Garmin GNC 250 Maintenance And Repair Manual page 11

Figure 5. Communication Transceiver Block Diagram
The VHF transmitter is a 3 stage RF power amplifier capable of delivering 5 watts minimum of CW
power to the antenna connector. The input to the transmitter is supplied by the receiver
synthesizer and is amplified from a level of 100 mW to the final output of 5 watts. The receiver board
AM modulates the transmitter using microphone input via the J2 connector to collector modulate the
final two stages of the transmitter through power transistor Q34.
The microphone circuits contain a compressor which limits the AM modulation depth over a 20 dB
microphone input range. In addition to the communications functions, the receiver board contains
the Gillham/Greycode altitude decoder circuitry. Encoded pressure and altitude data from a parallel
altimeter device is received as a 10 bit data word at the 26 pin J2 connector. The data is decoded and
output to the CPU via connector J13.
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