Transmitter - Honeywell Bendix/King KX 165A Maintenance Manual

BENDIX/KING
put directing the signal three places. The output of U3012 goes back to the synthesizer IC, U3011,
to keep the loop locked. It also goes to the first mixer to serve as the first LO injection (Gate 2 of
Q3011 in a unit without Mod 5 and to U3028 in units with Mod 5). In the transmit mode, U3012
also drives the TX Driver stage Q3015. Approximately +15 dBm of output power is available at
J3005 when Q3012 is turned on in transmit providing power to Q3015. This unmodulated signal
out of the TX driver stage is fed to the Transmitter board. In receive, when Q3012 is off, there is
less than 0 dBm at J3005.
A pin diode switch consisting of CR3033, is placed between the output of U3012 and the base of
Q3015 to remove the drive power to Q3015 in the receive mode.
In a normal synthesizer locked state, the LD output of U3011 (pin 7) will be high. Low going pulses
will be seen at the LD output if the synthesizer is unlocked. These pulses are filtered by C3092
and applied to comparator U3016A. The output of U3016 and the Com Synth Lock line at P3001
pin 16 is low in a locked condition and high in an unlocked condition.
4.3.2

TRANSMITTER

Refer to figure 6-13 for the transmitter schematics. Some transmitter functions appear on the main
board schematics in figure
4.3.2.1 RF PATH
The RECEIVER BOARD lineup of Q3020, U3012 and Q3015 produces an unmodulated RF drive
level of 30 mW (+15 dBm) at J3005 on the desired frequency of operation in the COM BAND.
The drive signal is conducted by 50 ohm transmission line and fed to the 2N4427 PREDRIVER,
Q4004. This stage provides approximately 9 dB of gain, boosting the level of the signal to 250
mW (+24 dBm) for the JT2468 DRIVER, Q4005. Approximately 14 dB of gain is achieved in this
stage, to a level of 6 watts for the MRF 173 FINAL, Q4006. The final stage produces 9 dB of
gain, to a level of 50 watts peak power. There is a loss of about 1 dB in the HARMONIC FILTER
which comprises the Low Pass Filter and Directional Coupler, so the peak output power at the
COM ANT is 40 watts for full AM modulation of a 10 watt carrier.
4.3.2.2 AUDIO PATH
Audio input from the MIC input is processed by U1023-A which provides bias for carbon mic ele-
ments, and buffering to the digital pot, U1032-A, for mic gain adjustment. The circuitry of U1034-
C, U1032-B, U1034-B, and Q1042 comprise the MIC COMPRESSOR to ensure modulation is
maintained at the highest average level as possible without overmodulating the carrier on modu-
lation peaks. The shape of the audio response in the BPF is determined in large part by U1035-
C to provide a 6 dB bandwidth from 350 to 2500 Hz. The KX165A has an additional audio filter
which is needed to meet the 8.33kHz channel spectral requirements. The KX165A TX Audio
Adapter Board provides this additonal audio filtering, reference
ing components form a switched capacitor filter having an elliptic response.
4.3.2.3 MODULATOR PATH
The audio signal from U1035-C TX Audio Adapter Board provides the reference to modulate the
carrier by comparing detected RF, which is buffered by U1035-D, and provides the feedback in
U1035-A, with the error amplified and applied to the transmitter final device's gate bias.
4.3.2.4 POWER CONTROL PATH
There are two control loops that monitor the modulator gate bias on TRANSMITTER BOARD
Q1006. The main loop for power control detects output power (TRANSMITTER BOARD Direc-
tional Coupler & CR4004, U1035-D) and compares in U1035-A this sample against a reference
for carrier power set, U1032-C buffered by U1035-C. This carrier power set reference is modulat-
ed by the processed audio from the MIC COMPRESSOR / BPF circuitry on the MAIN BOARD.
Rev 0, Jun/2000
6-8.
15610M00.RCD
Figure 6-10. U1 and its surround-
KX 165A
Page 4-33