Abell TH-308GB UHF, Servise Manual

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TH-308GB UHF SERVICE MANUAL
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6
）
Receive signaling
QT/DQT
300Hz and higher audio frequencies of the output signal from IF IC are cut by IC300A and
IC300D. The resulting signal enters the microprocessor (IC2). IC2 determines whether the
QT matches the preset value, and controls the AFCC and the speaker output sounds
according to the squelch results.
3.PLL frequency synthesizer
The PLL circuit generates the first local oscillator signal for reception and the RF signal for
transmission.
1 ） PLL
The frequency step of the PLL circuit is 5 or 6.25kHz. A 12.8mhz reference oscillator
signal is divided at IC3 by a fixed counter to produce the 5 or 6.25kHz reference frequency.
The voltage controlled oscillator (VCO) output signal is buffer amplified by Q47, then devided
in IC3 by a dual-module programmable counter. The devided signal is compared in phase
with the 5 or 6.25kHz reference signal in the phase comparator in IC3. The output signal
from the phase comparator is filtered through a low-pass filter and control the oscillator
frequency through VCO.
2
）
VOC
The operating frequency is generated by the oscillator with Q3. The control voltage is
applied to the varactor diodes D27 and D28 to rectify the oscillator frequency. The RX pin is
set low in transmit mode, Q30 and Q20 are turned off to adjust current frequency to transmit
frequency.
3 ） UNLOCKED DETECOR
If a pulse signal appears at the LD pin of IC3, an unlocked condition occurs,and the DC
voltage obtained form D7, R1 and C291 causes the voltage applied to the LOCK pin of the
microprocessor to go low. When the microprocessor detects this condition, the transmitter is
disabled and the error indicating alarm occurs, ignoring the push-to-talk switch input signal.
4.Transmitter
1
）
Transmit audio
The modulation signal from the microphone is amplified by IC5A, passes through a
preemphasis circuit, and amplified by the other IC5B to perform IDC operation. The signal
then passes through a low-pass filter (splatter filter)(Q13 and Q9) and cuts 3kHz and higher
frequencies. The resulting signal goes to the VCO through the VCO mudulation terminal for
direct FM modulation.
2 ） QT/DQT encoder