S1 Service manual
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to further remove spurious signals.
3）IF amplifier
The first IF signal is amplified by Q23, and then enters IC7(modulation processor IC). The
signal is heterodyned again with a second local oscillator signal within IC7 to create a 450MHz
second IF signal. The second IF signal is then fed through a 450MHz ceramic filter (CF2) to
further eliminate unwanted signals before it is amplified and FM detected in IC7.
4）AF Amplifier
The recovered AF signal obtained from IC7 is amplified by IC5, filtered by the high-pass filter
IC5 to eliminate signals under 300Hz. The processed AF signal passes through an AF volume
control and is amplified to a sufficient level to drive a loud speaker by an AF power amplifier
(IC6).
5）Squelch
Part of the AF signal from IC7 enters IC7 again, and the noise component is amplified and
rectified by a filter and an amplifier to produce a DC voltage by Q20 corresponding to the noise
level.
The DC signal from the FM IC goes to the analog port of the microprocessor(IC5).IC51
determines whether to output sounds from the speaker by checking whether the input voltage is
higher or lower than the preset value.
To output sounds from the speaker, IC5 sends a high signal to Q4 ( the AFCC lines) to turn on
Q13.
6）Receive signaling
300Hz and higher audio frequencies of the output signal from IF IC are cut by filter (IC5).
The resulting signal enters the microprocessor (IC5). IC5 determines whether the QT or DQT
matches the preset value, and controls the CPU and 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.
