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Claims  |
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What is claimed is:
1. A two-way, land mobile, single sideband, radio communciation system
comprising:
a push-to-talk transmitter for broadcasting a compressed signal, said
compressed signal including an unattenuated frequency modulated pilot tone
during an initial predetermined time interval and thereafter a composite
signal including an attenuated frequency modulated pilot tone and an audio
signal, said transmitter including means operative 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 signal,
pilot tone filter 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 filter means for (a) varying the filter characteristics of
said pilot tone filter means to thereby enhance acquisition of said
detected compressed composite signal, (b) varying the filter
characteristics of said phase lock loop means after acquisition of said
detected compressed composite signal to thereby enhance maintenance of
lock-on of said detected compressed composite signal,
audio signal filter means for separating said audio signal from said
compressed composite signal, and
means responsive to said pilot tone filter means for expanding and
amplifying said compressed audio signal without expanding and amplifying
said pilot tone.
2. The system of claim 1 wherein said receiver includes squelch means
responsive to the frequency modulation of said separated pilot tone signal
for adjusting the gain of said audio signal expanding and amplifying
means.
3. The system of claim 1 wherein said receiver includes means for detecting
the signal strength of said frequency modulated pilot tone and for
selectively delaying the variations of the filter characteristics of said
tone filter means and said phase lock loop filter means.
4. A method of communicating between a base station 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 including
an attenuated frequency modulated pilot tone and an audio signal, the
output power of said transmitter 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:
(1) detecting the compressed composite signal,
(2) separating the pilot tone from the detected compressed composite signal
in a pilot tone filter,
(3) locally generating a frequency modulated tone,
(4) detecting the phase difference between the locally generated tone and
the separated pilot tone,
(5) varying the filter characteristics of the pilot tone filter in response
to said phase difference to thereby enhance acquisition of said detected
compressed composite signal, and
(6) varying the filter cahracteristics of the pilot tone filter in response
to said phase difference after acquisition of lock-on of said detected
compressed composite signal to thereby enhance maintenance of lock-on of
said detected compressed composite signal, and
(7) expanding and amplifying the received compressed audio signal without
expanding or amplifying 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 tone.
6. The method of claim 4 including the further step of detecting the
strength of the separated frequency modualted pilot tone and selectively
adjusting the speed of response in varying the characteristics of the
pilot tone filter in response thereto.
7. In a method of enhancing reception of a composite radio frequency signal
including audio frequency components and a pilot tone component by a
receiver having a radio frequency oscillator, a phase lock loop, and radio
frequency and intermediate frequency amplifiers in which (a) the pilot
tone component is detected by a pilot filter and rectifier means and the
amplitude of the detected pilot tone component is used to control the gain
of the radio frequency and intermediate frequency amplifiers within the
receiver, (b) the audio frequency components are de-emphasized for
application to a speaker, and (c) the phase lock loop receives both the
detected pilot tone component and a locally generated pilot tone and
adjusts the radio frequency oscillator for frequency errors in the
received carrier frequency, the improvement wherein the frequency response
characteristics of the pilot filter are selectively varied as a function
of a phase lock loop lock-on.
8. The method of claim 7 wherein the selective variation of frequency
response characteristics includes,
(a) selecting narrow bandpass filter characteristics and narrow phase lock
loop filter characteristics in response to phase lock loop lock-on; and
(b) selecting wide phase lock loop filter characteristics after a first
predetermined time interval following the loss of lock-on in the phase
lock loop.
9. The method of claim 8 including the further step of adjusting the
duration of the first predetermined time interval as a function of signal
strength immediately preceding the loss of lock-on.
10. The method of claim 9 including the further step of delaying the
selection of wide phase lock loop filter characteristics for a second
predetermined time interval following loss of phase lock loop lock-on.
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;
providing a tone coded squelch signal responsively to the detection of the
modulation of the pilot tone; and
opening receiver squelch or breaking phase lock loop lock-on responsively
to the tone coded squelch signal.
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
opening receiver squelch or breaking phase lock loop lock-on responsively
to the tone coded squelch signal.
13. The method of claim 7 including the further steps of:
(a) delaying for a predetermined time interval following loss of phase lock
loop lock-on any variation in the frequency response characteristics of
the phase lock loop; and
(b) adjusting the duration of the predetermined time interval as a function
of the signal strength of the detected pilot tone component immediately
preceding 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.
15. The method of claim 7 wherein the radio frequency signal is a
compressed signal from a land mobile, two-way radio communication source
and including the further step of expanding the audio frequency components
both prior to and following de-emphasis.
16. A receiver comprising:
pilot tone filter means;
audio filter means;
means for detecting a composite pilot tone and audio signal and for
applying said composite signal to said pilot tone and audio filter means;
means responsive to said audio filter means for expanding, demphasizing and
amplifying said audio signal;
tone generating means;
phase detector means responsive to said tone generating means and to said
pilot tone filter means;
AFC means responsive to said phase detector means for varying the frequency
response characteristics of a filter associated with said AFC means to
thereby control the frequency response of said detecting means;
means responsive to said AFC means for adjusting the frequency response
characteristics of said pilot tone filter means; and
AGC means responsive to said pilot tone filter means for controlling the
gain of said detecting means.
17. The receiver of claim 16 wherein said expanding, de-emphasizing and
amplifying means includes first and second expanders and de-emphasis
means, and
wherein the audio signal is passed through said first expander prior to
said de-emphasis means and thereafter through said second expander.
18. The receiver of claim 16 wherein said AFC means is variable in its
frequency response characteristics;
including means for delaying any variation in the frequency response
characteristics of said AFC means for a first predetermined time interval
following loss of lock-on by said AFC means; and
means responsive to said pilot filter means for adjusting the duration of
said first predetermined time interval in response to the detection of a
weak pilot tone signal immediately preceding loss of lock-on.
19. The receiver of claim 16 wherein said pilot tone filter means frequency
response adjusting means includes means for delaying any broadening in the
filter characteristic for a predetermined time interval following loss of
lock-on by said AFC means.
20. The receiver of claim 19
including means for delaying any variation in the frequency response
adjusting means, said delaying means including means for delaying any
broadening in the filter characteristic for a predetermined time interval
following loss of lock-on by said AFC means; and
means responsive to said pilot filter means for adjusting the duration of
said first predetermined time interval in response to the detection of a
weak pilot tone signal immediately preceding loss of lock-on.
21. In a method of controlling the output power of a radio frequency
transmitter in transmitting a composite audio modulation signal including
audio signal components and a pilot tone component outside the audio
signal passband but within the bandpass of the transmitter, 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 remainder of the transmission irrespective of the amplitude of the
composite audio modulation signal.
22. The method of claim 21 including the step of 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 interval.
24. The method of claim 21 including the further step of limiting the
amplitude of the composite audio modulation signal to a value related to
the full output power of the transmitter.
25. A radio frequency transmitter comprising:
transmission initiating means;
summing means;
means responsive to said transmission initiation means for applying an
audio frequency signal to said summing means after a predetermined time
interval;
means responsive to said transmission initiating means for applying a pilot
tone to said summing means, said pilot tone having a frequency outside of
the bandwidth of said audio frequency signal applying means to thereby
provide a composite audio frequency and pilot tone signal;
means for providing a radio frequency signal;
means for modulating said radio frequency signal 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
providing means;
whereby the modulation of said radio frequency signal for a predetermined
period of time following the initiation of a transmission is solely by
said pilot tone and whereby the output power of the transmitter is
adjusted only during the initial transmission of the pilot tone.
26. The transmitter of claim 25 wherein said automatic level control means
adjusts the power of said radio frequency providing means to rated full
power during said predetermined time interval.
27. The transmitter of claim 26 including means for compressing said audio
frequency signal, means for pre-emphasing said compressed audio frequency
signal, and means for compressing said composite pilot tone and
compressed, pre-emphasized audio signal.
28. The transmitter of claim 27 including means for frequency modulating
said pilot tone.
29. The transmitter of claim 28 including means for attenuating said pilot
tone after said initial predetermined time interval.
30. The transmitter of claim 25 including means for attenuating said pilot
tone after said initial predetermined time interval. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to a radio frequency communications system
and method, and more particularly 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
difficult to achieve. In one aspect, the present invention greatly
simplifies the pre-emphasis circuit by compressing 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 fluctuate as a result of the widely varying
characteristics 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 transmitter thereafter limits the
amplitude of audio signals and thus prevents the transmitter power output
from exceeding its rated value.
In generally known systems, the pilot tone may be masked by audio frequency
components, or alternatively the phase lock loop of the receiver may try
to lock on a portion of the audio signal making initial acquisition
difficult. In one aspect, the present invention obviates this problem by
transmitting only the pilot tone for a period of time sufficient for
acquisition thereof by the phase lock loop of the receiver. Once
acquisition has been achieved, the frequency response characteristics of
the pilot tone filter are narrowed and the pilot tone attenuated to avoid
possible interference with the audio 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 extraneous noise until such time as a uniquely coded pilot
tone is received. In a further aspect, the present invention achieves
frequency modulation of the pilot tone by locating the modulating source
in the return end of the loop filter of a phase lock loop. Simplicity of
circuit design may thus be achieved when the frequency of modulation is
high with respect to the bandwidth of the loop filter associated with the
phase lock loop.
In receivers in systems of the type heretofore described, the frequency
response characteristics of the loop filter are varied as a function of
lock-on of the 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 inventions improves upon this feature by detecting the
strength of the detected pilot tone immediately prior to loss of lock-on,
and increasing the delay in reverting to the rapid acquisition mode under
conditions where the signal is weak and fades are likely to be longer in
duration.
In generally known prior art receivers, the amplitude of the pilot tone is
detected and used to control the gain of the receiver, i.e., to adjust the
strength of the composite 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 invention by using the signal strength of the detected pilot tone
to control only the gain of the audio signal components of the composite
signal.
In generally known systems, the speed of response in acquisition of the
pilot tone is a function of the bandwidth of the pilot filter. As
explained in connection with the transmitter of the present invention, the
initial transmission of a full power, unattenuated pilot tone greatly
facilitates lock-on. Thereafter, the pilot filter 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 response
characteristics of the pilot filter 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 adjusted as a function of signal strength
immediately prior to the loss of lock-on. The amount of delay in switching
to the wideband pilot filter is always greater than the amount of delay in
switching to the wideband phase lock loop filter. This allows the loop to
make rapid corrections if the received pilot signal drifts in frequency
without increasing the pilot filter 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
interval. In addition, the frequency response characteristics of the
wideband pilot tone filter are desirably selected such that noise tends to
drive the oscillator associated with the phase lock loop to one extreme,
thereby tending to center the pilot tone in the bandwidth of receiver's IF
filter (the primary selectivity element). In this way, the presence of the
tone is immediately detected even if that tone is not exactly on the
expected frequency. This minimizes the requirement for oscillator
stability in the transmitter and receiver and this reduces cost and
complexity.
The foregoing and many other features, objects and advantages of the
present invention will be readily apparent to one skilled in this art from
the claims and from a perusal of the following specification when read in
conjunction with the appended drawings.
THE DRAWINGS
FIG. 1 is a functional block diagram of a prior art transmitter;
FIG. 2 is a functional block diagram of one embodiment of the transmitter
of the present invention;
FIG. 3 is a functional block diagram of a prior art receiver;
FIG. 4 is a functional block diagram of one embodiment of the receiver of
the present invention;
FIG. 5 is a logic diagram of one embodiment of the logic circuit of the
receiver illustrated in FIG. 4;
FIG. 6 is a plot of the frequency spectrum illustrating the passband of the
transmitter of the present invention;
FIG. 7 is a schematic circuit diagram illustrating one embodiment of the
first expander in the receiver illustrated in FIG. 4;
FIG. 8 is a plot of the desired frequency response of the narrow band pilot
filter of FIG. 4;
FIG. 9 is a plot of the desired frequency response of the wide band pilot
filter of the receiver illustrated in FIG. 4; and
FIG. 10 is a circuit diagram illustrating the modulating of the pilot tone.
THE DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
An understanding of the transmitter of the present invention may be
facilitated by an understanding of the prior art transmitters. With
reference to FIG. 1 where a prior art transmitter is illustrated, a
suitable conventional microphone 10 may be used to pick up an audio
signal. The microphone 10 may be of the conventional push-to-talk type and
the output signal therefrom applied to a suitable conventional
pre-emphasis circuit 12. Inasmuch as most of the energy in an audio
frequency signal is concentrated at the low frequencies, the pre-emphasis
circuit desirably provides a 12 dB per octave gain so that the gain
applied to the signal is increased as a function of the frequency of the
signals passed therethrough.
The output signal from the pre-emphasis circuit 12 may be applied to a
suitable conventional compressor 14 where the entire audio signal is
compressed. The compressed signal is thereafter applied through a suitable
conventional notch filter 16 to one input terminal of a summing circuit 18
such as a summing amplifier. The notch filter 16 serves to remove audio
frequency components in a narrow band centered on the frequency of the
pilot tone, e.g. 3.1 KHz.
The other input terminal of the summing circuit 18 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 compressor 22 to the variable gain amplifier of an automatic
level control circuit 24 at the input of a conventional single sideband
transmitter 26. The output signal from the transmitter 26 is applied to a
suitable conventional antenna 28 for broadcast and is also fed back
through a rectifier 30 to control the gain of the automatic level
controlled amplifier 24.
The frequency of the pilot oscillator 20 may be modulated by a suitable
conventional modulator 32 for purposes 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 modulation 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 the SSB transmitter 26.
The system gain may vary considerably as the speaker's voice varies.
With reference now to FIG. 2 where one embodiment of the transmitter of the
present invention is illustrated, a suitable conventional microphone 34
may be used to provide an audio signal to be passed through an audio
response limiting filter 36 to a compressor 38. The compressed audio
signal is passed through a suitable conventional 6 dB per octave
pre-emphasis circuit 40, and through a limit circuit 42 to a suitable
conventional low pass filter 44. The limit circuit 42 is important because
the amplitude of the modulation signal effects the output power of a
single sideband transmitter.
The compressed audio output signal from the filter 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
composite signal is further compressed in compressor 48 and applied
through a variable gain automatic level control amplifier 50 of a suitable
conventional type to a single sideband transmitter 52 for transmission
from a conventional antenna 54. The output signal from the transmitter 52
is also passed through a rectifier 56 to a sample and hold circuit 58, the
output of which is used to control the gain of the ALC amplifier 50.
With continued reference to FIG. 2, the frequency modulated pilot tone from
the tone generator 60 is applied through a suitable conventional
attenuator 62 to the other input terminal of the summing circuit 46. A
suitable electronic shunt 64 is provided to selectively eliminate the
attenuator 62 and a suitable shunt 66 is provided at the output of the
lowpass filter 44 to selectively remove the audio signal from the input to
the summing circuit 46.
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.
In operation, the timer 68, when triggered by the initiation of a
transmission, provides for a first predetermined time period an output
signal which closes the shunts 64 and 66. Operation of the shunt 64
removes the attenuater 62 from the circuit and thus applies the frequency
modulated pilot tone to the summing circuit 46 undiminished in amplitude.
During the same period of time, operation of the shunt 66 shunts the audio
output signal from the lowpass filter 44 to ground and thus removes the
audio signal from the input to the summing circuit 46. Thus, for the
initial time interval as determined by the timer 68 at the beginning of
each transmission, the output signal of the summing circuit 46 will be 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 filter
44 to the summing circuit. Subsequently thereto, the output signal from
the summing circuit 46 will be a composite signal including the audio
signal and an attenuated frequency modulated pilot tone.
Also upon the timing out of the timer 68, the sample-and-hold circuit 58 is
operated to freeze or fix the level of the control signal applied to the
ALC amplifier 50. In this way, the automatic level control circuit for the
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
transmission. The gain of amplifier 48 is initially adjusted to produce
full rated power output from the transmitter when shunt 64 is closed. The
limiter 42 is designed such that the peak value of the audio signal that
it may pass does not exceed the output of tone generator 60. Thus 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 association 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 transmitter 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 transmitter remain unchanged for the duration of any single
transmission.
With continued reference to FIG. 2, the tone generator 60 may comprise a
pilot tone oscillator 70, a phase lock loop 72, a shaper 74 and a tone
coded squelch tone 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 loop 72 may be modulated for tone
coded squelch purposes by the application of the low frequency signal from
the TCS tone generator 76. With reference to FIG. 10, where the frequency
of the modulation is significantly greater than the bandwidth of the loop
filter, the frequency modulation of the pilot tone may be accomplished by
locating the modulating source at the point where the loop filter would
normally be returned to ground.
Note that the compressor 38 preceeds the pre-emphasis circuit 40. In this
way, a 6 dB per octave pre-emphasis after compression provides the
equivalent of 12 dB per octave prior to compression.
The receiver of the present invention may also be more easily understood
with reference to a prior art 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 amplifier 82 to a suitable conventional
mixer 84 where it is mixed with the output signal from an oscillator 86.
The output signal from the mixer 84 may be applied through a suitable
conventional intermediate frequency filter 88 and a conventional IF
variable gain amplifier 90 to a second mixer 92 for mixing with the output
signal from a conventional oscillator 94. The output signal from the mixer
92 may be passed through a variable gain audio amplifier 96 to a
conventional notch filter 98 where the pilot tone is removed.
The audio output signal from the notch filter 98 may then be expanded in a
suitable conventional expander circuit 100 and de-emphasized in a
de-emphasis circuit 102 to remove the effects of the pre-emphasis circuit
12 of FIG. 1. The expanded and de-emphasized audio signal may then be
passed through a suitable conventional variable gain amplifier 104 to a
speaker 106.
The composite output signal from the amplifier 96 may also be passed
through a pilot filter 108 to remove audio signal. The pilot signal
amplitude is detected in a level detector 110 and used to control the gain
of the amplifier 96 to provide a constant pilot amplitude at the pilot
filter output terminal.
The output signal from the level detector 110 may also be passed through a
low pass filter 112 and applied to the radio frequency and intermediate
amplifiers 82 and 90 respectively to control the gain thereof. Thus, the
amplitude of the pilot tone is used to dynamically control the gain of the
composite signal passing through the receiver.
The output signal from the pilot filter 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 amplifier to provide an output signal of
constant amplitude. The output signal from the shaper 114 may be applied
to one input terminal of a conventional 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 filters 122 and 124. The output terminals of the loop filters 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
frequency of the received signal into lock with the frequency of the
oscillator 118.
In operation, and in the absence of a signal from the lock detector 119,
the output signal of the phase lock loop 116 is applied through the wide
loop filter 122 to facilitate capture of the input signal. Once lock is
detected by the lock detector 119, the switch 120 is activated to apply
the output signal from the phase lock loop through the narrow loop filter
124. The response of the narrow loop filter is desirably very sluggish and
thus tends to maintain a constant value output signal for application to
the oscillator 86.
With continued reference to the prior art receiver of FIG. 3, the
oscillator 118 which provides one input signal to the phase lock loop 116
may be conveniently located in the tone coded squelch (TCS) circuit tone
detector 126. The TCS tone detector receives the shaped tone output signal
from the shaper 114 and includes a discriminator 128 and a detector 130 to
remove the frequency modulation from the frequency modulated pilot tone.
The modulation removed by the discriminator 128 and detector 130 may be
used to modulate the frequency of the output signal from the pilot tone
oscillator 118 so that the two input signals to the phase lock loop 116
are both frequency modulated in the same manner.
The modulation removed by the discriminator 128 and detector 130 may be
applied to a tone squelch decoder 132 which generates an output signal if
the detected tone is of the correct frequency. A squelch circuit 134 may
also be included that is responsive to the lock detector 119 output and/or
the TCS decoder 132 output to allow the receiver to be muted until a
correctly coded signal is received.
In a transceiver, as contrasted with separate transmitters and receivers,
the pilot oscillator 20 of the transmitter of FIG. 1 and the oscillator
118 in the receiver illustrated in FIG. 3 may be the same unit.
Now with reference to FIG. 4 where one embodiment of the receiver of the
present invention is illustrated, the signal broadcast by the antenna 54
of the transmitter of FIG. 2 may be received by the antenna 132. This
input signal is applied through a suitable conventional variable gain RF
amplifier 134 to a first mixer 136 where it is mixed with the output
signal from a suitable conventional oscillator 138. The output signal from
the first mixer 136 may be passed through a first IF filter 140 to a
second mixer 142 where it is mixed with the output signal from a
conventional oscillator 144. The output signal from the second mixer 142
may be passed through a second IF filter 146 and a suitable conventional
variable gain IF amplifier 148 to a third 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 amplifier 154 as the composite signal containing
compressed audio and frequency modulated pilot tone components.
The composite signal from the amplifier 154 may be applied through an audio
filter 156 which operates to remove the pilot tone components and to delay
the signal. This delayed audio signal is passed through a first expander
which may be of the type subsequently described in connection with FIG. 7,
and from there through a suitable conventional de-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 a second expander 162 and a suitable conventional
variable gain audio amplifier 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 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 filter 156, and
the compressed audio signal expanded, de-emphasized and expanded again for
application to the speaker 166.
Note that the delay of the signal passing through the narrow band pilot
filter 170 is desirably equal to the delay introduced by the audio filter
156 so that the expander 158 may be of the "feed forward" rather than the
"feed back" type. The operation of the first expander 156 is discussed
infra in more detail in connection with FIG. 7.
With continued reference to FIG. 4, the composite output signal from the
amplifier 154 is applied to a wide band pilot filter 168 and a narrow band
pilot filter 170. The output signal from the pilot filters 168 and 170 may
be selectively applied by way of a suitable electronic switch 172 to a
shaper 174. The shaper desirably includes a limiter to clip the amplitude
thereof to a constant low level, and a constant gain amplifier. The output
of the shaper 174 is thus a constant amplitude, frequency modulated pilot
tone.
The output signal from the shaper 174 is applied to one input terminal of a
phase detector 176. The reference input to phase detector 176 is the
output signal of pilot tone oscillator 178 applied through a second phase
lock loop 180. The output signal from the phase detector 176 is applied to
the input terminals of a wide AFC loop filter 182 and a narrow AFC loop
filter 184. The output signals from the AFC filters 182 and 184 are
applied through a suitable electronic switch 186 as the 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 rectifier 192 and a low pass filter 194. The function of
the detector circuit 188 is to detect the frequency modulation of the
pilot tone. The output signal from the detector 188 is applied to a tone
squelch decoder 196 wh | | |
