if it is kept at a constant
temperature.
In addition to the problem of producing highpower radiofrequency energy, solved by the use of
valves, a new problem arose: which frequency was
the most suitable for a given transmission?
69
100
Poulsen then experimented with arcs dipped into a
large tank of hydrogen. This type of generator
produced quasi-continuous waves which could be
amplitude modulated for telephony applications.
The system offered the great advantage of simplicity and economy, although it generated harmonics and consequently, RF interference. In
spite of its drawbacks, many researchers deemed
that the Poulsen arc was the successor of the sparkgap system and devoted themselves to improve it
even further. Prof. Majorana – who experimented
with multiple-spark generators for radiotelephony
applications in 1903-1904 – used the Poulsen arc
to transmit telephony signals over a range of about
400 km (the maximum distance that could be
reached by radiotelephony at the time).
5.3.
DC power
supply
C
Carbon
rods
Oscillatory
circuit
L
Isolating
choke
Figure 8
Circuit diagram of a
voltaic-arc generator.
parallel. An array of a further 300 valves with their
anodes in parallel was driven by the above circuits
and supplied its output energy to the high antenna
of the Arlington station.
Static frequency multipliers
Special alternators were developed to generate
continuous RF waves, but they had many
mechanical problems due to the very high frequencies that had to be reached. One method of
overcoming such problems was to use a “static
frequency multiplier” – a magnetic-core device,
similar to a peaking transformer, which provided
harmonics by distorting a sinewave. The multiplier was supplied directly by the alternator and
operated as an oscillation exciter in a circuit
which was tuned to the desired frequency and
coupled to the antenna.
This type of system was limited to longwave
applications. The cost of the installation was
prohibitively high because, in addition to the
alternators, several 200 m high towers were
required to support the horizontal longwave
radiators. This prompted the researchers to develop a simpler, more economic, generator of RF
oscillations.
6.
Isolating
choke
Thermionic valve
generators
It was around this time that the first thermionicvalve generators appeared. A lot of time was
devoted to their development, before the advantages of their application could be enjoyed. At that
point, there were many clashes between the
proponents of valve generators and the supporters
of systems which used RF alternators.
In 1915, considerable progress was obtained by the
stations at Arlington and Honolulu. An oscillatory
valve was coupled to the grid circuit of a valve of
greater power. The plate current of the latter drove
in turn the grid circuits of a further 10 valves in
EBU Technical Review Spring 1995
Lemme & Menicucci
Marconi had already experimented with valve
transmitters for the navy in 1914. With his own
experience and with that of Arlington and Honolulu, he became convinced that a transmitter which
used alternators, a voltaic-arc generator or rotary
discharger could not provide a commercial service
more convenient than the wired service. Therefore, he decided to develop systems that would
require new valve designs which were capable of
delivering power levels of greater than 1500 W.
In order to achieve the required power levels,
Marconi connected 48 valves in parallel and
increased the anode voltage to 10 kV. The resultant current in the aerial was then 330 A. A new
problem to be overcome was that of keeping the
frequency constant.
In 1921, Franklin devised the “Master Oscillator” consisting of an oscillating valve circuit
which generated signals of constant frequency,
independent of temperature changes. It was connected to a number of amplifier stages in order to
reach the desired power for supplying the aerial.
Thereafter, as it was easier to stabilize an oscillator that operated at a low frequency, the “drive
oscillator” was arranged to generate a frequency
which was a submultiple of the frequency of the
transmitter. The drive oscillator was followed
by several frequency doublers and triplers which
also operated as amplifiers. Very stable oscillators with “quartz crystals” (based upon piezoelectrical phenomena) were later developed: a
suitably-cut quartz plate is a resonator of excellent quality, especially if it is kept at a constant
temperature.
In addition to the problem of producing highpower radiofrequency energy, solved by the use of
valves, a new problem arose: which frequency was
the most suitable for a given transmission?
69
100
signals were strong and clear, even if the transmission power was reduced to only 1 kW. Under
such conditions, the signals were stronger than
those transmitted from Caernarvon (Wales) and
Nauen (Germany) on a wavelength several
hundred times greater, and with a power of
200 kW.
Thereafter, studies were made to improve the
modulation percentage (i.e. modulation index)
and to reduce the distortion. One such method
was grid modulation which was easier to produce
but which had low fidelity. Heising introduced
anode modulation where the signal, after suitable
amplification, was superimposed on the anode
voltage of the final radiofrequency valve. In contrast to the other methods, a modulated wave with
a high modulation percentage and a low distortion
was obtained.
Finally they had to solve the problem of how to
cool the valves which operated at power levels of
above 2500 W. Around 1930, the anode was
cooled by oil circulation. Later, after the solution
of anode-insulation problems, distilled water
circulation was used to keep the anode cool. Thus,
the high power required in the final stages of the
transmitter, especially for long waves, could now
be obtained by a lesser number of valves operating
in parallel. After a lot of improvements, the
tetrodes of today are capable of delivering several
hundreds of kilowatts.
7.
Figure 9
A high-voltage
mercury-arc rectifier
from 1933.
The first works of Marconi were based upon the
use of medium and long waves. Apparatus of high
power and large antennas supported by high
towers were needed, which required considerable
investment and resulted in a very expensive radio
service. The theory of Loewenstein (concerning
the composition of the atmosphere and the
influence of its layers upon the propagation of the
electrical waves), along with experiences made in
the short-wave field, prompted Marconi to confide
to one collaborator: “Only the short waves will be
able to save the radio from the blind alley of the
medium and long waves”.
In 1923, Marconi carried out important experiments with Franklin between the ship “Elettra” at
anchor by St. Vincent in the Cape Verde Islands,
(off the coast of West Africa) and the station at
Poldhu in Cornwall, England, where a short-wave
transmitter (25 – 90 metres) had also been
installed. At a distance of 4130 km the received
70
Modern transmitting
devices
Increases in the power needed for AM transmitters, accompanied by increases in the cost of
energy supply, has prompted the development of
high-efficiency RF power amplifiers and modulation systems (Pulse Duration Modulation,
Pulse Step Modulation, etc). More recently,
completely-digital AM transmitters with very
high efficiency have been developed.
Over a few decades, the increased application of
radio-telecommunication systems, and the growing need for more radiofrequency spectrum to
accommodate these systems, has led to the use of
ever higher frequencies: hundreds of MHz, tens of
GHz ... up to the frequencies of light. New devices,
based on the control of the electron beam current,
have taken the place of the old electronic tubes.
The maximum operating frequency of triodes,
tetrodes and pentodes is limited by the reactances
and resistances associated with the electrodes.
These effects, for instance, reduce the input and
EBU Technical Review Spring 1995
Lemme & Menicucci
100
signals were strong and clear, even if the transmission power was reduced to only 1 kW. Under
such conditions, the signals were stronger than
those transmitted from Caernarvon (Wales) and
Nauen (Germany) on a wavelength several
hundred times greater, and with a power of
200 kW.
Thereafter, studies were made to improve the
modulation percentage (i.e. modulation index)
and to reduce the distortion. One such method
was grid modulation which was easier to produce
but which had low fidelity. Heising introduced
anode modulation where the signal, after suitable
amplification, was superimposed on the anode
voltage of the final radiofrequency valve. In contrast to the other methods, a modulated wave with
a high modulation percentage and a low distortion
was obtained.
Finally they had to solve the problem of how to
cool the valves which operated at power levels of
above 2500 W. Around 1930, the anode was
cooled by oil circulation. Later, after the solution
of anode-insulation problems, distilled water
circulation was used to keep the anode cool. Thus,
the high power required in the final stages of the
transmitter, especially for long waves, could now
be obtained by a lesser number of valves operating
in parallel. After a lot of improvements, the
tetrodes of today are capable of delivering several
hundreds of kilowatts.
7.
Figure 9
A high-voltage
mercury-arc rectifier
from 1933.
The first works of Marconi were based upon the
use of medium and long waves. Apparatus of high
power and large antennas supported by high
towers were needed, which required considerable
investment and resulted in a very expensive radio
service. The theory of Loewenstein (concerning
the composition of the atmosphere and the
influence of its layers upon the propagation of the
electrical waves), along with experiences made in
the short-wave field, prompted Marconi to confide
to one collaborator: “Only the short waves will be
able to save the radio from the blind alley of the
medium and long waves”.
In 1923, Marconi carried out important experiments with Franklin between the ship “Elettra” at
anchor by St. Vincent in the Cape Verde Islands,
(off the coast of West Africa) and the station at
Poldhu in Cornwall, England, where a short-wave
transmitter (25 – 90 metres) had also been
installed. At a distance of 4130 km the received
70
Modern transmitting
devices
Increases in the power needed for AM transmitters, accompanied by increases in the cost of
energy supply, has prompted the development of
high-efficiency RF power amplifiers and modulation systems (Pulse Duration Modulation,
Pulse Step Modulation, etc). More recently,
completely-digital AM transmitters with very
high efficiency have been developed.
Over a few decades, the increased application of
radio-telecommunication systems, and the growing need for more radiofrequency spectrum to
accommodate these systems, has led to the use of
ever higher frequencies: hundreds of MHz, tens of
GHz ... up to the frequencies of light. New devices,
based on the control of the electron beam current,
have taken the place of the old electronic tubes.
The maximum operating frequency of triodes,
tetrodes and pentodes is limited by the reactances
and resistances associated with the electrodes.
These effects, for instance, reduce the input and
EBU Technical Review Spring 1995
Lemme & Menicucci
100
output impedances, and the efficiency as well.
Additionally, when the electron transit time is
comparable with a quarter of the period of the
microwave, the total efficiency of the valve, when
operating either as an oscillator or an amplifier, is
reduced to less than half. As a result, it is difficult
to produce triode or tetrode amplifiers for use
above 1 GHz which have an output power of
several kilowatts and an acceptable efficiency for
continuous operation.
7.1.
Electron-velocity control
tubes
Tubes which operate by modulating the velocity of
an electron beam have been developed for microwave use. Such tubes are mainly of two types:
– klystrons and travelling wave tubes, where the
electron beam flows linearly;
– magnetrons, where the electron beam follows a
curved path under the action of orthogonal electric and magnetic fields.
Klystrons and travelling wave tubes find a wide
application in broadcasting and microwave telecommunication systems. Both types of device use
an axial magnetic field to “collimate” the electron
beam, i.e. to prevent the beam from spreading out
as a result of small initial divergent angles, and the
repulsion effect of the negative charges which
make up the beam. The speed of each moving
electron is controlled by an electric field and is
calculated from the following equation for kinetic
energy:
7.1.1.
Klystron
The Klystron was invented in 1937 and basically
operates as follows (Fig. 10). Electrons are fired
from an electron gun which contains a cathode
heated by a filament, and a voltage source which
produces acceleration of the beam. The microwave input signal at the first cavity creates voltages across the gap traversed by the beam, causing
the electrons in the beam to accelerate or decelerate. As it travels towards the collector, the electron
beam is thus “velocity modulated” by the input
signal; groups of electrons bunch together to form
“electron packets”. When these electron packets
arrive at the output cavity, they generate a microwave resonance at an increased power level relative to the input signal, due to the energy supplied
by the electron beam.
Additional “intermediate” cavities are provided
on some klystrons to improve the bunching
process, resulting in a higher gain and improved
efficiency. By varying the size of a cavity, it is
possible to adjust its resonant frequency. Thus in
a multi-cavity device, if all the cavities are tuned
to the same frequency, the gain is maximized but
the bandwidth is reduced. On the other hand, if
the cavities are tuned to slightly different
frequencies, the gain is reduced but the bandwidth
is increased.
Klystrons with two or more cavities are used as
narrow-band linear power amplifiers. They can
generate about 10 kW under continuous operating
conditions, at a frequency of several GHz. The
Figure 10
maximum gain achieved by a klystron is some tens Schematic diagram of
of decibels and its efficiency ranges from about a two-cavity klystron
amplifier.
30 % to 50 %.
Ve 1 mv 2
2
where: V = the accelerating voltage acting on
the electron
e = the charge on the electron
(–1.6 x 10–19 C)
v = the velocity of the electron
m = the mass of the electron
(9.1 x 10–31 kg).
2Ve
m 10
7
Output cavity
Collector
Coaxial line
coupling
From this:
v
Input cavity
Electron gun
Waveguide
coupling
(cmsec)
Input
signal
And hence:
v 5.93 10 7 V (cmsec).
EBU Technical Review Spring 1995
Lemme & Menicucci
71
100
output impedances, and the efficiency as well.
Additionally, when the electron transit time is
comparable with a quarter of the period of the
microwave, the total efficiency of the valve, when
operating either as an oscillator or an amplifier, is
reduced to less than half. As a result, it is difficult
to produce triode or tetrode amplifiers for use
above 1 GHz which have an output power of
several kilowatts and an acceptable efficiency for
continuous operation.
7.1.
Electron-velocity control
tubes
Tubes which operate by modulating the velocity of
an electron beam have been developed for microwave use. Such tubes are mainly of two types:
– klystrons and travelling wave tubes, where the
electron beam flows linearly;
– magnetrons, where the electron beam follows a
curved path under the action of orthogonal electric and magnetic fields.
Klystrons and travelling wave tubes find a wide
application in broadcasting and microwave telecommunication systems. Both types of device use
an axial magnetic field to “collimate” the electron
beam, i.e. to prevent the beam from spreading out
as a result of small initial divergent angles, and the
repulsion effect of the negative charges which
make up the beam. The speed of each moving
electron is controlled by an electric field and is
calculated from the following equation for kinetic
energy:
7.1.1.
Klystron
The Klystron was invented in 1937 and basically
operates as follows (Fig. 10). Electrons are fired
from an electron gun which contains a cathode
heated by a filament, and a voltage source which
produces acceleration of the beam. The microwave input signal at the first cavity creates voltages across the gap traversed by the beam, causing
the electrons in the beam to accelerate or decelerate. As it travels towards the collector, the electron
beam is thus “velocity modulated” by the input
signal; groups of electrons bunch together to form
“electron packets”. When these electron packets
arrive at the output cavity, they generate a microwave resonance at an increased power level relative to the input signal, due to the energy supplied
by the electron beam.
Additional “intermediate” cavities are provided
on some klystrons to improve the bunching
process, resulting in a higher gain and improved
efficiency. By varying the size of a cavity, it is
possible to adjust its resonant frequency. Thus in
a multi-cavity device, if all the cavities are tuned
to the same frequency, the gain is maximized but
the bandwidth is reduced. On the other hand, if
the cavities are tuned to slightly different
frequencies, the gain is reduced but the bandwidth
is increased.
Klystrons with two or more cavities are used as
narrow-band linear power amplifiers. They can
generate about 10 kW under continuous operating
conditions, at a frequency of several GHz. The
Figure 10
maximum gain achieved by a klystron is some tens Schematic diagram of
of decibels and its efficiency ranges from about a two-cavity klystron
amplifier.
30 % to 50 %.
Ve 1 mv 2
2
where: V = the accelerating voltage acting on
the electron
e = the charge on the electron
(–1.6 x 10–19 C)
v = the velocity of the electron
m = the mass of the electron
(9.1 x 10–31 kg).
2Ve
m 10
7
Output cavity
Collector
Coaxial line
coupling
From this:
v
Input cavity
Electron gun
Waveguide
coupling
(cmsec)
Input
signal
And hence:
v 5.93 10 7 V (cmsec).
EBU Technical Review Spring 1995
Lemme & Menicucci
71
100
Cavity
Interaction
gap
Electron
gun
Repeller
Load
Figure 11
Schematic diagram of
a reflex klystron
amplifier.
Another version of the device, known as a reflex
klystron, is used as an RF oscillator (Fig. 11). In
this design, the output cavity has been eliminated
and an electrode (repeller), at a suitable negative
potential, is used to repel electron packets back to
the two grids of the sole cavity.
7.1.2.
Figure 12
Schematic diagram
of a travelling wave
tube.
Travelling wave tubes
close to the speed of light. However, along the axis
of the helix, the velocity of propagation (the socalled phase velocity) is much lower, typically 0.1
to 0.3 times the speed of light; it is equal to the
product of the velocity of light and the ratio
between the pitch and circumference of the helix.
The electrons generated by the TWT are accelerated by a high voltage such that, by the time they
reach the helix, their velocity is similar to that of
the axial phase velocity of the RF input signal
along the helix. When the RF signal enters the
helix, the longitudinal part of its field interacts
with the electron beam, causing some electrons to
accelerate and others to decelerate such that
electron packets are formed in a similar manner to
that of the klystron. The result is a progressive
rearrangement in phase of the electrons in relation
to the RF wave. In turn, the modulated electron
beam induces additional waves on the helix and
this process of mutual interaction continues along
the length of the tube. The outcome is that the DC
energy of the electron beam is transformed into
RF energy in the helix, and the RF wave is
amplified.
In a travelling wave tube (TWT) (Fig. 12), the
electrons generated by the heated cathode travel
along the axis of the tube, constrained by focussing
coils, until they reach the collector. Spaced closely
around the electron beam is a helix which is
capable of propagating a slow-moving electromagnetic wave.
In order to avoid self-oscillations, the amplified
RF wave should not be reflected back to the input
of the helix; any reflected portion of the signal
must be attenuated along the helix. To achieve this,
the wire of the helix is made of, or coated with, a
magnetic material. As a result, the potential gain
of a TWT is reduced but a good compromise between gain and stability can usually be reached.
The RF signal enters the tube via the input waveguide and travels along the helix wire at velocities
Unlike the klystron, a travelling wave tube has no
specific resonating elements. It can be fabricated
Input
waveguide
Gun anode
Output
waveguide
Helix
Glass
Collector
Heater
72
Cathode
Focusing coils
EBU Technical Review Spring 1995
Lemme & Menicucci
100
Cavity
Interaction
gap
Electron
gun
Repeller
Load
Figure 11
Schematic diagram of
a reflex klystron
amplifier.
Another version of the device, known as a reflex
klystron, is used as an RF oscillator (Fig. 11). In
this design, the output cavity has been eliminated
and an electrode (repeller), at a suitable negative
potential, is used to repel electron packets back to
the two grids of the sole cavity.
7.1.2.
Figure 12
Schematic diagram
of a travelling wave
tube.
Travelling wave tubes
close to the speed of light. However, along the axis
of the helix, the velocity of propagation (the socalled phase velocity) is much lower, typically 0.1
to 0.3 times the speed of light; it is equal to the
product of the velocity of light and the ratio
between the pitch and circumference of the helix.
The electrons generated by the TWT are accelerated by a high voltage such that, by the time they
reach the helix, their velocity is similar to that of
the axial phase velocity of the RF input signal
along the helix. When the RF signal enters the
helix, the longitudinal part of its field interacts
with the electron beam, causing some electrons to
accelerate and others to decelerate such that
electron packets are formed in a similar manner to
that of the klystron. The result is a progressive
rearrangement in phase of the electrons in relation
to the RF wave. In turn, the modulated electron
beam induces additional waves on the helix and
this process of mutual interaction continues along
the length of the tube. The outcome is that the DC
energy of the electron beam is transformed into
RF energy in the helix, and the RF wave is
amplified.
In a travelling wave tube (TWT) (Fig. 12), the
electrons generated by the heated cathode travel
along the axis of the tube, constrained by focussing
coils, until they reach the collector. Spaced closely
around the electron beam is a helix which is
capable of propagating a slow-moving electromagnetic wave.
In order to avoid self-oscillations, the amplified
RF wave should not be reflected back to the input
of the helix; any reflected portion of the signal
must be attenuated along the helix. To achieve this,
the wire of the helix is made of, or coated with, a
magnetic material. As a result, the potential gain
of a TWT is reduced but a good compromise between gain and stability can usually be reached.
The RF signal enters the tube via the input waveguide and travels along the helix wire at velocities
Unlike the klystron, a travelling wave tube has no
specific resonating elements. It can be fabricated
Input
waveguide
Gun anode
Output
waveguide
Helix
Glass
Collector
Heater
72
Cathode
Focusing coils
EBU Technical Review Spring 1995
Lemme & Menicucci
100
with a pass-band of 600 – 800 MHz and can yield
a gain of some tens of decibels. The maximum
power levels produced by a TWT are of the order
of several hundred watts at 10 GHz, and its efficiency is around 30 %.
8.
Transistors and other
semiconductors
Although we can refer to the cat’s whisker crystal
as the earliest ancestor of modern semiconductor
devices, reference should also be made to the
silicon point-contact diode, which was developed
during the Second World War for use in microwave
radar receivers, and the germanium junction diode
which was developed shortly after.
However, the new electronic age began in 1948
when W. Shockley, J. Berdeen and W.H. Brattain
of Bell Telephone Laboratories announced the
first transistor (a name abridged from “transfer
resistor”). This important electronic device had
a strong impact on the field of electronics, in both
engineering and economic terms. Transistors
and, in general, semiconductors are based upon
the controlled presence of imperfections in
otherwise nearly-perfect crystals. The materials
generally used are germanium and silicon.
8.1.
Semiconductor physics
The germanium or silicon atom may be considered
to consist of four valence electrons surrounding a
nucleus which has a positive charge of four
electron units. The atom as a whole is thus neutral.
This symmetry is broken if we introduce some
chemical impurities, the so-called donors or
acceptors, into silicon and germanium crystals.
The presence of these classes of impurities causes
an excess of electrons or holes, thus making the
crystal a conductor or rather a semiconductor.
Typical donor elements for silicon and germanium
are antimony and arsenic which each have five
valence electrons. Consequently, one electron for
every atom of impurity becomes an excess electron and wanders throughout the crystal (in the
n-type semiconductor).
Aluminum and gallium are typical acceptors for
silicon and germanium. These elements have three
valence electrons rather than four. As a result, they
cannot complete the paired electron structure of
the four bonds surrounding them, when they
replace an atom in the crystal lattice. If an electron
from a bond somewhere else is used to fill in this
hole, the acceptor atom will acquire a localized
negative charge and a new hole is created in the
EBU Technical Review Spring 1995
Lemme & Menicucci
atom which had given up its spare electron. The
crystal becomes a p-type semiconductor because a
positive particle wanders throughout the crystal
lattice.
It is evident that in a single crystal there may be
both p-type and n-type regions. The boundary
between such regions is called a p-n junction. Such
p-n junctions have interesting electrical and
optical properties. Due to the different charge on
the particles which wander throughout the two
regions (electrons and holes), an electric field is
established across the junction and no current will
flow across the junction under conditions of
thermal equilibrium.
The application of a negative voltage to the
p-region and a positive voltage to the n-region
(reverse bias) will increase the electrostatic potential difference between the two regions, so that no
current due to the majority carriers flows into the
device. If the reverse voltage increases to the
breakdown value (Zener voltage), the current
flowing into the device will rapidly increase to the
point of reverse saturation. This is due to the
minority carriers which are present in the crystal as
a result of breaking the covalent bonds. When
forward bias is applied, the current increases exponentially to the point of forward saturation.
8.2.
Junction transistor
The junction transistor consists of a single crystal
of germanium or silicon divided into three regions:
these regions are alternately n-type, p-type and
n-type and are called the emitter, base and
collector respectively. When an n-p-n transistor is
used as an amplifier, the positive bias is applied to
the collector region in order to bias the p-n junction
between the collector and the base regions in the
reverse direction. If a positive signal is applied
between the emitter and the base, a large number
of electrons will be able to diffuse into the base.
Because of the thickness of the base region, we
can consider that all the electron current entering
the base region from the emitter arrives at the
collector. Only a negligible fraction of the emitter
current flows in the base circuit. All changes in
the emitter current appear in the collector current
and, thus, the junction transistor behaves much
like an ideal triode but requires no cathode power
and offers the hope of practically unlimited life.
8.3.
Other transistor types
In addition to the junction or bipolar transistor
(so called because the conduction involves
electrical charges of both signs), unipolar tran-
73
100
with a pass-band of 600 – 800 MHz and can yield
a gain of some tens of decibels. The maximum
power levels produced by a TWT are of the order
of several hundred watts at 10 GHz, and its efficiency is around 30 %.
8.
Transistors and other
semiconductors
Although we can refer to the cat’s whisker crystal
as the earliest ancestor of modern semiconductor
devices, reference should also be made to the
silicon point-contact diode, which was developed
during the Second World War for use in microwave
radar receivers, and the germanium junction diode
which was developed shortly after.
However, the new electronic age began in 1948
when W. Shockley, J. Berdeen and W.H. Brattain
of Bell Telephone Laboratories announced the
first transistor (a name abridged from “transfer
resistor”). This important electronic device had
a strong impact on the field of electronics, in both
engineering and economic terms. Transistors
and, in general, semiconductors are based upon
the controlled presence of imperfections in
otherwise nearly-perfect crystals. The materials
generally used are germanium and silicon.
8.1.
Semiconductor physics
The germanium or silicon atom may be considered
to consist of four valence electrons surrounding a
nucleus which has a positive charge of four
electron units. The atom as a whole is thus neutral.
This symmetry is broken if we introduce some
chemical impurities, the so-called donors or
acceptors, into silicon and germanium crystals.
The presence of these classes of impurities causes
an excess of electrons or holes, thus making the
crystal a conductor or rather a semiconductor.
Typical donor elements for silicon and germanium
are antimony and arsenic which each have five
valence electrons. Consequently, one electron for
every atom of impurity becomes an excess electron and wanders throughout the crystal (in the
n-type semiconductor).
Aluminum and gallium are typical acceptors for
silicon and germanium. These elements have three
valence electrons rather than four. As a result, they
cannot complete the paired electron structure of
the four bonds surrounding them, when they
replace an atom in the crystal lattice. If an electron
from a bond somewhere else is used to fill in this
hole, the acceptor atom will acquire a localized
negative charge and a new hole is created in the
EBU Technical Review Spring 1995
Lemme & Menicucci
atom which had given up its spare electron. The
crystal becomes a p-type semiconductor because a
positive particle wanders throughout the crystal
lattice.
It is evident that in a single crystal there may be
both p-type and n-type regions. The boundary
between such regions is called a p-n junction. Such
p-n junctions have interesting electrical and
optical properties. Due to the different charge on
the particles which wander throughout the two
regions (electrons and holes), an electric field is
established across the junction and no current will
flow across the junction under conditions of
thermal equilibrium.
The application of a negative voltage to the
p-region and a positive voltage to the n-region
(reverse bias) will increase the electrostatic potential difference between the two regions, so that no
current due to the majority carriers flows into the
device. If the reverse voltage increases to the
breakdown value (Zener voltage), the current
flowing into the device will rapidly increase to the
point of reverse saturation. This is due to the
minority carriers which are present in the crystal as
a result of breaking the covalent bonds. When
forward bias is applied, the current increases exponentially to the point of forward saturation.
8.2.
Junction transistor
The junction transistor consists of a single crystal
of germanium or silicon divided into three regions:
these regions are alternately n-type, p-type and
n-type and are called the emitter, base and
collector respectively. When an n-p-n transistor is
used as an amplifier, the positive bias is applied to
the collector region in order to bias the p-n junction
between the collector and the base regions in the
reverse direction. If a positive signal is applied
between the emitter and the base, a large number
of electrons will be able to diffuse into the base.
Because of the thickness of the base region, we
can consider that all the electron current entering
the base region from the emitter arrives at the
collector. Only a negligible fraction of the emitter
current flows in the base circuit. All changes in
the emitter current appear in the collector current
and, thus, the junction transistor behaves much
like an ideal triode but requires no cathode power
and offers the hope of practically unlimited life.
8.3.
Other transistor types
In addition to the junction or bipolar transistor
(so called because the conduction involves
electrical charges of both signs), unipolar tran-
73
100
gate
source
gate
drain
source
drain
P
P
D
N
Figure 13
The basic structure
of a MOSFET.
D
N
P
substrate
Typical N-MOS
G
N
substrate
ID
S
sistors have also been developed. In these
devices, conduction is due to electrical charges
of only one sign; the junction field effect transistor (JFET) and metal oxide semiconductor
field effect transistor (MOSFET) are perhaps
the best-known examples.
JFETs are active devices with three terminals
(gate, source and drain) which come in two
versions: p-channel and n-channel. Both versions
can operate in either a linear or a non-linear mode.
Between the channel and the gate there is a p-n
junction, the polarization of which controls the
current flowing between the source and the drain.
In order to operate in the linear mode, the gatechannel junction should be reverse-biased. The
output and transfer characteristics of a JFET are
very similar to those of a triode.
The MOSFET transistor (Fig. 13) consists of a
p-type or an n-type substrate. The gate is insulated
from the substrate by silicon oxide, while the
source and drain are reverse-doped regions with
respect to the substrate. The source and substrate
are innerly connected so that they are at the same
potential. The channel is formed by the free
charges in the source region which are attracted to
the gate by its polarization voltage.
FET transistors are widely deployed in the integrated amplifiers used for microwave applications up to frequencies of around 14 GHz. The socalled monolithic microwave integrated circuit
(MMIC) is fabricated by using gallium arsenide
technology.
For higher frequency applications, parametric
amplifiers with negative resistance are used. A
typical negative-resistance element is the gallium
arsenide varactor.
Typical P-MOS
9.
74
S
Microchips
By the end of the 1940s, as electronic systems grew
in importance and size, more power was consumed, their weight and costs increased while their
reliability decreased. The cold war and the space
race both demanded cheaper, smaller and more
reliable electronic systems. Following on from the
invention of the transistor in 1948, J.S. Kilby
completed the first integrated circuit in 1958; it
consisted of a phase oscillator formed entirely of
semiconductors (transistors, resistors and capacitors) in a single block called a microchip.
The use of large computers to design ICs, together with rapid developments in the chemical
and photo-optical technologies applied to semiconductor electronics, has led to:
– greater purity of the silicon wafer used as the
substrate;
– more accurate control of the doping of the
different substrate regions which provide the
basic components of the electronic circuit
(transistors, diodes, resistors, capacitors, etc.);
– the highly-accurate fabrication of microscopic
material layers (junctions, connecting tracks,
connection soldering);
– continuous improvements in the performance
of chips due the use of new materials, such as
gallium arsenide and new manufacturing
technologies (e.g. planar process, MOS
technology);
– a higher integration capability to reduce the size
of the circuits so that a small chip can include
a greater number of functions and components.
Nowadays, integrated circuits are found in every
field of electronics; they have entirely replaced
circuits built from discrete components.
10.
Further important semiconductor devices are the
luminescent diodes (LED: light emitting diode)
and the opto-electronic components (photodiode,
phototransistor, etc.) which have been developed
especially for use with optical fibres.
ID
G
Digital signal processing
The processing of an electrical signal essentially
involves:
– signal processing both in the time and frequency domains;
EBU Technical Review Spring 1995
Lemme & Menicucci
100
gate
source
gate
drain
source
drain
P
P
D
N
Figure 13
The basic structure
of a MOSFET.
D
N
P
substrate
Typical N-MOS
G
N
substrate
ID
S
sistors have also been developed. In these
devices, conduction is due to electrical charges
of only one sign; the junction field effect transistor (JFET) and metal oxide semiconductor
field effect transistor (MOSFET) are perhaps
the best-known examples.
JFETs are active devices with three terminals
(gate, source and drain) which come in two
versions: p-channel and n-channel. Both versions
can operate in either a linear or a non-linear mode.
Between the channel and the gate there is a p-n
junction, the polarization of which controls the
current flowing between the source and the drain.
In order to operate in the linear mode, the gatechannel junction should be reverse-biased. The
output and transfer characteristics of a JFET are
very similar to those of a triode.
The MOSFET transistor (Fig. 13) consists of a
p-type or an n-type substrate. The gate is insulated
from the substrate by silicon oxide, while the
source and drain are reverse-doped regions with
respect to the substrate. The source and substrate
are innerly connected so that they are at the same
potential. The channel is formed by the free
charges in the source region which are attracted to
the gate by its polarization voltage.
FET transistors are widely deployed in the integrated amplifiers used for microwave applications up to frequencies of around 14 GHz. The socalled monolithic microwave integrated circuit
(MMIC) is fabricated by using gallium arsenide
technology.
For higher frequency applications, parametric
amplifiers with negative resistance are used. A
typical negative-resistance element is the gallium
arsenide varactor.
Typical P-MOS
9.
74
S
Microchips
By the end of the 1940s, as electronic systems grew
in importance and size, more power was consumed, their weight and costs increased while their
reliability decreased. The cold war and the space
race both demanded cheaper, smaller and more
reliable electronic systems. Following on from the
invention of the transistor in 1948, J.S. Kilby
completed the first integrated circuit in 1958; it
consisted of a phase oscillator formed entirely of
semiconductors (transistors, resistors and capacitors) in a single block called a microchip.
The use of large computers to design ICs, together with rapid developments in the chemical
and photo-optical technologies applied to semiconductor electronics, has led to:
– greater purity of the silicon wafer used as the
substrate;
– more accurate control of the doping of the
different substrate regions which provide the
basic components of the electronic circuit
(transistors, diodes, resistors, capacitors, etc.);
– the highly-accurate fabrication of microscopic
material layers (junctions, connecting tracks,
connection soldering);
– continuous improvements in the performance
of chips due the use of new materials, such as
gallium arsenide and new manufacturing
technologies (e.g. planar process, MOS
technology);
– a higher integration capability to reduce the size
of the circuits so that a small chip can include
a greater number of functions and components.
Nowadays, integrated circuits are found in every
field of electronics; they have entirely replaced
circuits built from discrete components.
10.
Further important semiconductor devices are the
luminescent diodes (LED: light emitting diode)
and the opto-electronic components (photodiode,
phototransistor, etc.) which have been developed
especially for use with optical fibres.
ID
G
Digital signal processing
The processing of an electrical signal essentially
involves:
– signal processing both in the time and frequency domains;
EBU Technical Review Spring 1995
Lemme & Menicucci
100
– deleting unwanted information (e.g. filtering
out noise);
– extracting specific information from a signal;
– deleting redundant and/or irrelevant information;
– adding extra information (e.g. encryption);
– storing, manipulating and transmitting signals
with the best fidelity and reliability.
The processing of analogue signals (amplification,
modulation, etc.) is very difficult and is restricted
by real limiting factors that can sometimes be improved upon, but never removed entirely:
– the circuits need to be very linear with low
noise;
– a special design is needed for almost every
application;
– the circuits depend on sensitive components
(resistors, capacitors) which need adjustment;
– circuits tend to drift in both the short-term and
the long-term;
– the signals are difficult to record and store;
– the signals are not easy to compress.
Digital signals, on the other hand, use devices (such
as amplifiers) which have only two operating states:
saturation (on) and cutoff (off). It is thus possible
to obtain better use, reliability and efficiency from
each device in the chain. Futhermore, the on-off
signals can be regenerated many times throughout
the chain, with no losses whatsoever.
In 1938, Alec H. Reeves patented an invention
concerning the transmission of voice signals by
using coded pulses of constant amplitude, similar
to those used in telegraphy. This invention was
called pulse code modulation (PCM). However, it
was not until 1962 that such an invention found an
application in the field of telephony, with the production of the first 24-channel PCM system by
Bell Systems of the USA.
In a PCM system, the conversion of the analogue
signal into a digital signal is carried out through
three different steps: sampling, quantizing and
coding.
In the sampling step, the analogue input signal is
transformed into a timed discrete signal, by pulse
amplitude modulation (PAM) of a carrier formed
of equally time-spaced pulses. The modulation is
carried out by changing the amplitude of the transmitted pulses in sympathy with changes in the
amplitude of the input analogue signal. The
resultant PAM signal is then “quantized” by
assigning to each sample a discrete level of amplitude, selected according to a definite level scale
(e.g. 256 quantized levels). The amplitudes of the
PAM samples are then “coded” according to a
succession of elementary bits (8-bit words in the
case of 256 quantized levels). The sampling may
also be carried out by either modulating the
duration or the position of the sampling pulse,
instead of its amplitude.
Michele Lemme graduated in Electrotechnical Engineering from the University of Naples in 1952 and
initially stayed on as a Research Assistant at the University.
In 1956, he joined Vatican Radio at a time when the St. Maria di Galeria transmitting centre was in the
process of being set up.
Since then, until his recent retirement, Mr Lemme has devoted his whole working life to St. Maria di
Galeria, where he was the Director and the prime motor behind all its developments.
Rolando Menicucci graduated in Radiocommunications from the G. Galilei Technical Industrial Institute
of Rome in 1958.
After completing a special course in Nuclear Physics at the National Research Council (CNR), he joined
the Selenia company – where he was granted a scholarship – and worked on the development of radar
systems in the project laboratory.
Mr Menicucci also worked in a company which produced car radio receivers before joining Vatican Radio
in 1964. He now heads the telecommunications section of Vatican Radio.
EBU Technical Review Spring 1995
Lemme & Menicucci
75
100
– deleting unwanted information (e.g. filtering
out noise);
– extracting specific information from a signal;
– deleting redundant and/or irrelevant information;
– adding extra information (e.g. encryption);
– storing, manipulating and transmitting signals
with the best fidelity and reliability.
The processing of analogue signals (amplification,
modulation, etc.) is very difficult and is restricted
by real limiting factors that can sometimes be improved upon, but never removed entirely:
– the circuits need to be very linear with low
noise;
– a special design is needed for almost every
application;
– the circuits depend on sensitive components
(resistors, capacitors) which need adjustment;
– circuits tend to drift in both the short-term and
the long-term;
– the signals are difficult to record and store;
– the signals are not easy to compress.
Digital signals, on the other hand, use devices (such
as amplifiers) which have only two operating states:
saturation (on) and cutoff (off). It is thus possible
to obtain better use, reliability and efficiency from
each device in the chain. Futhermore, the on-off
signals can be regenerated many times throughout
the chain, with no losses whatsoever.
In 1938, Alec H. Reeves patented an invention
concerning the transmission of voice signals by
using coded pulses of constant amplitude, similar
to those used in telegraphy. This invention was
called pulse code modulation (PCM). However, it
was not until 1962 that such an invention found an
application in the field of telephony, with the production of the first 24-channel PCM system by
Bell Systems of the USA.
In a PCM system, the conversion of the analogue
signal into a digital signal is carried out through
three different steps: sampling, quantizing and
coding.
In the sampling step, the analogue input signal is
transformed into a timed discrete signal, by pulse
amplitude modulation (PAM) of a carrier formed
of equally time-spaced pulses. The modulation is
carried out by changing the amplitude of the transmitted pulses in sympathy with changes in the
amplitude of the input analogue signal. The
resultant PAM signal is then “quantized” by
assigning to each sample a discrete level of amplitude, selected according to a definite level scale
(e.g. 256 quantized levels). The amplitudes of the
PAM samples are then “coded” according to a
succession of elementary bits (8-bit words in the
case of 256 quantized levels). The sampling may
also be carried out by either modulating the
duration or the position of the sampling pulse,
instead of its amplitude.
Michele Lemme graduated in Electrotechnical Engineering from the University of Naples in 1952 and
initially stayed on as a Research Assistant at the University.
In 1956, he joined Vatican Radio at a time when the St. Maria di Galeria transmitting centre was in the
process of being set up.
Since then, until his recent retirement, Mr Lemme has devoted his whole working life to St. Maria di
Galeria, where he was the Director and the prime motor behind all its developments.
Rolando Menicucci graduated in Radiocommunications from the G. Galilei Technical Industrial Institute
of Rome in 1958.
After completing a special course in Nuclear Physics at the National Research Council (CNR), he joined
the Selenia company – where he was granted a scholarship – and worked on the development of radar
systems in the project laboratory.
Mr Menicucci also worked in a company which produced car radio receivers before joining Vatican Radio
in 1964. He now heads the telecommunications section of Vatican Radio.
EBU Technical Review Spring 1995
Lemme & Menicucci
75
100
Analogue
IN
Figure 14
Schematic diagram
of a DSP channel.
lowpass
filter
A/D
convertor
DSP
processor
chip
D/A
convertor
Analogue
OUT
The PCM system was the forerunner to the modern
technique of digitally processing an analogue signal
in real time. Digital signal processing (DSP) techniques use the same main procedures as the PCM
technique (i.e. sampling, quantizing and coding).
– lower sensitivity to the environment and to
component ageing;
Coding is usually carried out by data compression,
i.e. by reducing the redundancy which is inherent
in the source analogue signal. Such compression
is often based upon so-called adaptive differential
coding, in which the difference between the input
signal and its most recent estimate is coded, not the
absolute value of the input signal.
– mathematical equations can be implemented
directly as signal processing algorithms;
The digital values of the coded signals are processed by software techniques (i.e. operations such
as filtering, modulation and delay, which were
typically executed by hardware, are now executed
by software). Thus, once the signal is converted
into digital form, it is possible to process it in both
the time and frequency domains, by applying the
Fourier transform and other mathematical algorithms.
A typical DSP channel is illustrated in Fig. 14.
The input analogue signal enters the channel via
a low-pass filter and is next passed through an
A/D converter to ensure that the DSP unit
receives a digital, band-limited version of the
original input signal.
The DSP chip processes the digital data with the
algorithm stored in its memory and then passes the
modified data onto a D/A converter. As the output
of the D/A converter includes discrete values
rather than a pure analogue value, the signal must
be passed through a smoothing filter to remove
unwanted high-frequency components.
– no need to tune the components;
– availability of sophisticated functions not
covered by conventional analogue techniques;
– less sensitivity to noise.
11.
– computer simulation to check the analogue
system performance;
– increased compactness of the equipment;
Summary
The first communication systems were based on
the presence or absence of a transmitted signal
(e.g. Morse code, telegraphy) and thus were truly
digital systems. The work of Marconi and other
pioneers, however, was devoted to finding better
ways to transmit speech, music and other sounds as
analogue signals and, consequently, a wide range
of linear electronic devices were developed over
the years to increase the range and quality of these
analogue broadcasts.
Now, as the first century of wireless communication draws to a close, development efforts are
once again being aimed vigorously in the direction
of digital technologies.
Bibliography
[1]
Solari, L.: Storia della Radio
Treves, 1939.
[2]
Riley, J.P.: Who started ripples in the ether?
Electronics World + Wireless World,
September 1994.
[3]
Pickworth, G.: The spark that gave Radio to
the world
Electronics World + Wireless World,
July 1994.
[4]
O’Dell, D.H.: Marconi’s magnetic domain that
stretches into the ether
Electronics World + Wireless World,
August 1993.
[5]
Shockley, W.: Transistor Electronics; Imperfections, Unipolar and Analog Transistors
Proceedings of the I.R.E., Vol. 40, No. 11,
November 1952.
DSP techniques offer the following advantages
over analogue processing methods:
76
smoothing
filter
EBU Technical Review Spring 1995
Lemme & Menicucci
100
Analogue
IN
Figure 14
Schematic diagram
of a DSP channel.
lowpass
filter
A/D
convertor
DSP
processor
chip
D/A
convertor
Analogue
OUT
The PCM system was the forerunner to the modern
technique of digitally processing an analogue signal
in real time. Digital signal processing (DSP) techniques use the same main procedures as the PCM
technique (i.e. sampling, quantizing and coding).
– lower sensitivity to the environment and to
component ageing;
Coding is usually carried out by data compression,
i.e. by reducing the redundancy which is inherent
in the source analogue signal. Such compression
is often based upon so-called adaptive differential
coding, in which the difference between the input
signal and its most recent estimate is coded, not the
absolute value of the input signal.
– mathematical equations can be implemented
directly as signal processing algorithms;
The digital values of the coded signals are processed by software techniques (i.e. operations such
as filtering, modulation and delay, which were
typically executed by hardware, are now executed
by software). Thus, once the signal is converted
into digital form, it is possible to process it in both
the time and frequency domains, by applying the
Fourier transform and other mathematical algorithms.
A typical DSP channel is illustrated in Fig. 14.
The input analogue signal enters the channel via
a low-pass filter and is next passed through an
A/D converter to ensure that the DSP unit
receives a digital, band-limited version of the
original input signal.
The DSP chip processes the digital data with the
algorithm stored in its memory and then passes the
modified data onto a D/A converter. As the output
of the D/A converter includes discrete values
rather than a pure analogue value, the signal must
be passed through a smoothing filter to remove
unwanted high-frequency components.
– no need to tune the components;
– availability of sophisticated functions not
covered by conventional analogue techniques;
– less sensitivity to noise.
11.
– computer simulation to check the analogue
system performance;
– increased compactness of the equipment;
Summary
The first communication systems were based on
the presence or absence of a transmitted signal
(e.g. Morse code, telegraphy) and thus were truly
digital systems. The work of Marconi and other
pioneers, however, was devoted to finding better
ways to transmit speech, music and other sounds as
analogue signals and, consequently, a wide range
of linear electronic devices were developed over
the years to increase the range and quality of these
analogue broadcasts.
Now, as the first century of wireless communication draws to a close, development efforts are
once again being aimed vigorously in the direction
of digital technologies.
Bibliography
[1]
Solari, L.: Storia della Radio
Treves, 1939.
[2]
Riley, J.P.: Who started ripples in the ether?
Electronics World + Wireless World,
September 1994.
[3]
Pickworth, G.: The spark that gave Radio to
the world
Electronics World + Wireless World,
July 1994.
[4]
O’Dell, D.H.: Marconi’s magnetic domain that
stretches into the ether
Electronics World + Wireless World,
August 1993.
[5]
Shockley, W.: Transistor Electronics; Imperfections, Unipolar and Analog Transistors
Proceedings of the I.R.E., Vol. 40, No. 11,
November 1952.
DSP techniques offer the following advantages
over analogue processing methods:
76
smoothing
filter
EBU Technical Review Spring 1995
Lemme & Menicucci
100
[6]
Fuselli, D.: Elettronica verso l’Integrazione
Zanichelli, 1990.
[7]
Setti, G.: Manuale di Reti di Telecomuni-
cazioni e Trasmissione Dati,
Calderini, 1991.
[8]
Texas Instruments Seminar on
DSP, 1993.
Twin HF antenna
towers on a rotating
platform in the
Netherlands (1938).
EBU Technical Review Spring 1995
Lemme & Menicucci
77
100
[6]
Fuselli, D.: Elettronica verso l’Integrazione
Zanichelli, 1990.
[7]
Setti, G.: Manuale di Reti di Telecomuni-
cazioni e Trasmissione Dati,
Calderini, 1991.
[8]
Texas Instruments Seminar on
DSP, 1993.
Twin HF antenna
towers on a rotating
platform in the
Netherlands (1938).
EBU Technical Review Spring 1995
Lemme & Menicucci
77
100
[6]
Fuselli, D.: Elettronica verso l’Integrazione
Zanichelli, 1990.
[7]
Setti, G.: Manuale di Reti di Telecomuni-
cazioni e Trasmissione Dati,
Calderini, 1991.
[8]
Texas Instruments Seminar on
DSP, 1993.
Twin HF antenna
towers on a rotating
platform in the
Netherlands (1938).
EBU Technical Review Spring 1995
Lemme & Menicucci
77
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