Abell S1, Service Manual

Page: 11 / 43

S1 Service manual
11
1
）
PLL
The frequency step of the PLL circuit is 2.5 or 3.125 or 5 or 6.25KHz. A 13MHz reference
oscillator signal is divided at IC4 by a fixed counter to produce the 2.5 or 3.125 or 5 or 6.25 KHz
reference frequency. The voltage controlled oscillator (VCO) output signal is buffer amplified by
Q14, then divided in IC14 by a dual-module programmable counter. The divided signal is
compared in phase with the 2.5 or 3.125 or 5 or 6.25 kHz reference signal in the phase
comparator in IC4. The output signal from the phase comparator is filtered through a low-pass
filter and passed to the VCO to control the oscillator frequency.
2
）
VCO
The operating frequency is generated by the oscillator which centre component is Q8.
The oscillator frequency is controlled by applying the VCO control voltage, to the varactor
diodes (VD1 and VD3). The RX power is set low in receive mode.
3
）
UNLOCK DETECTOR
If a pulse signal appears at the CP1 pin of IC4, an unlocked condition occurs, the voltage
applied to the LOCK pin of the microprocessor to go low. When the microprocessor detects this
condition, the transmitter and receiver are 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 IC5, passes through a
pre-emphasis circuit, and amplified by the other IC400 (1/2) to perform IDC operation. The
signal then passes through a low-pass filter (splatter filter) (Q401 and Q400) and cuts 3 kHz
and higher frequencies. The resulting signal goes to the VCO through the VCO modulation
terminal for direct FM modulation
The audio signal from microphone enters into IC5 from 97 pin is amplified and disposed of
(voice comprador and scramble are optional). Then the signal from 99 pin enters into VCO
through R32, R74, C55, R15M R22 and C142 been modulated.
MIC
M2
IC5
pre-EMPHASIS
BPF
COMPAND
SCRAMBLER
C142