Com Receiver - Honeywell Bendix/King KX 165A Maintenance Manual

BENDIX/KING KX 165A
4.2.1

COM RECEIVER

Refer to figure 4-2. The front end of the Com receiver employs a dual-gate FET RF amplifier em-
bedded in a 4-pole tracking preselector. This RF amplifier FET also provides RF AGC action. A
doubly-balanced mixer converts the RF to a first IF at 21.4 MHz and is followed by a bipolar IF
amplifier. In radios which use 25 kHz bandwidths exclusively, the IF amplifier is followed by six
poles of crystal filtering. The IF amplifier is followed by an IF filter board in radios which are both
25 kHz and 8.33 kHz channel capable. This IF filter board switches the IF between a six pole 25
kHz (wide) crystal filter and an 8.33 kHz (narrow) eight pole filter. Selecting wide or narrow chan-
nel tuning determines which filter is used. The filtered 21.4 MHz output is amplified by another IF
amplifier transistor and passed to the inputs of an AM Receiver IC.
This IC amplifies the first IF signal, mixes it down to 450 kHz, passes the signal through an exter-
nal ceramic filter before it is further amplified in the receiver IC and detected. The detected signal
is used to develop IF AGC voltage and the detected signal is also output from the IC to drive audio
filters and amplifiers. RF AGC voltage is derived from the IF AGC voltage. A high-side-injection
first LO signal is derived from a synthesizer circuit which employs a dual-modulus Synthesizer IC,
an active integrator, a loop filter and a VCO. Amplification of the VCO output is accomplished with
a MMIC. In transmit the synthesizer locks up on the desired transmit frequency and a discrete
bipolar amplifier is used to amplify the MMIC output to provide a drive signal to the transmitter.
The 20.95MHz synthesizer reference frequency is generated by a crystal oscillator on 25kHz
units, and by a TCXO on radios which are 25/8.33kHz capable. This reference frequency also
serves as the second LO injection signal. The detected audio signal from the AM IC is lowpass
filtered and amplified. A compressor circuit provides a relatively constant audio output level re-
gardless of the percent of modulation of the incoming signal. Squelch action is controlled by both
an audio noise detector squelch and a carrier squelch signal derived off the RF AGC. Squelching
is accomplished in the audio path on the RECEIVER BOARD as well as by another squelch gate
on the Main Board. On the Main Board, the audio signal passes through the Com volume control
to the audio amplifier.
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