All hams know a bit about modulation, or they did when

All hams know a bit about modulation, or they did when
BACK TO BASICS – MODULATION BY ANDREW BARRON ZL3DW
All hams know a bit about modulation, or they did when they sat the exam anyway. So we will treat
this as a refresher on different modulation types and why we need to understand how they affect
our signal. Our radio transmitters all generate RF energy on frequencies within the allocated
amateur radio bands, whether that is around 146MHz to talk on a 2m repeater, 3.6Mhz for an 80m
net, or 14.010MHz for a cw contact. Sending the RF wave on its own is not enough because it carries
no ‘intelligence’, for example keying up your FM handheld on the local repeater and not talking, just
sends the message “I am being a nuisance” and very little else. We need to add some information to
the signal such as an audio voice signal or digital data. The process of adding an information signal
to the RF energy is called modulation. The RF energy or ‘wave’ is called a ‘carrier’ wave because it
carries the information signal that you want to send.
There are only three ways to modulate an RF wave so that it can carry information. You can change
the amplitude (level) of the RF signal, you can change the frequency, and you can change the phase.
All modulation types use changes of one or more of these factors to carry the information
intelligence. I know about 50% of you are saying “what about cw then”. But you can consider cw to
be an, amplitude modulated digital mode. When you send cw, the RF carrier amplitude is changed
from full output level to zero output level and that is enough to convey the digital code, in this case
Morse code. Spark transmitters transmitted wide band noise as RF energy, but to convey
information it was turned on and off using the Morse key and so ‘spark’ is still effectively an
amplitude modulated signal.
It does not matter whether we are talking about analogue modulation where the information, for
example your voice, modulates the amplitude, frequency, or phase of the RF carrier signal, or digital
modulation where the information is coded into data bits and then that data is used to modulate the
amplitude, frequency and/or phase of the RF carrier, those three methods are all there are. I know
you are thinking that in SSB no carrier wave is transmitted, but the RF is still generated by the
oscillator in the rig. What happens is that only the modulated RF energy is transmitted and that
brings us to the issue of how does the modulation affect the RF signal being transmitted and why
should we care about that.
A pure RF sine wave signal has a very narrow bandwidth (theoretically zero), phase noise, minor
amplitude variations and jitter cause the bandwidth to be a little wider because they cause noise
modulation of the pure wave. The wanted modulation increases the bandwidth further. Bandwidth
is the range of frequencies that are required to transmit your information. Generally the more
information you want to send, the wider the signal. For example hi fidelity music needs a wider
bandwidth than 300Hz-3kHz communications quality voice audio, higher data speeds require more
bandwidth as well. An example of this is looking at PSK31, PSK63 and PSK125 signals on the
waterfall of your digi-mode software. You find that the faster modes are wider. In most cases if you
over modulate the RF wave you will cause distortion which usually makes the bandwidth of your
signal much wider, potentially causing interference to other users, or even out of band to other
services. In (analogue) voice modes this is called splatter. Your signal would sound distorted and
might be heard across a wide area of the band. In digital modes the distortion caused by over
modulation can cause the signal to be wider or fuzzy. Severe distortion could cause the phase
transitions to be masked causing poor or no decoding of your signal even when the receive level is
strong. Most digital modes use phase transitions or frequency shifts to convey the data information.
High capacity digital systems usually use QAM (quadrature amplitude modulation) which is a
combination of amplitude and phase changes.
Why is bandwidth important? The main reasons are that you must not transmit outside of the
allocated bands and you must try to not cause interference to other users of the band. If you are
transmitting an USB signal on a frequency of 14.347MHz and the audio you are using for modulation
has been limited to a maximum of 3kHz then all is fine, but what about if you change those mic
equaliser settings in your fancy new transceiver to a 4kHz filter? Yes it will sound more hi-fi but you
will be transmitting out of band. Further down the band, you will interfere with other stations 3kHz
above you and unless the ham at the other end of the QSO has a wide receiver filter they won’t hear
the high notes in your signal anyway. Another downside of wide signals is that your RF power is
distributed over a wider spectrum so your signal will have a lower signal to noise ratio at the
receiver.
As the bandwidth of the RF signal becomes wider due to modulation the amount of RF spectrum
used increases, usually on frequencies both above and below the carrier frequency. These bands of
RF energy on each side of the centre carrier frequency are known as ‘side bands’. As explained at
the beginning, the carrier on its own contains no useful information. All of the useful information is
contained in the sidebands. So some modulation schemes minimise or eliminate the RF power sent
on the carrier frequency in order to maximise the RF power containing the useful information. In
single sideband transmission we only transmit one sideband and no carrier, so all of the RF energy
transmitted contains useful information (speech or data). This is why the recovered audio from an
SSB signal is much stronger than from an AM signal, where much of the power radiated is carrier
signal. This increase in received audio signal is the reason that most hams use SSB rather than AM
on the HF bands.
Modulating signal
AM modulated RF signal
FM modulated signal
With an analogue FM (frequency modulation) transmission the transmitter radiates a fixed level of
RF energy, the amplitude of the audio signal changes the frequency of the transmission. In other
words the louder you talk the more the frequency deviates from the carrier frequency. The
maximum amount that the RF signal can be deviated is always limited to avoid using too much
bandwidth and again hi-fi signals with large dynamic range need more bandwidth. For example a
narrow band FM signal from your hand held radio will be limited to +-3kHz deviation while an FM
broadcast station will be limited to +-75kHz deviation. The frequency of your audio signal affects the
rate that the signal changes. Because changing the instantaneous frequency of the signal also
changes the phase of the wave, there is essentially no difference between analogue Frequency
Modulation and analogue Phase Modulation. Many FM transmitters actually use phase modulators,
check the specification page on your hand-held radio manual. There are differences in the frequency
response of the received signal when frequency rather than phase modulators are used and these
are dealt with using pre-emphasis at the transmitter with de-emphasis at the receiver.
FM transmission is more immune to noise than amplitude modulated signals like SSB. That is
because the amplitude of the FM signal is not very important (as long as it is not zero!). Noise spikes
don’t affect the deviation much so you don’t hear them in the received audio. The main
disadvantage is that FM signals take more bandwidth to carry the same quality of audio. This would
be a problem on the narrow HF bands, but is less important on the higher bands because there is
more room. That is the reason that FM is only used on the 10m ham band and above.
In the digital world FM (frequency modulation) is not the same as PM (phase modulation). The RTTY
mode is a good example of a frequency keyed digital mode, two tones are transmitted at the same
level. Your transmitter only sends one tone at a time, so the tones don’t mix. The way the tones are
alternated sends the digital information, in this case the Baudot code. This means that an RTTY
signal is narrow bandwidth and because the tone levels are constant an RTTY signal can be amplified
with a non linear amplifier. Modes like Domino, Ros and Olivia are also frequency keyed. In these
cases a variety of tones are used, but only one tone is transmitted at a time. The sequence of the
tones carries the digital information. In other words a transition from tone (a) to tone (b) may mean
“0110” and a transition from tone (a) to tone (c) may mean “1011”.
Frequency Keyed modulated signal
Modulating signal
PSK31 is the simplest phase modulated mode. PSK stands for ‘phase shift keying’. In PSK a single
tone is transmitted. 180 degree phase changes are used to send the digital information. A more
complicated phase modulation method is 4PSK. In that mode 90 degree phase changes each send
two bits of the digital signal. Each of the four possible phase states is called a symbol. Because PSK
modes only use one frequency they are narrow bandwidth and can be amplified with a non linear
amplifier.
Un-modulated RF signal
Modulating signal
Phase Keyed modulated signal
Quadrature modulation is usually a combination of analogue and phase modulation. In 4QAM a two
voltage level signal (like a standard binary stream of 1s and 0s) is combined with a second a two
voltage level signal which is adjusted to be 90 degrees out of phase with the first stream. This
creates an output with four possible phase states 90 degrees apart, each phase state or ‘symbol’
carries 2 bits of digital information. If you are still with me, you will be thinking “huh that’s just like
4PSK” and you would be right! There is no functional difference between a 4PSK signal (with 90
degree shifts) and a 4QAM signal. Modern digital radio systems use quadrature modulation to
combine two, 4 level data signals, with each voltage level equivalent to 2 bits of binary data to
create a 16QAM signal with 16 phase states (symbols). Each symbol represents four bits of digital
data, two from each of the input data streams. Because you have the voltage levels as well as the
phase states, 16QAM mod schemes and above are a combination of amplitude modulation and
phase modulation. High capacity radio systems carry on with this idea and use, 64QAM, 256QAM or
even 512QAM modulation. The higher the complexity of the modulation the wider the bandwidth
that is required. Because 16QAM and above includes amplitude modulation you must use a linear
amplifier. Non linearity would cause the received data streams to have the wrong voltage levels
which could cause the data to be decoded incorrectly. Also the amplifiers must have good phase
linearity because a phase error on the signal can also cause the received data to be decoded
incorrectly.
This is the phase diagram showing the 16 phase states
generated by a 16QAM modulator.
Note that each phase state symbol carries 4 bits of data.
The three rings illustrate the three amplitude levels. So
each symbol has a phase angle with respect to the
carrier wave and also one of the three possible
amplitude levels.
FINALLY A WORD OR TWO ON AMPLITUDE LINEARITY. Amplitude linearity often referred to as just ‘linearity’
is a measurement of how accurately the output of a device follows changes in the level of the input
signal. With amplitude modulated signals, particularly digital AM signals, it is important that the
output signal of stages like your final amplifier and any additional amplifiers is an accurate, but
larger copy of the input signal. Severe amplitude distortions could cause your signal to be distorted
and splatter. Many hams use a Linear Amplifier on the HF bands where perhaps the most commonly
used modulation method is SSB. Many digital modes and cw don’t actually need the amp to be
particularly linear and some linear amps have an alternative bias setting for cw. On the VHF bands
when you are using FM modulation or digi-modes like JT65, class C amplifiers may be used. Of
course for SSB you still need a linear amplifier.
Drawings were gathered from various Internet sources.
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