Rev. 1.0
FMX30/FMX100/FMX250 User’s Manual
4—12
the synthesizer to the selected frequency to produce an intermediate frequency of
10.7 MHz.
The 10.7 MHz IF frequency comes out of the tuner module and is coupled into the
first filter FL1; passes through FL1 and into the IF decoder system. 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 an integrated circuit buffer gain stage. From there the
signal passes through a second filter for further removal of unwanted products, and
then routed to the IF stage of that integrated circuit. The quadrature detector
detects the FM signal and converts it to a low distortion audio signal. The audio, still
a composite at this point, goes to the first buffer amplifier. Then it goes through a
compensation network and on to the stereo decoder chip.
When a stereo signal is present, Led 1 illuminates which indicates that left and right
audio is available. The stereo signals go to gain amplifiers and then to the RCA jacks
on the back of the transmitter. 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, stereo decoder, and gain stages have no effect on the
signal that goes through to the transmitter. This section along with the composite
signal is totally separate from the transmitter section.
A muting circuit switches the output audio off when the incoming RF signal is too
weak to produce a useful audio signal. The strength of the signal muted is
determined by the adjustment of control R15. The audio signal is then routed to
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 of a
buffer amplifier. The output of the buffer is adjusted by control R20 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
then goes through an adjustable compensation network. 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 receiver is powered from both on-board 9 volt and 5 volt supplies and the +12
and -12 volts from the motherboard.