Honeywell Bendix/King KX 165A Maintenance Manual page 35

BENDIX/KING
Com receiver audio is output from the RECEIVER BOARD via J1002 pin 1. The signal routes to
U1023C on the main board which has a gain of about 0.5. The signal goes to U1029A which has
a gain of about 2.5, and then is directed to digital pots U1028A&D which control volume. After
the volume control, the audio signal is fed to U1029, an amplifier circuit that has a gain of about
0.3, and then to U1027B which has a gain of about 35. T1001 transforms the signal in order to
drive a 500 ohm load.
The com receiver audio is squelched when the gates of Q1024 and Q1029 are pulled high.
4.3.1.12 Noise Squelch
Detected Com audio from U3015 pin 6 is applied to the squelch noise bandpass filter U3013-D.
Squelch operation differs depending on whether the unit is tuned to a 25kHz or a 8.33kHz channel.
When tuned to a 25kHz channel U3010-C, U3017-C and Q3018 are off, and U3010-D is on. this
sets the BPF U3013-D to a center frequency of 8kHz and routes the output directly to the noise
detector CR3016. When no desired signal is present the amount of 8kHz noise will be high, charg-
ing C3152 up to about 2.5Vdc and exceeding the threshold on pin 9 of squelch comparator
U3013C. This allows audio to pass by turning on Q3016 through inverted transistor Q3007. When
a desired signal is present the 8kHz noise will drop, and allow Q3016 to turn on and pass audio
to the audio LPF U3008-B.
When tuned to an 8.33kHz channel Q3018 is turned on which shifts the center frequency of BPF
U3013-D to approximately 4kHz. Because of this the 8.33kHz squelch operates off of 4kHz noise.
The output of the BPF is switched to the HPF U3013-A so as to eliminate possible squelch action
on desired audio tones. The output of the HPF is routed to the noise detector and the squelch
comparator. U3017-C is used to lower the hysteresis of the squelch comparator while on an
8.33kHz channel.
4.3.1.13 RF AGC and Carrier Squelch
The AM receiver IC, U3015, outputs an IF AGC voltage on pin 7. This voltage will be about 1 VDC
with no input signal to the unit and rise to about 2 VDC with a strong input signal. U3018-D is used
to monitor the IF AGC voltage, and when it reaches about 1.6 VDC, turns Q9 on, pulling the Com
RF AGC voltage down below 8VDC. The reduced level of RF AGC voltage reduces the gain of
RF amp Q10 to prevent the following stages from being overloaded.
As the IF AGC voltage out of U3015 increases above about 1.5V, the output of the carrier-squelch
comparator U3013-B goes positive. This positive voltage is directed to the input of squelch com-
parator, U3013-C, to force the squelch to open on strong signals.
This carrier squelch (or backup squelch) is required when receiving signals that might contain a
strong 8 kHz modulation component which would prevent the noise squelch circuitry from opening
the squelch.
4.3.1.14 Com Synthesizer and TX Driver
Serial data from the microprocessor is sent to program the synthesizer IC, U3011. U3011 com-
pares VCO output on pin 8 with the 20.95 MHz reference oscillator signal on pin 1 and generates
high or low pulses out of pin 5 to the active integrator, U3016-B. The combined action of U3016-
B and the RC loop filter apply a DC tuning voltage to CR3014 in the VCO. The voltage will vary
from about 2.5 V at 118 MHz transmit to about 8.4 V at 136.9 MHz receive.
Internal to U3011 are two dividers which divide the 20.95 MHz reference frequency and the VCO
frequency down to a lower reference frequency of 25 kHz. The divided reference pulses appear
on U3011 pin 13 and the divided VCO frequency pulses appear on pin 14; these pulses are only
about 50 ns wide on pin 13 and 300 ns wide on pin 14 and will occur at a 25 kHz rate.
In the transmit mode, the VCO frequency is the same as the unit's operating frequency. In receive,
the VCO frequency is 21.4 MHz higher than the operating frequency. U3012 buffers the VCO out-
Page 4-32 15610M00.RCD
KX 165A
Rev 0, Jun/2000