User Manual

SECTION 1: THEORY OF OPERATION
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After the final band-pass filter, the signal is applied to a mixer where it is mixed with the receiver injection
signal (45 MHz below the received signal) to produce a first IF of 45 MHz. The 45 MHz IF is passed by a
45 MHz crystal filter and is routed to the 45 MHz receiver section for further processing.
45 MHz Receiver Section
Functional block diagram of the 45 MHz receiver.
The 45 MHz IF from the front-end is mixed with a 44.545 MHz injection frequency to produce a 455 kHz
second IF which is passed by a 455 kHz filter. This second IF is then amplified and limited by an IF
amplifier and limiting amplifier respectively. A 455 kHz filter provides interstage filtering between the IF
amplifier and the IF limiting amplifier.
This section includes the current-to-voltage converters, which produce the RSSI signal (a DC voltage
proportional to the log of the received signal strength). The RSSI responds to signals as low as -125 dBm
and rises monotonically over a range of approximately 90 dB. RSSI is used by the DRC to select the best
receiver at any particular point in time.
The output of the limiting amplifier is applied to a tuned quadrature detector, which outputs recovered FM.