Honeywell Bendix/King KX 165A Maintenance Manual page 34

BENDIX/KING
A doubly-balanced mixer, U3028, converts the incoming signal to the 21.4 MHz first IF frequency.
The first local oscillator signal is applied to pin 6 of the mixer. The First IF amplifier Q3011 ampli-
fies the 21.4 MHz signal and feeds it to either the first IF filter FL3001 or the IF Filter board. On
radios which are 25/8.33kHz capable the IF Filter board is installed. It switches between an eight
pole narrow bandwidth crystal filter or a six pole wide bandwidth crystal filter depending on the
radio being tuned to a wide (25kHz) or narrow (8.33kHz) channel. Radios which use 25kHz band-
widths exclusively use FL3001 and the IF Filter board is not installed. The filtered 21.4MHz IF
signal is amplified by IF amp Q3017 before being passed to pin 18 of U3015, an AM receiver IC.
4.3.1.9 AM Receiver IC and Second IF
U3015, includes an additional gain controlled 21.4 MHz amplifier, a second mixer, amplification
at 450 kHz, a detector and AGC circuitry.
plified then mixed down to 450 kHz, the second IF frequency. The 20.95 MHz reference oscillator
is applied to U3015 pin 16 to serve as the second LO.
The 450 kHz second IF signal output at pin 1 of U3015 is impedance matched by L3015 and
C3066 to FL3006, a 450 kHz ceramic filter. The filtered signal is then returned to U3015 at pin 3,
amplified within the IC, then passed to an internal detector. The gain of both the 21.4 MHz and
450 kHz amplifiers internal to U3015 is controlled by the AGC voltage on pin 7. AGC time con-
stants are established by C3120 and C3134. The amplified 450 kHz signal is available at pin 12
of U3015 (TP3005) to allow easy viewing of the signal with an oscilloscope.
4.3.1.10 Reference Oscillator
In 25kHz radios a 20.95 MHz reference frequency oscillator consisting of Y3002 and Q3022 gen-
erates a stable reference frequency to be utilized by the synthesizers. Varactor CR3013 circuitry
provides a variable capacitance to fine tune the frequency of Y3002. Units that are both 25kHz
and 8.33kHz channel capable have Y3001 installed. This is temperature compensated oscillator
used so that the radio remains in specification despite the narrower bandwidths of 8.33kHz chan-
nels. This reference frequency is buffered by Q3021 and fed to U3015 to serve as the second
local oscillator for U3015. U3015 also outputs the 20.95 MHz signal on pin 14. Q3023 amplifies
and buffers this 20.95 MHz reference signal for distribution to other receiver circuits.
4.3.1.11 Com Audio and Receiver Compressor
Detected Com audio from U3015 pin 6 is sent to buffer transistor Q3025 to provide an un-
squelched-constant-level output to the unit's rear connector for use by external equipment. U3015
also outputs detected audio on pin 9, which is routed through R3180 then to compressor FET,
Q3024. The purpose of the receive audio compressor is to hold the audio level the pilot hears at
a nearly constant level regardless of the percent of modulation of the received signal. CR3015 is
a clipper that limits the level of noise and transients.
If squelch gate Q3016 is turned on, the signal is fed through to the Audio LPF which amplifies the
signals below 2.7 kHz and attenuates higher frequencies. U3008-B and U3008-A stages both
have a gain of about 3. Audio out of U3008-A is fed to the Main board via P3002 pin 1 and also
the compressor amp, U3008-C. The compressor amp inverts the signal and then feeds it to the
compressor detector consisting of CR3017 and C3151. If the incoming signal has a modulation
percentage of less than about 20%, the voltage at the gate of Q3024 is high enough to hold Q3024
off, letting the audio signal pass unattenuated to the audio LPF. On signals that are modulated at
greater than about 20%, the detected voltage across C3151 will drop enough to turn on Q3024.
Q3024 then acts as a variable resistor, attenuating the detected signal out of U3015 as necessary
to hold the audio output at a constant level. For testing purposes it may be desirable to disable
the compressor; this is accomplished by Q3005. When the input to Q3005 is driven high by the
microprocessor via U3002, the compressor is disabled.
Rev 0, Jun/2000
The 21.4 MHz signal applied to U3015 pin 18 is am-
15610M00.RCD
KX 165A
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