Specifications
4–16
FM500 User's Manual
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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.