Compressed single side band communications system and method

Compressed single side band communications system and method
United States Patent [19]
[11]
Patent Number:
4,539,707
Jacobs et a1.
[45]
Date of Patent:
Sep. 3, 1985
[54]
3,518,566
6/1970
Vogel ........................... .. 455/43
3,667,047
5/1972
lwasaki et a1.
4.271,499
6/1981
Leveque .............................. .. 455/72
[75]
COMPRESSED SINGLE SIDE BAND
COMMUNICATIONS SYSTEM AND
METHOD
Inventors: Paul H. Jacobs, Fairport; Douglas P.
Collette, Stafford, both of NY.
Primary Examiner—Marc E. Bookbinder
Attorney, Agent, or F1'rm--L. Lawton Rogers, III
Assignee: Aerotron, Inc., Raleigh, NC.
Appl. No.: 384,148
{57]
ABSTRACT
A compressed single side band communication system
and method in which the audio signal is compressed
Filed:
Jun. 1, 1982
prior to pre-emphasis and thereafter summed with a
pilot tone for further compression prior to transmission.
lnitally, only the pilot tone is transmitted at full rated
power to aid in acquisition of the signal by the receiver.
Int. Cl.3 ....................... .. H04B 1/00; H04B 1/16;
H04B 1/76
US. Cl. ...................................... .. 455/47; 455/71;
Thereafter, the transmitter ALC is disabled and the
pilot tone is attenuated. The receiver adjusts the fre
quency characteristics of the pilot tone ?lter and phase
455/72; 455/109; 455/116; 455/203; 455/265;
455/266; 455/260
Field of Search ..................... .. 455/70, 71, 72, 47,
lock loop ?lter in the detector as a function of lock-on.
455/260, 265, 266, 68, 45, 46, 108, 109, 116,
126, 202, 203; 370/74, 7; 375/97, 111, 119, 120;
The delay after loss oflock-on in reverting to wide band
pilot tone and \videband loop ?lters is varied as a func
tion of signal strength, The pilot tone may be modulated
for tone coded squelch. The modulating source is lo
cated in the return end of the phase lock loop ?lter. A
179/170 A
[56]
455/43
References Cited
U.S. PATENT DOCUMENTS
2,248,757
2,349,870
7/1941
5/1944
Herold
Koch ................ ..
3,084,328 4/1963
3,112,462 ll/l963
3,217,255 ll/l965
Groeneveld et al. ..
De Jager et al. ........ ..
Broadhead, Jr. et al. .
3,271,681
McNair ............................... .. 455/71
9/1966
unique ?lter is provided to insure acquisition ofthe pilot
......... .. 455/72
455/72
tone. Automatic gain control of the audio signal is re
sponsive to the tone signal without effecting the c0m~
455/47
455/72
.. 455/71
posite audio and tone signal.
30 Claims, 10 Drawing Figures
PUSH ~10 -
@8 TALK
|
I TBNE GEN.
1
4,539,707
2
signal is weak and fades are likely to be longer in dura
COMPRESSED SINGLE SIDE BAND
COMMUNICATIONS SYSTEM AND METHOD
BACKGROUND OF THE INVENTION
tion.
.
In generally known prior art receivers, the amplitude
5
The present invention relates to a radio frequency
communications system and method, and more particu
larly to a two-way single sideband, land mobile system
in which a pilot tone is transmitted with an audio signal
and a phase lock loop used to acquire the pilot tone and
thus the audio signal.
Known prior art systems of this type have pre
emphasized the audio signal prior to compression.
However, the desired degree of pre-emphasis has been
dif?cult to achieve. In one aspect, the present invention
greatly simpli?es the pre-emphasis circuit by compress
ing the audio signal before pre-emphasis.
Automatic level control circuits (ALC) are well
known and generally operate to maintain a constant
output power from the transmitter. Since the output
power of a single sideband transmitter is a function of
the amplitude of the modulation signal applied thereto,
the output power of known transmitters tend to widely
?uctuate as a result of the widely varying characteris
tics of a typical audio signal. In another aspect, the
present invention obviates this problem by adjusting the
power of the transmitter only during an initial time
interval when a constant amplitude signal is present,
and thereafter maintaining the gain of the transmitter
constant. A limiter in the audio circuits of the transmit
ter thereafter limits the amplitude of audio signals and
thus prevents the transmitter power output from ex
ceeding its rated value.
In generally known systems, the pilot tone may be
masked by audio frequency components, or alterna
tively the phase lock loop of the receiver may try to
lock on a portion of the audio signal making initial
acquisition dif?cult. In one aspect, the present invention
obviates this problem by transmitting only the pilot tone
for a period of time suf?cient for acquisition thereof by
the phase lock loop of the receiver. Once acquisition has
been achieved, the frequency response characteristics
of the pilot .tone is detected and used to control the gain
of the receiver, i.e., to adjust the strength of the com
posite audio and tone signal to bring the tone signal up
to a predetermined level. Since the amplitude of the
pilot tone is being adjusted in such receivers in response
to detection of the pilot tone, undesirable “hunting”
may result. This problem is avoided in the present in
vention by using the signal strength of the detected pilot
tone to control only the gain of the audio signal compo
nents of the composite signal.
In generally known systems, the speed of response in
acquisition of the pilot tone is a function of the band
width of the pilot ?lter. As explained in connection with
the transmitter of the present invention, the initial trans
mission of a full power, unattenuated pilot tone greatly
facilitates lock-on. Thereafter, the pilot ?lter may be
switched to a narrow band mode and the amplitude of
the pilot tone reduced without the loss of lock-on as a
result of the presence of a high amplitude audio signal.
Thus, the present invention controls the frequency re
sponse characteristics of the pilot ?lter as a function of
phase lock loop lock-on.
In addition, the amount of delay in switching to the
acquisition mode following loss of lock-on may be ad
justed as a function of signal strength immediately prior
to the loss of lock-on. The amount of delay in switching
to the wideband pilot ?lter is always greater than the
amount of delay in switching to the wideband phase
lock loop ?lter. This allows the loop to make rapid
corrections if the received pilot signal drifts in fre
quency without increasing the pilot ?lter bandwidth
and thus subjecting the loop to possible interference
from audio components of the received signal.
An additional problem in generally known receivers
is the acquisition of the pilot tone in the presence of an
audio signal. As earlier explained, the present invention
transmits the pilot tone only during an initial time inter
val. In addition, the frequency response characteristics
of the wideband pilot tone ?lter are desirably selected
such that noise tends to drive the oscillator associated
of the pilot tone filter are narrowed and the pilot tone
with the phase lock loop to one extreme, thereby tend
attenuated to avoid possible interference with the audio 45 ing to center the pilot tone in the bandwidth of receiv
signal without loss of lock-on.
Frequency modulation of the pilot tone for tone
coded squelch purposes is known. In this way, the audio
signal of a particular receiver may be gated off to avoid
er’s IF ?lter (the primary selectivity element). In this
pilot tone is received. In a further aspect, the present
way, the presence of the tone is immediately detected
even if that tone is not exactly on the expected fre
quency. This minimizes the requirement for oscillator
stability in the transmitter and receiver and this reduces
cost and complexity.
invention achieves frequency modulation of the pilot
The foregoing and many other features, objects and
extraneous noise until such time as a uniquely coded
tone by locating the modulating source in the return end
advantages of the present invention will be readily ap
of the loop ?lter of a phase lock loop. Simplicity of
parent to one skilled in this art from the claims and from
circuit design may thus be achieved when the frequency 55 a perusal of the following speci?cation when read in
of modulation is high with respect to the bandwidth of
conjunction with the appended drawings.
the loop ?lter associated with the phase lock loop.
THE DRAWINGS
In receivers in systems of the type heretofore de
scribed, the frequency response characteristics of the
FIG. 1 is a functional block diagram of a prior art
loop ?lter are varied as a function of lock-on of the 60
transmitter;
phase lock loop. In this way, the pilot tone may be
rapidly acquired and thereafter maintained in the event
of the temporary fades characteristic of two-way, land
mobile communications. In another aspect, the present
ment of the transmitter of the present invention;
FIG. 2 is a functional block diagram of one embodi
FIG. 3 is a functional block diagram of a prior art
receiver;
inventions improves upon this feature by detecting the 65 FIG. 4 is a functional block diagram of one embodi
strength of the detected pilot tone immediately prior to
ment of the receiver of the present invention;
loss of lock-on, and increasing the delay in reverting to
FIG. 5 is a logic diagram of one embodiment of the
the rapid acquisition mode under conditions where the
logic circuit of the receiver illustrated in FIG. 4;
3
4,539,707
compressed audio signal is passed through a suitable
FIG. 7 is a schematic circuit diagram illustrating one
embodiment of the ?rst expander in the receiver illus
trated in FIG. 4;
FIG. 8 is a plot of the desired frequency response of
the narrow band pilot ?lter of FIG. 4;
FIG. 9 is a plot of the desired frequency response of
the wide band pilot ?lter of the receiver illustrated in
FIG. 4; and
FIG. 10 is a circuit diagram illustrating the modulat
conventional 6 dB per octave pre-emphasis circuit 40,
and through a limit circuit 42 to a suitable conventional
low pass ?lter 44. The limit circuit 42 is important be
cause the amplitude of the modulation signal effects the
output power of a single sideband transmitter.
The compressed audio output signal from the ?lter 44
is applied to a conventional summing circuit 46. As is
subsequently explained, the audio signal is combined
with the pilot tone to form a composite signal. The
ing of the pilot tone.
THE DETAILED DESCRIPTION OF
PREFERRED EMBODIMENTS
4
used to provide an audio signal to be passed through an
audio response limiting ?lter 36 to a compressor 38. The
FIG. 6 is a plot of the frequency spectrum illustrating
the passband of the transmitter of the present invention;
composite signal is further compressed in compressor
48 and applied through a variable gain automatic level
15 control ampli?er 50 of a suitable conventional type to a
An understanding of the transmitter of the present
invention may be facilitated by an understanding of the
single sideband transmitter 52 for transmission from a
conventional antenna 54. The output signal from the
transmitter 52 is also passed through a recti?er 56 to a
sample and hold circuit 58, the output of which is used
prior art transmitters. With reference to FIG. 1 where a
prior art transmitter is illustrated, ‘a suitable conven
tional microphone 10 may be used to pick up an audio 20 to control the gain of the ALC ampli?er 50.
With continued reference to FIG. 2, the frequency
modulated pilot tone from the tone generator 60 is ap
plied through a suitable conventional attenuator 62 to
the other input terminal of the summing circuit 46. A
Inasmuch as most of the energy in an audio frequency
signal is concentrated at the low frequencies, the pre 25 suitable electronic shunt 64 is provided to selectively
eliminate the attenuator 62 and a suitable shunt 66 is
emphasis circuit desirably provides a 12 dB per octave
provided at the output of the lowpass ?lter 44 to selec
gain so that the gain applied to the signal is increased as
tively remove the audio signal from the input to the
a function of the frequency of the signals passed there
signal. The microphone 10 may be of the conventional
push-to-talk type and the output signal therefrom ap
plied to a suitable conventional pre-emphasis circuit 12.
through.
summing circuit 46.
The output signal from the pre-emphasis circuit 12 30
may be applied to a suitable conventional compressor
14 where the entire audio signal is compressed. The
compressed signal is thereafter applied through a suit
Control of the shunts 64 and 66 as well as the sample
time of the sample-and-hold circuit 58 may be under
control of a suitable conventional timer 68 responsive to
the push-to-talk button of the transmitter.
able conventional notch ?lter 16 to one input terminal
In operation, the timer 68, when triggered by the
of a summing circuit 18 such as a summing ampli?er.
initiation of a transmission, provides for a ?rst predeter
mined time period an output signal which closes the
The notch ?lter 16 serves to remove audio frequency
components in a narrow band centered on the fre
quency of the pilot tone, e.g. 3.1 KHZ.
The other input terminal of the summing circuit 18
shunts 64 and 66. Operation of the shunt 64 removes the
attenuater 62 from the circuit and thus applies the fre
quency modulated pilot tone to the summing circuit 46
compressor 22 to the variable gain ampli?er of an auto
undiminished in amplitude. During the same period of
time, operation of the shunt 66 shunts the audio output
signal from the lowpass ?lter 44 to ground and thus
removes the audio signal from the input to the summing
matic level control circuit 24 at the input of a conven
circuit 46. Thus, for the initial time interval as deter
receives a pilot tone from a pilot oscillator 20 and the
composite output signal, i.e., the pilot tone and audio
signal, is applied through a second suitable conventional
tional single sideband transmitter 26. The output signal 45 mined by the timer 68 at the beginning of each transmis
from the transmitter 26 is applied to a suitable conven
sion, the output signal of the summing circuit 46 will be
tional antenna 28 for broadcast and is also fed back
through a recti?er 30 to control the gain of the auto
matic level controlled ampli?er 24.
The frequency of the pilot oscillator 20 may be modu
lated by a suitable conventional modulator 32 for pur
poses of providing a tone coded squelch signal at the
receiver.
In operation, the audio signal from the microphone
10 is pre-emphasized, compressed and combined with
the frequency modulated pilot tone. This composite
signal is further compressed in the compressor 22 and
applied to the single sideband transmitter as the modula
tion signal thereof for transmission. The output power
of the transmitter 26 is controlled continuously during
the transmission by means of the automatic level control
circuit so that the peak value of output signal from the
attenna 28 does not exceed the rated power capability of
an unattenuated pilot tone.
When the timer 68 times out, the shunts 64 and 66 are
opened to respectively attenuate the amplitude of the
frequency modulated pilot tone from the tone generator
60 and to apply the audio signal from the ?lter 44 to the
summing circuit. Subsequently thereto, the output sig
nal from the summing circuit 46 will be a composite
signal including the audio signal and an attenuated fre
quency modulated pilot tone.
Also upon the timing out of the timer 68, the sample
and-hold circuit 58 is operated to freeze or ?x the level
of the control signal applied to the ALC ampli?er 50. In
this way, the automatic level control circuit for the
60 transmitter 52 is operative to adjust the power gain of
the transmitter only during the initial period of the
timer 68, after which the power gain of the transmitter
will remain unchanged for the duration of the transmis
the SSB transmitter 26. The system gain may vary con
sion. The gain of ampli?er 48 is initially adjusted to
siderably as the speaker’s voice varies.
65 produce full rated power output from the transmitter
With reference now to FIG. 2 where one embodi
when shunt 64 is closed. The limiter 42 is designed such
ment of the transmitter of the present invention is illus
that the peak value of the audio signal that it may pass
trated, a suitable conventional microphone 34 may be
does not exceed the output of tone generator 60. Thus
5
4,539,707
the peak output of the transmitter during audio passages
does not exceed the preset value (full rated power).
The use of the automatic level control circuit in asso
ciation with the transmitter 52 is desirable in that the
output power of the single sideband transmitter is a
function of the amplitude of the input signal as well as
being subject to changes in the response of the transmit
ter as a function of parameters such as temperature. In
order to obtain a natural sounding communications
system, it is desirable that the overall gain of the trans
mitter remain unchanged for the duration of any single
transmission.
With continued reference to FIG. 2, the tone genera
tor 60 may comprise a pilot tone oscillator 70, a phase
lock loop 72, a shaper 74 and a tone coded squelch tone
6
The output signal from the pilot ?lter 108 may also be
applied to a shaper 114 where it is limited or clipped to
a predetermined level and thereafter passed through a
constant gain ampli?er to provide an output signal of
constant amplitude. The output signal from the shaper
114 may be applied to one input terminal of a conven
tional phase lock loop 116 to which the output signal
from an oscillator 118 is applied. Lock-on of the phase
lock loop 116 may be detected by a conventional lock
detector 119 and the output signal therefrom used to
control a switch 120 which controls the application of
the output signal from the phase lock loop 116 to one of
two loop ?lters 122 and 124. The output terminals of the
loop ?lters 122 and 124 are applied to the oscillator 86
as an automatic frequency control signal to vary the
output frequency of the oscillator 86 to bring the fre
quency of the received signal into lock with the fre
quency of the oscillator 118.
In operation, and in the absence of a signal from the
loop 72 may be modulated for tone coded squelch pur
lock detector 119, the output signal of the phase lock
poses by the application of the low frequency signal
loop 116 is applied through the wide loop ?lter 122 to
from the TCS tone generator 76. With reference to
facilitate capture of the input signal. Once lock is de
FIG. 10, where the frequency of the modulation is sig
tected by the lock detector 119, the switch 120 is acti
ni?cantly greater than the bandwidth of the loop ?lter,
vated to apply the output signal from the phase lock
the frequency modulation of the pilot tone may be ac
25 loop through the narrow loop ?lter 124. The response
complished by locating the modulating source at the
of the narrow loop ?lter is desirably very sluggish and
point where the loop ?lter would normally be returned
thus tends to maintain a constant value output signal for
to ground.
application to the oscillator 86.
Note that the compressor 38 preceeds the pre-empha
With continued reference to the prior art receiver of
sis circuit 40. In this way, a 6 dB per octave pre-ernpha
FIG. 3, the oscillator 118 which provides one input
sis after compression provides the equivalent of 12 dB
signal to the phase lock loop 116 may be conveniently
per octave prior to compression.
located in the tone coded squelch (TCS) circuit tone
The receiver of the present invention may also be
detector 126. The TCS tone detector receives the
more easily understood with reference to a prior art
shaped tone output signal from the shaper 114 and in
generator 76. In operation, the output signal from the
pilot tone oscillator 70 is applied to a phase lock loop.
The frequency of the output signal from the phase lock
receiver. With reference to FIG. 3 where a prior art
receiver is illustrated, the signal broadcast from the
transmitter of FIG. 1 may be received by the antenna-80
and passed through a variable gain, radio frequency
ampli?er 82 to a suitable conventional mixer 84 where it
is mixed with the output signal from an oscillator 86.
cludes a discriminator 128 and a detector 130 to remove
the frequency modulation from the frequency modu
lated pilot tone. The modulation removed by the dis
criminator 128 and detector 130 may be used to modu
late the frequency of the output signal from the pilot
tone oscillator 118 so that the two input signals to the
The output signal from the mixer 84 may be applied
phase lock loop 116 are both frequency modulated in
through a suitable conventional intermediate frequency
the same manner.
?lter 88 and a conventional IF variable gain ampli?er 90
The modulation removed by the discriminator 128
to a second mixer 92 for mixing with the output signal
and detector 130 may be applied to a tone squelch de
from a conventional oscillator 94. The output signal 45 coder 132 which generates an output signal if the de
from the mixer 92 may be passed through a variable
tected tone is of the correct frequency. A squelch cir
gain audio ampli?er 96 to a conventional notch ?lter 98
cuit 134 may also be included that is responsive to the
where the pilot tone is removed.
lock detector 119 output and/or the TCS decoder 132
The audio output signal from the notch ?lter 98 may
output to allow the receiver to be muted until a cor
then be expanded in a suitable conventional expander 50 rectly coded signal is received.
circuit 100 and de-emphasized in a de-emphasis circuit
In a transceiver, as contrasted with separate transmit
102 to remove the effects of the pre-emphasis circuit 12
ters and receivers, the pilot oscillator 20 of the transmit
of FIG. 1. The expanded and de-emphasized audio
ter of FIG. 1 and the oscillator 118 in the receiver illus
signal may then be passed through a suitable conven
trated in FIG. 3 may be the same unit.
tional variable gain ampli?er 104 to a speaker 106.
55
Now with reference to FIG. 4 where one embodi
The composite output signal from the ampli?er 96
may also be passed through a pilot ?lter 108 to remove
ment of the receiver of the present invention is illus
trated, the signal broadcast by the antenna 54 of the
audio signal. The pilot signal amplitude is detected in a
transmitter of FIG. 2 may be received by the antenna
level detector 110 and used to control the gain of the
ampli?er 96 to provide a constant pilot amplitude at the
132. This input signal is applied through a suitable con
ventional variable gain RF ampli?er 134 to a ?rst mixer
136 where it is mixed with the output signal from a
pilot ?lter output terminal.
The output signal from the level detector 110 may
suitable conventional oscillator 138. The output signal
also be passed through a low pass ?lter 112 and applied
from the ?rst mixer 136 may be passed through a ?rst IF
to the radio frequency and intermediate ampli?ers 82
?lter 140 to a second mixer 142 where it is mixed with
and 90 respectively to control the gain thereof. Thus, 65 the output signal from a conventional oscillator 144.
the amplitude of the pilot tone is used to dynamically
The output signal from the second mixer 142 may be
control the gain of the composite signal passing through
passed through a second IF ?lter 146 and a suitable
the receiver.
conventional variable gain IF ampli?er 148 to a third
7
4,539,707
mixer 150 where it is mixed with the output signal from
a conventional oscillator 152 and thus converted to
audio frequency signals. The output signal from the
mixer 150 may be passed through an ampli?er 154 as the
composite signal containing compressed audio and fre
quency modulated pilot tone components.
The composite signal from the ampli?er 154 may be
applied through an audio ?lter 156 which operates to
remove the pilot tone components and to delay the
signal. This delayed audio signal is passed through a
?rst expander which may be of the type subsequently
output signal from the phase detector 176 by way of the
narrow AFC loop ?lter 184 instead of to the wide AFC
described in connection with FIG. 7, and from there
through a suitable conventional tie-emphasis circuit 160
where the effects of the pre—emphasis circuit 40 of the
transmitter of FIG. 2 are reversed. The output signal
from the de-emphasis circuit 160 may be passed through
loop ?lter 182.
A ?rst delay circuit 202 may be employed between
the lock detector 200 and the switch 186, upon loss of
lock-on, to delay the operation of the switch 186 for a
predetermined time interval. This delay in switching
a second expander 162 and a suitable conventional vari
able gain audio ampli?er 164 to a conventional speaker
166. Note that the second expansion occurs after the
de-emphasis circuit 160.
In operation, the composite signal received by the
8
the output signal from the pilot tone oscillator 178 as it
is applied to the phase detector 176.
In addition, the tone squelch decoder 196 provides a
binary signal TCS LOGIC for application to a logic
circuit 198 subsequently described in more detail in
connection with FIG. 5.
With continued reference to FIG. 4, lock-on of the
phase lock loop may be detected by a conventional lock
detector 200 and the output signal therefrom used to
0 control the position of the switch 186 and thus select the
from the narrow AFC loop filter 184 back to the wide
AFC loop ?lter 182 is desirable where the loss of lock
20 on may be temporary due to multipath propagation
causing signal cancellation or temporary fades. The
response of the narrow loop ?lter is desirably slow such
that the tuning of the oscillator 144 cannot change sig
ni?cantly during the interval of a temporary fade as
156, and the compressed audio signal expanded, de-em 25 long as ?lter 184 is selected.
The output signal from the ?rst delay circuit 202 may
phasized and expanded again for application to the
also be passed through a second delay circuit 204 and
speaker 166.
the AFC LOCK output signal therefrom used to con
Note that the delay of the signal passing through the
trol the operation of the switch 172 which selects be
narrow band pilot ?lter 170 is desirably equal to the
tween the wide band pilot ?lter 168 and the narrow
delay introduced by the audio ?lter 156 so that the
band pilot ?lter 170. Both the ?rst and second delay
expander 158 may be of the “feed forward” rather than
circuits provide their respective delays only when loss
the “feed back” type. The operation of the ?rst expan
antenna 132 is detected by the circuit elements indicated
generally within the dashed lines 168 on FIG. 4. The
pilot tone components are removed by the audio ?lter
oflock is indicated by lock detector 200, and act quickly
when lock-on is indicated.
der 156 is discussed infra in more detail in connection
with FIG. 7.
In operation, the detection of lock-on by the lock
detector 200 is thus effective to quickly switch the input
signal into the shaper 174 through the narrow band pilot
?lter 170. The ?rst delay circuit 202 and the second
delay circuit 204 are operative to delay, upon loss of
With continued reference to FIG. 4, the composite
output signal from the ampli?er 154 is applied to a wide
band pilot ?lter 168 and a narrow band pilot ?lter 170.
The output signal from the pilot ?lters 168 and 170 may
be selectively applied by way of a suitable electronic
switch 172 to a shaper 174. The shaper desirably in
" cludes a limiter to clip the amplitude thereof to a con
stant low level, and a constant gain ampli?er. The out
put of the shaper 174 is thus a constant amplitude, fre
quency modulated pilot tone.
40
lock-on the operation of the switch 172 to re-insert the
wide band pilot ?lter 168.
This second delay is desirable when the pilot signal
being received is changing or drifting in frequency.
Such a change will cause lock detector 200 to indicate
The output signal from the shaper 174 is applied to
45 an out-of-lock condition; because the slow response of
one input terminal of a phase detector 176. The refer
ence input to phase detector 176 is the output signal of
pilot tone oscillator 178 applied through a second phase
drifting signal. At the end of the delay period intro
duced by ?rst delay circuit 202, switch 186 will operate
narrow ?lter 184 will not allow the loop to follow a
lock loop 180. The output signal from the phase detec
to select wide AFC loop ?lter 182 and thus allowing the
tor 176 is applied to the input terminals of a wide AFC
loop ?lter 182 and a narrow AFC loop ?lter 184. The
output signals from the AFC ?lters 182 and 184 are
applied through a suitable electronic switch 186 as the
loop to make a rapid correction to the new frequency.
automatic frequency control or AFC signal applied to
the oscillator 144 to bring the pilot tone in the detected
composite signal into lock with the locally generated
pilot tone from oscillator 178.
The output signal from the shaper 174 may also be
applied to a detector (FM discriminator) circuit 188
which desirably comprises a differentiating circuit 190,
a recti?er 192 and a low pass ?lter 194. The function of
However, the second delay circuit 204 prevents switch
172 from selecting wideband pilot ?lter 168 and this
protects the loop from interference due to audio signal
portions of the received signal. When the correction is
complete, the lock-on condition is again sensed by lock
detector 200 and switch 186 again selects the narrow
AFC ?lter 184.
The AFC LOCK output signal of the second delay
circuit 204 may also be applied to the logic circuit 198
of FIG. 5 as will subsequently explained.
With continued reference to FIG. 4, the output signal
from the narrow band pilot ?lter 170 may be applied
the detector circuit 188 is to detect the frequency modu~
lation of the pilot tone. The output signal from the
through a recti?er 206 and a low pass ?lter 208 to a
detector 188 is applied to a tone squelch decoder 196
comparator 210 to which a variable AGC REF signal is
which acts a very high Q ?lter and provides a TCS 65 also applied as a standard. The output signal of the
TONE signal for application to the phase lock loop 180.
comparator 210 serves as the AGC signal and is applied
In this way, the modulation ofthe pilot tone as detected
to the RF ampli?er 134 and the IF ampli?er 148 to
by the detector 188 may be used to frequency modulate
control the gain thereof in a conventional manner.
4,539,707
The AGC output signal from the comparator 210
10
continue to supply a disabling signal to the input termi
may also be applied to a suitable conventional weak
nal 222 of the NAND gate 224 and thus continue to
signal detector 212 and the DELAY ADJ output signal
therefrom applied to the ?rst delay circuit 202 to
thereby vary the delay in the operation of the switch
insure the absence of the BK LOCK signal.
The BK LOCK signal may also be enabled if the
AFC signal falls below prescribed limits. The AFC
signal is applied to an AFC out-of-range timer 228, the
output signal from which is applied to the other input
terminal 230 of the NAND gate 224.
In operation, the AFC signal dropping below a set
level, e.g. l.5 volts, will trigger the AFC out-of-range
timer 228 to apply a BK LOCK disabling signal to the
input terminal 230 of the NAND gate 224 for a period
of time, e.g. 10 milliseconds. A BK LOCK signal is
186 and in the switch 172 in response to the loss of
lock-on.
The recti?ed and ?ltered output signal from the nar
row band pilot ?lter 170 is also applied to one input
terminal of a second comparator 214 to which a manu
O
ally variable squelch reference signal SQ REF is ap
plied. The output signal from the comparator 214 is
applied as the PILOT SQ signal to the logic circuit 198
described in connetion with FIG. 5.
In operation, the adjustment of the reference signal
5
for the comparator 214 may be used to selectively block
reception of weak signals. This may be desirable where
the operator of a mobile receiver is receiving signals of
good signal strength from a desired source as well as
occasional signals of weak signal strength from an unde
sirable source.
As will be explained in greater detail in connection
with FIG. 5, the presence of the AFC LOCK, PILOT
With reference to FIG. 6, the pass band of the trans
mitter is desirably between about 300 Hz and 3,300 Hz
with the pilot tone at 3100 Hz and the audio band be
tween 300 Hz and about 2,600 Hz with the high pass
?lter 36 and the low pass ?lter 44 of FIG. 2 providing
the limits. In this way, the pilot tone frequency of 3,l00
Hz as determined by the oscillator 70 in FIG. 2 is out
side of the audio passband but within the passband of
the transmitter.
With reference to FIG. 7 where the ?rst expander
158 of the receiver illustrated in FIG. 4 is shown, the
output signal from the audio ?lter 156 of FIG. 4 may be
SQ and TCS LOGIC signals are operative in the logic
circuit 198 to generate an SQ GATE signal to unlock
the squelch of the receiver by control the gain of the
ampli?er 164. In other words, the detection of lock-on
by the phase detector 176 acts to switch the phase lock
loop 176 loop ?lter to its narrow bandwidth and to
switch the pilot ?lter to its narrow bandwidth and to
generate the AFC LOCK signal applied to logic circuit
198. The presence of a tone signal of suf?cient ampli
tude from the narrow band pilot ?lter 170 will thus be
operative to open the squelch of the ampli?er 164
through the generation of the SQ GATE signal.
generated which will actuate switch 185 in FIG. 4 to
ground the output of phase detector 176 and thus break
AFC LOCK and return the AFC signal to its quiescent
high voltage state where it will again attempt to search
out and lock to the pilot tone. Thus, the generation of
the BK LOCK signal is inhibited for a predetermined
period of time following loss of the AFC signal irre~
spective of the presence of the TCS LOGIC signal.
applied through a resistor 240 to the negative input
35 terminal of an operational ampli?er 242 to which an
It is necessary for the generation of the SQ GATE
appropriate reference voltage is applied to the positive
signal (required to open the squelch of ampli?er 164) for
input terminal 244 thereof. The output signal from the
operational ampli?er 242 may be fed back through a
variable gain feedback circuit 246 to the negative input
the TCS LOGIC signal to be applied to the logic circuit
198 when tone coded squelch is incorporated within the
system.
The presence of the AFC LOCK signal may be ap
plied through the logic circuit 198 in the form of a CH
terminal 242.
The output signal from the narrow band pilot ?lter
170 of the receiver of FIG. 4 may be recti?ed in a recti
?er 248 and used to vary the gain of the circuit 246 in
BUSY signal to a suitable indicater 216.
With reference to the logic circuit illustrated in FIG.
the feedback path of the ampli?er 242. In this way, the
5, the AFC LOCK signal is applied to a tone coded 45 amplitude of the pilot tone may be used to vary the gain
squelch (TCS) timer 218 which applies an enabling
signal to the OR gate 220 for a predetermined time
interval controlled by the RC time constant of the timer
218. At the end of the predetermined time interval, the
output signal from the TCS timer 218 is thus removed
from the OR gate 220. The other input signal to the OR
gate 220 is the TCS LOGIC signal from the tone’
squelch decoder 196 of the receiver of FIG. 4.
In operation, the presence of PLL lock-on as de
tected by the lock detector 200 will trigger the TCS
timer 218 to apply a signal to one input terminal 222 of
a NAND gate 224. The presence of this signal inhibits
the generation of the BK LOCK signal for that initial
period of time. As a result, the switch 186 in the re
ceiver of FIG. 4 remains in the narrow AFC loop ?lter 60
position.
The predetermined time interval of the TCS timer
218 is of sufficient duration, e.g. 250 milliseconds, to
permit the tone squelch decoder 196 of the receiver of
of the audio signal without varying the gain of the re
ceiver per se or the composite audio and pilot tone
signal as it passes through the receiver.
With reference to FIG. 8, the desired frequency re
sponse of the narrow band pilot ?lter 170 is illustrated as
having a peak at 3,100 cycles and an effective bandpass
of approximately 300 Hz. In this way, the audio band
pass between 300 Hz and 2,600 Hz as illustrated in FIG.
6 may be excluded.
With reference to FIG. 9, the desired frequency re
sponse of the wide band pilot ?lter 168 of the receiver of
FIG. 4 is illustrated. As earlier explained in connection
with FIG. 6, the passband of the transmitter is only
slightly wider than the bandwidth of the combined pilot
tone and the audio signal passband. By the selection of
an appropriate ?lter such as an under damped, lowpass
?lter with a peak in response at the high end, the high
frequency noise will drive the phase lock loop to the
upper limit of the oscillator associated therewith. This
FIG. 4 to determine the presence of the correct fre 65 effective shifting of the phase lock loop to the upper
quency modulation of the pilot tone. If the TCS
LOGIC signal is present at the time that the signal from
the TCS timer 218 is removed, the OR gate 220 will
frequency limit of the oscillator will insure the presence
of the 3,100 Hz pilot tone within the receiver’s bandpass
and thus enhance the acquisition thereof.
11
4,539,707
With reference to FIG. 10 and to the tone generator
60 of FIG. 2 by which the tone coded squelch modula
tion of pilot tone is effected, the TCS TONE output
signal from the TCS tone generator 76 may be applied
to the loop ?lter of the phase locked loop 72 as illus
trated.
ADVANTAGES AND SCOPE OF THE
I
-
‘
INVENTION
The transmitter of the present invention operates to
broadcast only the pilot tone during an initial time inter
val. The presence of the pilot tone without the audio
signal greatly facilitates the detection thereof and the
lock-on of the receiver. In addition the pilot tone is
transmitted at full rated power of the transmitter during 5
this initial period and thereafter, when lock-on has been
12
only during said initial predetermined time interval
for adjusting said transmitter to produce full rated
power; and
a receiver for receiving said broadcast signal, said
receiver including:
means for detecting said compressed composite sig
nal,
pilot tone ?lter means for separating the pilot tone
from said compressed signal,
means for locally generating a frequency modulated
tone,
phase lock loop means responsive to said local tone
generating means and to said pilot tone ?lter means
for (a) varying the ?lter characteristics of said pilot
tone ?lter means to thereby enhance acquisition of
said detected compressed composite signal, (b)
accomplished, the pilot tone amplitude is attenuated to
varying the ?lter characteristics of said phase lock
reduce the likelihood of interference with the audio
signal. In addition, the automatic level control of the
transmitter is effective to adjust the transmitter to its full 20
rated power during the initial time interval when the
pilot tone is present at its predetermined maximum am
loop means after acquisition of said detected com
audio signal ?lter means for separating said audio
plitude, and thereafter the gain is unchanged. Thus the
transmitter gain has been automatically adjusted so the
means responsive to said pilot tone ?lter means for
peaks of the audio signal will drive the transmitter to its 25
full rated power but not beyond.
The frequency modulation of the pilot tone by the
location of the modulation source in the return end of
pressed composite signal to thereby enhance main
tenance of lock-on of said detected compressed
composite signal,
signal from said compressed composite signal, and
expanding and amplifying said compressed audio
signal without expanding and amplifying said pilot
tone.
2. The system of claim 1 wherein said receiver in
the loop ?lter is advantageous where the bandwidth of
loop ?lter is signifcantly lower than the frequency of
cludes squelch means responsive to the frequency mod
ulation of said separated pilot tone signal for adjusting
the modulation.
In the receiver, the selection of a wide band pilot
the gain of said audio signal expanding and amplifying
?lter or a narrow band pilot ?lter as a function of lock
3. The system of claim 1 wherein said receiver in
cludes means for detecting the signal strength of said
on of the phase lock loop is a signi?cant advantage in
means.
that the acquisition of the pilot tone is facilitated. The
frequency modulated pilot tone and for selectively de
use of a time delay between selection of the Wide AFC
laying the variations of the ?lter characteristics of said
loop ?lter and reinsertion of the wideband pilot ?lter,
provides additional security against the need for reac
quisition in the event the incoming signal shifts slightly
in frequency. The probability of drop out in weak signal
tone ?lter means and said phase lock loop ?lter means.
4. A method of communicating between a base sta
areas is signi?cantly reduced through the use of the
weak signal detector to vary the time delays after loss of
lock-on in controlling the automatic frequency control
circuits of the receiver.
Further, the selection of the frequency response char 45
acteristics of the wide band pilot ?lter such that the
presence of noise tends to drive the phase lock loop
associated therewith to one frequency limit of the oscil
lator sign?cantly enhances acquisition of the pilot tone,
as this technique assures the pilot tone lies within the
reciver passband.
While preferred embodiments of the present inven
tion and land mobile receiver comprising:
(a) broadcasting a compressed signal from a base
station by a push-to-talk single sideband, radio
communication transmitter, said compressed signal
including an unattenuated frequency modulated
pilot tone during an initial predetermined time
interval; and thereafter a composite signal includ
ing an attenuated frequency modulated pilot tone
and an audio signal, the output power of said trans
mitter being adjusted to full rated transmitted
power only during said initial predetermined time
interval; and
(b) receiving the broadcast signal at a receiver by the
steps of:
(l) detecting the compressed composite signal,
tion have been described, it is to be understood that the
embodiments described are illustrative only and that the
scope of the invention is to be de?ned solely by the
appended claims when accorded a full range of equiva~
lents, many variations and modi?cations naturally oc
curring to those skilled in the art from a perusal hereof.
What is claimed is:
pressed composite signal in a pilot tone ?lter,
(3) locally generating a frequency modulated tone,
(4) detecting the phase difference between the 10
cally generated tone and the separated pilot tone,
(5) varying the ?lter characteristics of the pilot
1. A two-way, land mobile, single sideband, radio
communciation system comprising:
thereby enhance acquisition of said detected
a push-to-talk transmitter for broadcasting a com
pressed signal, said compressed signal including an
(2) separating the pilot tone from the detected com
tone ?lter in response to said phase difference to
compressed composite signal, and
(6) varying the ?lter cahracteristics of the pilot
unattenuated frequency modulated pilot tone dur
ing an initial predetermined time interval and 65
thereafter a composite signal including an attenu
ated frequency modulated pilot tone and an audio
maintenance of lock-on of said detected com
signal, said transmitter including means operative
pressed composite signal, and
tone ?lter in response to said phase difference
after acquisition of lock-on of said detected com
pressed composite signal to thereby enhance
4,539,707
13
(7) expanding and amplifying the received com
pressed audio signal without expanding or ampli
fying the received pilot tone.
5. The method of claim 4 including the further step of
adjusting the gain of said audio signal expanding and
amplifying means responsively to the separated pilot
lock loop; and
interval as a function of the signal strength of the
5
6. The method of claim 4 including the further step of
detecting the strength of the separated frequency mo
dualted pilot tone and selectively adjusting the speed of
response in varying the characteristics of the pilot tone
715. The method of claim 7 wherein the radio fre
'quency signal is a compressed signal from a land mobile,
7. In a method of enhancing reception of a composite
two-way radio communication source and including the
radio frequency signal including audio frequency com
further step of expanding the audio frequency compo
nents both prior to and following de-emphasis.
16. A receiver comprising:
ponents and a pilot tone component by a receiver hav
ing a radio frequency oscillator, a phase lock loop, and
radio frequency and intermediate frequency ampli?ers
pilot tone ?lter means;
audio ?lter means;
means for detecting a composite pilot tone and audio
in which (a) the pilot tone component is detected by a
pilot ?lter and recti?er means and the amplitude of the
detected pilot tone component is used to control the
signal and for applying said composite signal to
gain of the radio frequency and intermediate frequency
ampli?ers within the receiver, (b) the audio frequency
said pilot tone and audio ?lter means;
means responsive to said audio ?lter means for ex
components are de-emphasized for application to a
panding, demphasizing and amplifying said audio
speaker, and (c) the phase lock loop receives both the
detected pilot tone component and a locally generated 25
pilot tone and adjusts the radio frequency oscillator for
frequency errors in the received carrier frequency, the
improvement wherein the frequency response charac
signal;
tone generating means;
phase detector means responsive to said tone generat
ing means and to said pilot tone ?lter means;
AFC means responsive to said phase detector means
teristics of the pilot ?lter are selectively varied as a
function of a phase lock loop lock-on.
8. The‘ method of claim 7 wherein the selective varia
for varying the frequency response characteristics
of a ?lter associated with said AFC means to
thereby control the frequency response of said
detecting means;
tion of frequency response characteristics includes,
(a) selecting narrow bandpass ?lter characteristics
and narrow phase lock loop ?lter characteristics in
means responsive to said AFC means for adjusting
35
the frequency response characteristics of said pilot
tone ?lter means; and
AGC means responsive to said pilot tone ?lter means
tics after a ?rst predetermined time interval follow
for controlling the gain of said detecting means.
ing the loss of lock-on in the phase lock loop.
17. The receiver of claim 16 wherein said expanding,
de-emphasizing and amplifying means includes ?rst and
second expanders and de-emphasis means, and
9. The method of claim 8 including the further step of
adjusting the duration of the ?rst predetermined time
interval as a function of signal strength immediately
wherein the audio signal is passed through said ?rst
preceding the loss of lock-on.
expander prior to said de-emphasis means and
thereafter through said second expander.
10. The method of claim 9 including the further step
of delaying the selection of wide phase lock loop ?lter
characteristics for a second predetermined time interval 45
18. The receiver of claim 16 wherein said AFC means
is variable in its frequency response characteristics;
following loss of phase lock loop lock-on.
including means for delaying any variation in the
frequency response characteristics of said AFC
11. The method of claim 10 wherein the pilot tone is
frequency modulated and including the further steps of:
detecting the modulation of the pilot tone;
means for a ?rst predetermined time interval fol
lowing loss of lock-on by said AFC means; and
means responsive to said pilot ?lter means for adjust
providing a tone coded squelch signal responsively to
the detection of the modulation of the pilot tone;
ing the duration of said ?rst predetermined time
and
opening receiver squelch or breaking phase lock loop
interval in response to the detection of a weak pilot
tone signal immediately preceding loss of lock-on.
lock-on responsively to the tone coded squelch
signal.
detected pilot tone component immediately pre
ceding the loss of lock-on.
14. The method of claim 13 wherein the composite
signal is compressed and including the further step of
expanding the audio frequency components prior to
de-emphasis.
’
response to phase lock loop lock-on; and
(b) selecting wide phase lock loop ?lter characteris
-
(b) adjusting the duration of the predetermined time
tone.
?lter in response thereto.
14
the frequency response characteristics of the phase
55
12. The method of claim 7 wherein the pilot tone is
frequency modulated,
detecting the modulation of the pilot tone;
providing a tone coded squelch signal responsively to
the detection of the modulation of the pilot tone;
and
19. The receiver of claim 16 wherein said pilot tone
?lter means frequency response adjusting means in
cludes means for delaying any broadening in the ?lter
characteristic for a predetermined time interval follow
ing loss of lock-on by said AFC means.
20. The receiver of claim 19
including means for delaying any variation in the
opening receiver squelch or breaking phase lock loop
frequency response adjusting means, said delaying
lock-on responsively to the tone coded squelch
means including means for delaying any broaden
ing in the ?lter characteristic for a predetermined
signal.
13. The method of claim 7 including the further steps
Of:
(a) delaying for a predetermined time interval follow
ing loss of phase lock loop lock-on any variation in
65
time interval following loss of lock-on by said AFC
means; and
means responsive to said pilot ?lter means for adjust
ing the duration of said ?rst predetermined time
15
4,539,707
16
interval in response to the detection of a weak pilot
bandwidth of said audio frequency signal applying
tone signal immediately preceding loss of lock-on.
means to thereby provide a composite audio fre
quency and pilot tone signal;
21. In a method of controlling the output power of a
radio frequency transmitter in transmitting a composite
audio modulation signal including audio signal compo—
means for providing a radio frequency signal;
means for modulating said radio frequency signal
nents and a pilot tone component outside the audio
with said composite signal; and
automatic level control means operable only during
said predetermined time interval for adjusting the
output power of said radio frequency signal pro
viding means;
whereby the modulation of said radio frequency sig
nal for a predetermined period of time following
the initiation of a transmission is solely by said pilot
signal passband but within the bandpass of the transmit
ter, the improvement including the steps of
(a) transmitting only the pilot tone component for a
predetermined initial time interval, and
(b) adjusting the power level of the transmitter during
the initial time interval and thereafter refraining
from adjustment of the power level for the remain
der of the transmission irrespective of the ampli
tude of the composite audio modulation signal.
10
tone and whereby the output power of the trans
mitter is adjusted only during the initial transmis
22. The method of claim 21 including the step of
sion of the pilot tone.
26. The transmitter of claim 25 wherein said auto
matic level control means adjusts the power of said
radio frequency providing means to rated full power
compressing the audio signal prior to pre-emphasis to
effectively double the effective pre-emphasis achieved
by compression after pre-emphasis.
23. The method of claim 21 including the further step
of attenuating the pilot tone after the initial time inter
during said predetermined time interval.
27. The transmitter of claim 26 including means for
val.
compressing said audio frequency signal, means for
24. The method of claim 21 including the further step
pre-emphasing said compressed audio frequency signal,
of limiting the amplitude of the composite audio modu
lation signal to a value related to the full output power 25 and means for compressing said composite pilot tone
and compressed, pre-emphasized audio signal.
of the transmitter.
28. The transmitter of claim 27 including means for
25. A radio frequency transmitter comprising:
frequency modulating said pilot tone.
transmission initiating means;
summing means;
29. The transmitter of claim 28 including means for
attenuating said pilot tone after said initial. predeter
means responsive to said transmission initiation means
mined time interval.
for applying an audio frequency signal to said sum
30. The transmitter of claim 25 including means for
ming means after a predetermined time interval;
attenuating said pilot tone after said initial predeter
means responsive to said transmission initiating means
mined time interval.
for applying a pilot tone to said summing means,
*
*
*
*
=|<
said pilot tone having a frequency outside of the 35
45
55
65
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