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and into the IF decoder system of IC LM1865 (U1). The FL1 filter sets the
bandwidth or everything outside of the bandwidth depending on the filter that is
selected. It could be a bandwidth of 180 kHz where everything outside of that is
filtered out depending on the filter characteristics. A second filter (F3) is available
when the signal has a great amount of interference from an adjacent signal. In
such a case, remove the jumper cap that is in the F3 position, then remove the
ceramic filter that is in the F4 storage position and place it into the F3 position.
Then the signal goes to a buffer gain stage at pin 1 of LM1865 (U1). From there
the signal passes through F2, which is a second filter for further removal of
unwanted products, and then it goes on to the IF of that chip. The quadrature coil
L4 is tuned to 10.7 MHz as per calibration procedures. This results in a low
distortion of around 0.2 to 0.3% on the audio. The audio, still a composite at this
point, will come out of pin 15 of that IC (U1) and go to the first buffer U9. Then it
goes through a compensation network R54 and C26, and on to the stereo decoder
chip at pin 2 of U5.
When a stereo signal is present, Led 1 illuminates which indicates that left and
right audio is available. Then the stereo signals go to gain stages U6A and U6B and
out to the RCA jacks on the back of the cabinet. These can be used for off-air
monitoring of the audio signal. Incoming frequency can be monitored from the
frequency monitor BNC jack on the back. The stereo buffer U9, stereo decoder U5,
and gain stages U6A and U6B have no effect on the signal that goes through the
transmitter. This section along with the composite signal coming out of pin 15 of
LM1865 (U1) is totally separate from the transmitter section.
A muting circuit, consisting of C22, a 1N914 diode, R14, and varible resistor R15
mutes the output when a signal is too weak to be understood. The strength of the
signal muted is determined by the adjustment of R15. Any signal below the setting
of R15 is shorted to +VCC through C22 by the current drawn through R14 and the
diode. The audio signal above this setting goes through C17 to the connector P3.
The P3 connector block allows jumpering to either internal circuitry or to external
signal processing such as advertisement injection or other forms of altering the
signal. If the jumper is installed for internal circuitry, the signal will go through
R39 to the input of U2A. This is a buffer that drives the R20 pot located on the top
left hand corner of the board. R20 sets signal gain for 100% modulation if
adjusted correctly with a full incoming 75 kHz deviation signal. Then the signal
goes through R21, R22, and C20 which, along with adjustable pot R24 and C21,
forms a compensation network with some phase shifting. This allows the best
stereo separation possible by adjusting and compensating for differences in FM
exciter boards. The signal is buffered through U2B and finally reaches the output
connectors P1 and P2, and on to the transmit circuitry.
The power supply is fairly straight forward. The incoming 12 volt supply goes to a
7809, 9 volt regulator (VR1) which supplies all 9–volt needs on the board. The 9
volts also supplies a 7805, 5 volt regulator (VR2) which supplies all 5–volt needs on
the board. Plus and minus 12 volts from the motherboard is filtered and supplies
various needs on the board. Finally there is a precision reference voltage supplied
through R50 by U7 and U8. These two 2.5 volt reference shunts act very much like
a very accurate zenor diode to provide precision 5 volts to the metering board.
4–16
FM30/FM100/FM250 User's Manual
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