Honeywell Bendix/King KX 165A Maintenance Manual page 19

BENDIX/KING KX 165A
4.2.3.3 NAV (VOR/LOC) Receiver
Refer to figure 4-2 for the Nav receiver description. In order to satisfy the requirements for immu-
nity from interference caused by FM Broadcast stations, the front end of the Nav receiver employs
a high-dynamic-range RF amplifier embedded in a 5-pole-band-switched preselector. The prese-
lector also includes pin diodes to provide RF AGC action. A high-level-doubly-balanced mixer
converts the incoming signal to a 20.5 MHz first IF frequency. Eight poles of crystal filtering follow
the mixer. The filtered 20.5 MHz signal is amplified by a first IF amplifier transistor then is applied
to the input of an AM Receiver IC. This IC amplifies the first IF signal, mixes it down to 450 kHz,
passes the signal through an external ceramic filter and JFET amplifier, then further amplifies the
signal at 450 kHz and finally detects the signal. 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. The low-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 both a MMIC and a
discrete bipolar amplifier. The synthesizer reference frequency of 20.95 MHz is taken from the
Com section of the RECEIVER BOARD. This reference frequency also serves as the second LO
injection signal. The detected signal is applied to a buffer-amplifier without any audio filtering to
provide the VOR/LOC composite signal to the Main Board. On the Main Board the Nav composite
signal then splits into three paths: 1) 30 Hz reference, 2) anti-alias filters, and 3) Nav audio.
For the 30 Hz reference, the Nav composite passes through a 9960 Hz bandpass filter and a lim-
iter stage before going to the phase locked loop (PLL) FM demodulator. The PLL extracts the 30
Hz reference modulation from the composite signal so it can be passed to the microprocessor.
The anti-alias filters are low pass filters that filter the composite signal before it is passed to the
microprocessor for signal processing used in the converter function.
The Nav composite signal also passes through a low pass filter to strip off the 9960 Hz sub-carrier
leaving the 30 Hz "variable" signal. The resultant audio signal passes through a bandpass filter
that establishes the frequency response of the signal. The signal passes through a front panel
selectable ident filter before being routed to amplifier stages, volume control, and finally the audio
amplifier.
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