Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
The present invention relates in general to a
frequency modulated radio frequency synchronous repeater
system for the transmission of frequency modulated
broadcast signals,~and~more particularly to a frequency
modulated radio frequency repeater system employing a
synchronous frequency modulated booster system for the
re-transmission;of frequency modulated broadcast signals.
Frequency modulated~broadcast~transmissions have been
limited~to audiences within a~reception area. In order to
increase the reception area for the frequency modulated
broadcast transmission to reach a greater audience,
re-transmission sites have been installed in areas
remotely located from the originating transmitter. At the
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~re-transmission sites were booster transmitters and
synchronous transmitter exciters to increase the power
level of the frequency modulated broadcast signals at a
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re-transmission site.
Heretofore, a demodulatlon process was used at a
re-transmission site ~hich caused phase shifts, delays and
- ~ inconsistent modulation levels between the originating
frequency modulated broadcast signal and the frequency
modulated broadcast signal at the re-transmission site.
Phase shifts, delays and inconsistent modulation levels
between the originating frequency modulated radio
frequency signal and the re-transmitted frequency
modulated radio~frequency signal resulted in signal
degradation and noise interference in the coverage area
intended for improvement by the re-transmitted frequency
modulated broadcast signal of the booster transmitter.
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Heretofore, a device was employed to produce a
reference signal for synchronizing and stabilizing the
output of frequency modulated radio frequency signals
transmitted at a re-transmission booster site. The device
did not use a reference signal generated at the site of
the originating transmitter or re-use the FM modulation in
the original carrier. Hence, the output of the frequency
modulated radio fre~uency signal of the transmitter at the
re-transmission booster site was modified from the -
original FM sig~al in frequency and in phase. -
In the U.S. Patent to Wu et al., No. 4,710,970, issued
on December 1, 1987, for Method Of And Apparatus For
Generating A Frequency Modulated Ultrahigh Frequency Radio
Transmission Signal, there is disclosed an ultra high
radio frequency transmitter. The output of a very high
frequency voltage controlled generator is phase locked
through a phase detector with a voltage controlled crystal
oscillator producing a reference signal for the
stabilizing of the output transmission frequency of the
ultra high radio fre~uency transmission frequency
oscillator.
In the U.S. Patent to Martinez, No. 4,208,630, issued
on June 17, 1980, for Narrow Band Paging Or Control Radio
System, there is disclosed a radio system for paging in
which a central transmitting device and remote receiving
devices are phase locked to a local broadcast station
radio frequency carrier so as to provide a means to
synchronize the transmitting device with the receiving
device.
The 8ritish Patent to McGraw-Edison Company,
No. 2,061,581B, published on May 18, 1983, for
Communication System For Distribution Automation And
Remote Metering, discloses a phase detector to which is
applied the output of a limiter-amplifier and a re~erence
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signal from a voltage controlled crystal oscillator. The
output of the phase detector is a control signal which is
proportional to the phase differences of the input signals
to the phase detector. The error signal is applied to the
voltage controlled crystal oscillator. The circuit
described is part of a phase locked loop circuit.
In an article published by Omega International of
Irvine, California, entitled Synchronous Repeaters, there
is mentioned that the output frequency of a booster is
phase locked with the originating station through analog
j simulation of a digital control signal derived from the
i originating station.
Heretofore, FM exciters were sold to accept the
~ composite baseband signal from a stero generator, and STL
', 15 system or monaural audio and SCA programming, and to
generate its operating frequency with a digitally
programmed, phase-locked frequency synthesis system. Such
an FM exciter was sold by Continental Electronics Mfg. Co.
of Dallas, Texas, as the Continental Type 802A FM exciter,
~ 20 and by Broadcast Electronics of Quincy, Illinois, as the
3~ Model FX-30.
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SUMMARY OF THE INVENTION
A synchronous frequency modulated booster system for a
transmitter that re-transmits frequency modulated radio
frequency signals at a booster site away from the
originating program source. The synchronous frequency
modulated booster system includes a synchronous FM
transmitter exciter that converts incoming frequency
modulated intermediate frequency signals transmitted
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at the site of an originating transmitter to the frequency
modulated radio frequency signals transmitted by the
originating transmitter and applies the frequency
converted frequency modulated radio frequency modulated
radio frequency signals to a booster transmitter that
re-transmits the frequency modulated radio frequency
signals.
A synchronous frequency modulated booster system
comprises a transmitter that re-transmits frequency
modulated radio frequency signals. The synchronous
frequency modulated booster system includes a synchronous
FM exciter that receives an IF signal and a reference
pilot signal generated at the originating transmitter for
synchronizing the carrier frequency and modulation level
of frequency modulated radio frequency signals applied to
the transmitter for re-transmission with the fre~uency
modulated radio frequency signals transmitted at the
: originating transmitter. ~ :
An object of the present invention is to provide a
method of synchronizing frequency modulated booster system
for a transmitter that re-transmits frequency modulated
radio frequency signals, which booster system includes a
synchronous FM exciter for stabilizing the frequency of
the booster transmitter, the baseband, group delays, and
inconsistent modulation levels between frequency modulated
radio frequency signals transmitted by an originating -:
transmitter and the frequency modulated radio frequency
signals re-transmitted from a remote transmitter.
Another object of the present invention is to provide
a synchronous frequency modulated booster system for a
transmitter that re-transmits frequency modulated radio
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frequency signals which booster system includes a
synchronous FM exciter for receiving a reference signal
from the site of the originating transmitter along with
frequency modulated intermediate fre~uency signals for
synchronizing the re-transmitted frequency modulated radio
frequency signals with the frequency modulated radio
frequency signals transmitted by the originating
transmitter to preserve signal integrity and stability.
Another object of the present invention is to provide
an economical arrangement for intermediate frequency
repeating transmitter links.
A feature of the present invention is to obviate need
for an additional subcarrier frequency which has been
heretofore used for transmitting a synchronizing tone to
lock the carrier frequency of the FM transmitter.
Another feature of the present invention is that the
modulation levels at all remote transmitters are
synchronized.
Another feature of the present invention is the
minimization of audio phase delays for enhancing stereo
quality.
Another feature of the present invention is the -
elimination of an FM subcarrier to transmit a reference
signal for synchronizing the frequency of the booster
transmitter.
Another feature of the present invention is that a
synchronous FM exciter can be employed to modify the
modulation level of incoming modulation depth of the FM
carrier by a constant, for example:
l.S (input = + SS KHz; output = + 82.5 ~Hz).
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DESCRIPTION OF THE DRAWINGS
Figure 1 is a block diagram of a transmitter system
~: for originating frequency modulated radio frequency
signals and transmitter systems for re-transmitting
frequency modulated radio frequency signals.
Figure 2 is a block diagram of a modification of the
transmitter system shown in Figure 1.
Figure 3 is a block diagram of a further modification
of the transmitter system shown in Figure 1.
i 10 Figure 4 is a block diagram of a still further
modification of the transmitter system shown in Figure 1.
Figure 5 is a schematic diagram of a synchronous FM
exciter employed in the transmitter system shown in
Figures 1-4. j~
Figure 6 is a schematic diagram of a synchronous FM
exciter, which is a modification of the synchronous FM
exciter shown in Figure 5.
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:: 20 DESCRIPTION OF THE PREFERRED EMBODIMENTS
Illustrated in Figure 1 is a serial feed frequency
modulated radio frequency broadcast system or network 10
with boosters, which comprises an originating broadcast :.
station 15 for broadcasting an originating frequency
:~ 25 modulated radio fre~uency signal (FM) and re-transmitting ;~
broadcast stations 20 and 25~for re-transmitting,
respectively, the frequency modulated radio fre~uency
signals (FM) transmitted by the originating broadcast
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station 15. The re-transmitting broadcast stations 20 and
25 are on-site remotely located from the site of the
originating broadcast station 15.
The originating FM broadcast station 15 comprises a
suitable studio 30, a conventional studio transmitter link
~:~ (STL) receiver~35 with an intermediate frequency output, a
synchronous FM exciter 40, and a conventional originating
FM transmitter 45. Included in the studio 30 are an FM
broadcast stereo generator 46, and conventional FM
subcarrier generators for subsidiary communication
authorization ~SCA) equipment 47 and 48, all of which are
connected to a conventional studio link ~ STL ) transmitter
50 for transmitting a composite FM baseband signal from
the studio to the originating broadcast station 15.
15The FM broadcast stereo generator 46 is modified to
accept a highly stable time base reference signal. This
signal is 19 KHz or multiple of 19 KHz generated by a
highly stable crystal oscillator, not shown, with a long
term stability of l ppm per year. The highly stable
crystal oscillator is included in the STL transmitter 50.
In this manner, a highly stable time base reference signal
is provided for phase locking the originating broadcast
frequency and~the rebroadcasting frequencies in the entire
network 10 so that stability and accuracy of the broadcast
frequencies are achieved.
The output of the STL transmitter 50 is an FM signal
which, in the exemplary embodiment, is a 950 megahertz
~~MHz) signal. The stereo generator 46 and the SCA
¦~equipment 47 and 48 modulate the carrier frequency
generated by the STL transmitter 50 with the program to be
broadcast to provide the FM signal of 950 MHz. The STL
transmitter 50 applies a reference signal to the stereo
generator 46. In the exemplary embodiment, the reference
signal is a 19 kilohertz (KHz) signal or a multiple of a
19 KHz frequency signal.
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The STL receiver 35 receives through a suitable studio
link 55, the FM signal from the STL transmitter 50, and
applies an FM signal to the synchronous FM exciter 40 and
an FM signal to a conventional intermediate frequency (IF)
STL transmitter or a one-way repeater 60. In the
exemplary embodiment, the FM signal applied to the
synchronous FM exciter 40 from the STL receiver 35 is a
10.7 MHz signal and the FM signal applied to the IF
repeater STL transmitter 60 is a 63 MHz signal, which is
also the IF signal of the STL receiver 35. The studio '--
link 55, in the exemplary embodiment, may be a
conventional microwave link, telephone lines, or the
like. The reference signal from the stereo generator 46
to the STL transmitter 50 is received by the STL receiver
35 with the 950 MHz signal and is present in both the ~-
10.7 MHz signal applied to the synchronous FM exciter 40
and the 63 MHz signal applied to the IF STL transmitter
60.
The synchronous FM exciter 40, which will be described
in detail hereinafter, applies to the originating FM :
transmitter 45, an FM broadcast signal, the carrier
fre~uency of which is generated by the synchronous FM
exciter 40. In the exemplary embodiment, the FM
transmitter 45 transmits an FM broadcast signal in the
range of 88 MHZ-108 MHz.
; A conventional link 65, such as a microwave link, VHF
or UHF link, inter-city relay and the like is employed for
transmitting an FM IF signal to an STL receiver 70 of the
re-transmitting FM broadcast booster station 20. In the
exemplary embodiment, the FM IF signal transmitted by the
IF STL transmitter 60 is an FM 950 MHZ signal. The IF STL
transmitter 60 increases the carrier frequency from 63 MHz
to 950 MHz, in the exemplary embodiment.
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The STL receiver 70 applies an FM signal to a
synchronous FM exciter 75 of the re-transmitting station
20 and an FM signal to an IF repeater STL transmitter or
one-way repeater 80 of the re-transmitting station 20.
The signal applied to the synchronous FM exciter 75, in
the exemplary embodiment, is 10.7 MHz FM signal with a 19
: KHz reference signal in the composite baseband signals
which is also the IF siqnal of the STL receiver 70.
The signal applied to the IF repeater STL transmitter 80,
in the preferred embodiment, is 63 M~z FM signal with a 19
KHz reference signal in the composite baseband signals.
The synchronous FM exciter 75, which will be described in
detail hereinafter, is similar in construction and
operation to the synchronous FM exciter 40 of the
: : 15 originating transmission station 15. In the exemplary
embodiment, the output of the synchronous FM exciter 75 is
an FM signal having a frequency in the range of 88 MHz-108
MHz.
Connected to the output of the synchronous FM exciter
75 is a suitable booster 85 of the re-transmission station
: 20, which, in a conventional manner, amplifies the power
of the FM signal applied to a re-transmitting or repeater
transmitter 90 of the re-transmitting station 20. Thus,
; the output of the FM signal from the synchronous FM
exciter 75 has the power level thereof amplified by the
booster 85 to compensate for losses that may occur from
the transmission of signals from the broadcast station 15
` ~ to the broadcast station 20 and increase the signal
strength of the originating transmitter 15 without causing
transmission interference with other booster stations of
the network 10. The output signal of the re-transmitter
90, in the preferred embodiment, is in the range of
88 MHz-108 MHz.
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A conventional link 95, such as a microwave link, -
VHF or UHF link, inter-city relay and the like is employed
for transmitting an FM IF signal to an STL receiver 100 of
the re-transmitting FM broadcast booster station 25. In
the exemplary embodiment, the FM IF signal transmitted by
the-IF repeater STL transmitter 80 is an FM 950 MHz ;~
signal. The IF repeater STL transmitter increases the
carrier frequency from 63 MHz to 960 ~Hz in the exemplary
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embodiment.
The STL receiver 100 applies an FM signal to a
synchronous FM exciter 105 of the re-transmitting station
25 and an FM signal to an IF repeater STL transmitter or
one-way repeater 110 of a succeeding station. The signal
applied to the synchronous FM exciter 105, in the
exemplary embodiment, is a 10.7 MHz FM signal with a 19
~ KHz reference signal in the composite baseband
I fre~uencies. The signal applied to the IF repeater STL
transmitter 110, in the preferred embodiment, is a 63 MHz
~ FM signal with a 19 KHz reference signal in the composite
l 20 baseband signals which is also the IF signal of the STL
receiver 100. The synchronous FM exciter 105, which will
be described in detail hereinafter, is similar in
I construction and operation to the synchronous FM exciter -~
~ 75 of the re-transmission station 20. In the exemplary
; 25 embodiment, the output of the synchronous FM exciter 105
~l is an FM signal having a frequency in the range of
88 MHz-108 MHz.
~i~ i Connected to the output of the synchronous FM exciter
3 105 is a suitable booster 115 of the re-transmission ~-
station 20, which, in a conventional manner, amplifies the
power of the FM signal applied to a re-transmitting or
repeater transmitter 120 of the re-transmitting station
25. Thus, the output of the FM signal from the
synchronous FM exciter 105 has the power level thereof
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amplified by the booster 115 to compensate for losses that
may occur from the transmission of signals from the
originating broadcast station 15 to the re-broadcast
station 25 and, perhaps, increase the signal strength of
the originating transmitter 15 without causing
transmission interference with other booster stations of
the network 10. The output signal of the re-transmitter
120, in the preferred embodiment, is in the range of 88
MHz-108 MHz.
In the FM broadcast network 10, the studio STL
transmitter 50 is in communication with the originating
broadcast station 15, but is isolated from the FM
re-broadcast stations 20 and 25 or any succeeding
; re-broadcast station of the network 10. The FM signal is
~; 15 transmitted successively from one transmitter to the
;~ succeeding transmitter in a serial fashion starting with
the originating transmitter 45 and proceeding successively
or in series in the order of proximity to the originating
transmitter 45.
Illustrated in Figure 2 is a parallel feed fre~uency
modulated radio frequency broadcast system or network 125
~with boosters, which is a modification of the FM radio
frequency broadcast system or network 10 shown in
Figure 1. Elements of the FM radio frequency system 125
similar in construction and operation to the elements of
the FM radio frequency system or network 10 are designated
with the same reference numeral but with a prime suffix.
The FM radio frequency broadcast system 125 differs
from the FM radio frequency broadcast system 10 in that
~; 30 the IF repeater STL transmitter 60 has been omitted. As a
consequence thereof, the STL transmitter 50' has the -~
output thereof transmit the baseband composite signals
directly to the STL receiver 70' of the re-transmitting
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broadcast station 20'. Thus, the originating transmitter .
45' is boosted by the re-transmitting stations 20', 25'
and any succeeding re-transmission station. In this -~
arrangement, the STL transmitter 50' of the studio 30' of
the originating broadcast station 10' applies an FM signal
: to the STL receiver 35' of the originating broadcast
: station 10' and to the succeeding STL receiver 70' of the
re-transmitting broadcast station 20'. Thus, the STL
transmitter 50' applies an FM signal to the originating
transmitter station 15' and the succeeding re-transmitter
and repeater station 20'. The re-transmitting station 25'
operates in the manner heretofore described in connection
with the re-transmitting station 25 in Figure 1 and its
reception in relation to re-broadcast station 20' is
similar to that heretofore described in relation to
re-broadcast station 25 and re-broadcast station 20 of :
Figure 1.
Illustrated in Figure 3 is a parallel and serial feed
FM radio frequency broadcast system or network 130 with
boosters, which is a further modification of the FM radio
l frequency broadcast system 10 shown in Figure 1. Elements
¦ of the FM radio frequency system 130 or network similar in
¦~ construction and operation to the elements of the radioI frequency system 10 are designated with the same reference
~! 25 numeral but with a double prime suffix.
The FM radio frequency broadcast system 130 differs
from the FM radio broadcast system 10 in that the STL
transmitter 50" of the studio 30" for the originating
broadcast station 15" has the output thereof additionally
transmit the baseband composite signals to the STL
receiver 100" of the re-transmitting station 25" through a
suitable link 135. Thus, the input of the IF repeating
STL receiver 100" of the re-transmitting station 25" does
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not receive the output of the IF repeater STL receiver 70"
of the re-transmitting station 20" but receives the output
of the STL transmitter 50" of the studio 30" of the
originating broadcast statlon 15" through the link 135.
The link 135 may be a microwave link, VHF or UHF link,
inter-city relays or the like.
The FM broadcast system 130 shown in Figure 3 is
employed when the terrain ~isolates the FM re-transmitting
system 25" from the~FM re-transmitting system 20". In
; 10 this arrangement, the STL transmitter 50" in the studio
30~i sends an FM signal to ~the originating broadcast
station 15" and to the nearest unobstructed
re-transmitting broadcast station 20" and the originating
broadcast station 15". The FM signal is then relayed to
the obstructed re-transmitting broadcast station 25" from
the STL transmitter 50" at the originating broadcast
station 15".
Illustrated in Figure 4 is an FM radio frequency
broadcast system or network 140 with boosters, which is a
still further modification of the FM radio frequency
broadcast system or network 10. Elements of the FM radio
frequency system or network 140 similar in construction
~;~ and operation to the elements of the radio frequency
system or network 10 are designated with the same --
; 25 reference numeral but with a triple prime suffix. ~-
The FM radio frequency broadcast system or network 140
differs from the FM radio frequency broadcast system or
network 10 in that the reference time base signal from the
STL receiver 35"' is applied over a separate path to an FM -
modulating exciter 40a. ~An IF repeater STL receiver-
transmitter 60a is located at a relay point between the
originating transmitter station 15"' and the booster
station 20"'. In the exemplary embodiment, the FM signal
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received by the IF repeater STL receiver-transmitter 60a
from the originating transmitters 45"' is in the range of
88 MHz-108 MHz. Similarly, the IF signal from the STL
receiver 70"' is applied to the synchronous FM exciter
75"'. The FM exciter 40a is a modification of a
conventional FM broadcast exciter and is arranged to
accept an external time base reference signal for phase
locking the broadcast frequency carrier in the range of
~ 88-108 MHz. The external time base re~erence signal is in
3 lO the frequency range of 2.5 KHz to 19 KHz and is generated
from a conventional stable crystal oscillator, not shown,
~ having a long term stability of a ppm per year. The time
;j base reference signal for locking the FM broadcast exciter
.~ 40a is generated in the STL receiver 35"'.
~ 15 Illustrated in Figure 5 is the synchronous FM exciter
.1 75 shown in block diagram in Figure 1. The synchronous FM
exciters 40 and 105 are similar in construction and
~ operation to the synchronous FM exciter 75 and, hence,
:~' only the synchronous FM exciter 75 will be described in
.j 20 detail. It follows that the synchronous FM exciters 40',
75', 105', 40", 75", 105", and 75"' are similar in
construction and operation to the synchronous FM exciter
75.
The synchronous FM exciter 75 has applied thereto from
the STL receiver 70 an FM signal, which, in the exemplary
embodiment, is a 10.7 MHz FM signal modulated by the
composite signals for the FM broadcasting including a
reference signal of 19 KHz, in the exemplary embodiment.
The 10.7 MHz FM signal is applied to a conventional FM
discriminator 141, which demodulates the 10.7 MHz signal
into the reference signal of 19 KHz. The 19 KHz reference
signal is applied to one input of a conventional phase
detector of a phase locked loop circuit 143.
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A suitable reference signal is generated by a
conventional voltage controlled crystal oscillator (VCXO)
~ 145 of the phase locked loop circuit 143. One output of: the VCXO 145 is applied to a conventional frequency
divider network 146 of the phase locked loop circuit 143.
The output of the frequency divider network 146 which is,
~ in the exemplary embodiment, a 19 KHz reference signal, is
:~ applied to the other input of the phase detector-142.
: The phase detector 142 produces in the output thereof
an error .signal whose amplitude is proportional to the
difference in phase between the input signals applied to
the phase detector 142. The phase error signal is applied
to a phase error amplifier 150 of the phase locked loop
circuit 143. The phase error signal produced in the
output of phase error amplifier 150 is applied to the VCXO ~ .
145 to compensate for the phase error or difference :-~
applied to the inputs of the phase detector 142.
When the phase difference of the reference signal
applied to the inputs of the phase detector 142 approaches
~: 20 or approximates zero, the VCXo 145 is phase locked to the
~: 19 KHz reference signal contained in the composite signal
~: in the output of the FM discriminator 141. The stable ~
reference signal in the output of the VCXo 145 is now ~ :
phase locked with the reference signal of the composite :~;
:~ .
: 25 signal in the output of the FM discriminator 141 and the
IF frequency modulated signal sent by the originating
broadcast station 15.
The output of the VCXO 145 is applied to a ~
conventional frequency divider network 154 of a phase . ;
locked loop circuit 155. The reference signal in the
output of the frequency divider network 154, in the
exemplary embodiment, is 10 KHz. The 10 KHz reference
signal of the divider network 154 is applied to one input
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of a conventional phase detector 165 of a phase locked
loop circuit 155.
A conventional voltage controlled oscillator (VCO) 156
of the phase locked loop circuit 155 applies a frequency
~; 5 to one input of a conventional mixer circuit 160. In the
exemplary embodiment, the output frequency of the VCO 156,
in the range of 77.3 MHz-97.3 MHz, is the difference
between the input frequency from the STL receiver 70 and
the broadcast frequency of the re-transmitting transmitter
90. Applied to another input of the mixer circuit 160 is
the intermediate frequency FM signal from the STL receiver
70. In the exemplary embodiment, the intermediate
frequency FM signal of the STL receiver 70 is a 10.7 MHz
FM signal. The mixer circuit 160 combines by adding the
output signal from the VCO 156 and the output signal from
the STL receiver 70 and produces in the output thereof the
FM broadcast fre~uency signal in the range of 88 MHz-108
MHZ. :~
Connected to the output of the mixer circuit 160 is a
suitable band-pass filter 161, which passes FM signals in
the frequency range of 88 MHZ-108 MHZ. The output of the
band-pass filter 161 is applied to a conventional
frequency divider 162 of the phase locked loop circuit
;~ 155.
i~ ~25 The frequency divider 162 produces in its output a
10 KHZ signal, which is applied to one input oE a phase
detector 165 of the phase locked loop circuit 155. The
~,.? ' ' ' output of the frequency divider 154, which, in the
exemplary embodiment, is 10 KHZ, is applied to the other
input of the phase detector 165. The 10 KHz output signal
from the fre~uency divider 154 is a reference signal. The
10 KHz output signal from the fre~uency divider 162 is a
comparison signal.
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The phase detector 165 produces in the output thereof
an error signal whose amplitude is proportional to the
difference in phase between the input signals applied to
the phase detector 165. The phase error signal is applied
to a phase error amplifier 163 of the phase locked loop
circuit 155. The phase error signal produced in the
output of the phase error amplifier 163 is applied to the
~- VCO 156 to compensate for the phase error or differences ~ -
~; of the signals applied to the inputs of the phase detector
165.
When the phase differences of the reference and
comparison signals~applied to the inputs of the phase
detector 165 approximate or approach zero, the sum of the -
frequency of the VCO 156 and the incoming IF signal, in
the exemplary embodiment, of 10.7 MHz, is phase locked to
the 10 KHz reference signal in the output of the frequency
~-~ divider 162 and is derived from the broadcast frequency by ~ ~
programming the programmable divider 162 in a well-known ~ -
~ manner. The stable reference signal in the output of the -
:~ 20 VCXo 145 is now phas~e locked with the reference signal
sampled from the output of the band-pass filter 161, which
passes the range of broadcast frequencies. The FM
broadcast signals in the output of the band-pass filter
161 have been stabilized in frequency and phase as well as
modulation level with the IF frequency modulated signals ~-
sent by the originating broadcast station 15.
The phase locked loop circuits 143 and 155 serve to -
maintain modulation~and frequency integrity for the
re-transmission station 20. The stations 15, 20 and 25 -
are synchronized to eliminate interference. Heterodyne
and noise to the unsynchronized transmitters, phase shifts
on the baseband, group delays and inconsistent modulation
levels have been obviated to enhance broadcast program
quality.
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The FM broadcast output signals of the band-pass
filter 161 are applied to a suitable power amplifier 170,
which amplifies the power level of the FM broadcast output
signals of the band-pass filter 161. Connected to the
5 output of the power amplifier 170 is a suitable band-pass
filter 171, which passes FM broadcast signals in the range
of 88 MHz to 108 MHz. A suitable power amplifer 172 is
connected to the band-pass filter 171 and its FM broadcast
output signal is applied to the booster 85. The FM
lO broadcast output signal of- the booster 85 is applied to
the re-transmitter 90 for broadcasting the FM signal that
has originated from the broadcast studio 30.
A conventional automatic level control ~ALC) circuit
175 is coupled to the output of the power amplifier 172
15 and applies a compensating control voltage to the power
amplifier 172 to automatically maintain power output level
applied to the booster 85. Included in the ALC circuit
175 is an adjustable variable resistor 176 for setting a
reference voltage applied to the level control circuiting
20 of power amplifier 172 in a well-known manner.
While reference herein may be made to standard FM
broadcast frequency signals, it appears that the invention
disclosed herein is also applicable to any FM radio
frequency signals, including VHF, UHF and microwave
25 signals, FM inter-city relay link signals, and any FM
broadcast signals. It is also apparent that the
synchronous FM exciters for the originating broadcast
stations 15, 15', and 15" need not employ all the
operational features of the synchronous FM exciter 75,
30 which were described in detail.
Illustrated in Figure 6 is a synchronous FM exciter
180, which is a modification oi the synchronous FM
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exciter 75 shown in Figure 5. Components of the
synchronous FM exciter 180 similar in construction and
l~ operation to the components of the synchronous FM exciter
!: 75 have been designated with the same reference numerals
f 5 but with a suffix "a".
The synchronous FM exciter 180 differs from the
synchronous FM exciter 75 to the extent that a phase -
locked loop circuit 185 is interposed between the STL
receiver 70 with the mixer 160a. The phase locked loop
circuit 185 incIudes a phase detector 186. The STL
receiver 70 applies the FM intermediate frequency
modulated signal having a modulation level of _ fl -
in the output thereof to a suitable frequency divider
circuit 187 of the phase locked loop circuit 185. The ~,
I5 output of the frequency divider circuit 187 having a
dividing ratio of N4 is applied to one input of the
phase detector 186. In the exemplary embodiment, the
~ output of the frequency divider circuit 187 is an FM 5.35
;~ MHz signal having a modulation level of 2 _ fl, since
N4 is chosen to be 2.
Also included in the phase locked loop circuit 185 is
;~ a suitable voltage controlled oscillator 188, which
applies a stable comparison signal to another input of the
phase detector 186 through a suitable frequency divider
circuit 189 having a dividing ratio of N5. In the
exemplary embodiment, the output of VCO 188 is 16.05 MHz
and the stable comparison signal applied to the phase
` ! detector 186 is a 5.35 MHz having a modulation level of
3/2 fl, since N5 is chosen to be 3. The phase
detector 186 compares the phase of the signals applied to
the inputs thereof. If there is a phase difference
therebetween, the error or difference therebetween
produces a voltage amplitude in the output of the phase
2~19~1
.
-21-
detector 186 commensurate with such error or difference.
The phase difference signal produced in the output of the
phase detector 186 is applied to an error correction
amplifier 190. The output of the error correction
amplifier 190 is applied to the voltage controlled
oscillator 188 to correct the phase error between the
signals applied to the::inputs of the phase detector 186.
When the phase error or phase difference between the
signals to the input of the phase detector 186 approaches
or is approximately zero, the frequency and the modulation
characteristics of the VCO 188 may be calculated in the
~: following manner: -
: '
(1) VCO frequency = IF frequency
N5 N4
or
: : VCo frequency = N5 IF frequency
: 20 (2) VCo modulatLon = N5 IF modulation
N4
In this mannerj the moduIation level of an incoming FM
carrier can be modified to a level of modulation
acceptable to:the succeeding STL receiver. The level of
:~
modulation of an FM signal applied to a transmitter for
re-transmission can be synchronized and stabilized with
~ thellevel o~ modulation of an FM radio frequency signal
¦~: transmitted at an originating transmitter, when the FM
intermediate frequency signal is modified to a level of
modulation acceptable to the succeeding STL receiver.
~:~
~ .
