The audio signal from MIC is amplified by IC405 before being pre-emphasized and
encoded by IC401. The output audio is added into signalling and then is fed through VCO
for modulation.
RF Driver and Final Power Amplification Circuit
The TX-RF signal outputted from Q703 in VCO circuit is amplified by Q101, driver Q110
and Q109, The amplified signal is then fed to IC101 (final PA) and passes through LPF
before reaching the antenna terminal.
3.3 APC
The APC circuit stablizes the output power at a predetermined level. D101 and D102
transform the signal from detector to DC voltage, which is then compared with the
reference voltage from CPU in IC104 and outputted as DC control voltage. The DC control
voltage controls the output power by controlling the grid of IC101.
4. PLL Circuit
The transmit signal and the receive first L.O. signal are generated by PLL circuit. PLL
circuit consists of TX frequency oscillator (Q701), RX frequency oscillator (Q702), buffer
amplifier (Q703), RF amplifier (Q102), PLL IC (IC801), LPF and TX/RX VCO control
switch (Q704, Q706).
In transmit mode, IC120 transmits the frequency data to PLL IC. Q704 is turned on to
activate TX VCO. The outputted signal is amplified by Q703, Q102, and then divided by
PLL IC into 2.5KHz, 5KHz or 6.25KHz signal. The divided signal is compared with 2.5KHz,
5KHz or 6.25KHz reference signal from 16.8MHz crystal oscillator (2.5 PPM frequency
stability) in the phase comparator. The frequency control voltage outputted from the phase
comparator is sent to TX VCO after passing through LPF (Q802, Q803). In the meantime,
modulation signal (TX) is passed to TX VCO for frequency modulation.
The working principle in receive mode is similar to that in transmit mode.
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