HYT TC-618, Руководство по обслуживанию

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TC-618/626 Service Manual
14
MCU outputs appropriate PWM waveform according to the selected channel. The waveform is then
rectified by the RC filter circuit (R441, R442, C440 and C441) to generate level of APC/TUNE control
signal. One flow of APC/TUNE control signals go to R500 to be used as control voltage of the Rx
electrically tunable circuit. In Tx mode, the signals go to R439 and R440 for voltage division, and the
APC reference voltage is derived.
The error detect voltage is derived after the Tx current goes through R420, R421 and R422. Then the
voltage is compared and amplified by the operational amplifier U430, and further compared with the APC
reference voltage. After these operations, the APC control voltage is output, and the close-loop negative
feedback control power is generated when the Tx current changes.
5. Audio and Signaling Processing Circuit
The baseband processor (AN29160) is highly integrated and allows multiple functions (e.g. VOX Level
Detect & Output, SQ Level Detect & Output, Rx/Tx Audio Process & Convert, Audio Amplify, etc.) to be
realized in it. In addition, it can be used for both transmission and reception.
1) Block diagram of Tx audio & signaling processing circuit
0
sp Amp
SF
AMP(TX)
T
o
n
e
AFMOD
Mic amp
U200
AN29160AA
PROCESSER
signalling input
AF
HPF
5th-HPF
Pre
Emphasis
mic
M IC3
AF
mute
volume
mic mute
LIM
(TX)
AUDIO OUT
CPU
TX VCO
Figure 6
a) Tx Audio Process: The audio signals from MIC are converted to electrical signals via acoustoelectric
conversion of MIC. After pre-emphasized, the audio signals are processed by a limiting amplifier (at
U200). Then the processed signals go to the low-pass filter circuit to remove frequencies over 3KHz.
Finally, the signal goes to the potentiometer VR200 and then to VCO for direct frequency
modulation.
b) Tx Signaling Process: MCU outputs, via the QTO port, signaling encoded waveform, which is
divided via the RC circuit into two flows. One flow modulates the reference frequency oscillator of
PLL directly, while the other flow modulates VCO.
Note: VR260 adjusts the amplitude proportion between the two flows, to optimize the signaling
waveform modulated on the carrier.