Honeywell Bendix/King KX 165A Maintenance Manual page 43

BENDIX/KING
4.3.4.12 Detector
Q4 is the active device in the 30 kHz IF detector. Q3 supplies a temperature compensated bias
for Q4. C34 and R86 form an RC filter that filters off residual 30 kHz IF but does not filter any of
the 90/150 Hz composite audio.
4.3.4.13 Composite Audio Buffer Amplifier
Op-Amp U3-A is used to amplify and DC level shift the 90/150 Hz audio before it goes to the main
board. C106 and R36 introduce a low pass response to filter out some of the higher frequency
noise.
4.3.4.14 IF AGC Amplifier
Op-Amp U3-B is configured as an integrator and is the IF AGC Amplifier. The DC level from the
Detector is used by the AGC loop to maintain a constant carrier input power into the Detector. The
output of this Amplifier is connected to the IF VGA, to adjust the IF gain, and is the input to the RF
AGC Amplifier. The IF AGC voltage varies from 3.6 V with no RF input, to 6 V with -33 dBm RF
input.
4.3.4.15 RF AGC Amplifier
Op-Amp U3-D is the RF AGC Amplifier with a gain of 16 and an adjustable DC offset. While the
RF AGC is in its linear region, it varies 16 times faster than the IF AGC. That is why the RF AGC
predominates while it is active. R25 is used to set the point at which the RF AGC becomes active.
This point is factory set at -87 dBm. The RF AGC voltage varies from 7.5 V with no RF input, to
0.1 V with -33 dBm RF input.
4.3.4.16 20.95 MHz Crystal Oscillator
Q8 is the active device in this common collector Synthesizer Reference Crystal Oscillator. Y1 is
an AT cut crystal that is the Oscillator's frequency determining device. C66 is a variable NPO ca-
pacitor that adjusts the operating frequency of the Crystal Oscillator. Q9 is used in a buffer am-
plifier that increases the level of the crystal oscillator to the level needed by the Synthesizer IC U6.
The buffer amplifier also provides isolation to the Crystal Oscillator.
4.3.4.17 Synthesizer
Serial Data from the Main Board is sent to program the Synthesizer IC, U6. The synthesizer com-
pares a divided down 20.95 MHz reference with a divided down sample of the VCO output. U6
divides the reference and VCO down to 10 kHz for phase/frequency comparison. U6 is a dual
modulus synthesizer, so that the VCO can be tuned in 10 kHz steps throughout the GS band. Op-
eration is limited to the GS channels less the 30 kHz IF offset (low side injection). The range of
operation is therefore 329.12 MHz to 339.97 MHz. U6 pin 5 is the synthesizer charge pump output
and is used as the input to the loop filter/integrator U5-A.
4.3.4.18 LO Frequency Dither
A sample of the 10 kHz comparison frequency is taken from U6-14 and divided by 16 in U8. the
resulting 625 Hz square wave is used to dither (frequency modulate) the VCO. The 625 Hz square
wave is present on U8 pin 11 and is changed into a ramp waveform by the RC combination of R89
and C83. It is this ramp that varies the VCO/LO frequency at a rate of 625 Hz and a deviation of
1 kHz. Dithering eliminates the possibility of zero beat problems in the IF. This would be the case
of a receiver or ground station failure causing a receive or transmit frequency error that allows the
LO and RF to be only 90 Hz or 150 Hz offset.
4.3.4.19 332 MHz VCO
Q10 is the active device in this common base VCO (Voltage Controlled Oscillator). C75 is a me-
chanical variable capacitor that is adjusted so that the electronic tuning range is centered in the
Page 4-40 15610M00.RCD
KX 165A
Rev 0, Jun/2000