QUANTUN QP650U Plus, Service Manual

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QP-650U Plus Circuit Description
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again in U200 to create a 450 KHz second IF signal. The second IF signal then passes through a
450KHz ceramic filter to eliminate unwanted signals before it is amplified and detected in U200.
4. Transmitter
AF and signaling
AF signal from the microphone is amplified and low-pass-filtered in U200, which also enters
CYC/CDC/DTMF etc signaling generated by CPU. The U200 processed mixing signal enters VCO for
direct FM modulation (see fig.5).
RF Power Amplifier
The transmit signal from VCO buffer amplifier
（
Q102
）
is amplified by Q104 and Q400. The
amplified signal is then amplified by the power amplifier Q402 and Q403 (including a two-stage FET
amplifier) to create 5.0W RF power (see Fig. 4).
Fig. 4 APC System
Antenna switch and LPF
Output signal from RF amplifier passes through a low-pass filter network and a transmit/receive
switch circuit comprised of D401
，
D501 and D502 before it reaches the antenna terminal. D501 and
D502 is turned on (conductive) in transmit mode and off (isolated) in receive mode.
APC
The automatic power control (APC) circuit stabilizes the transmit output power by detecting the
drain current of final stage amplifier FET. U430 (2/2) compares the preset reference voltage with the
voltage obtained from final current. APC voltage is proportional to the difference between auto detect
voltage and reference voltage output from U430
（
1/2
）
. The output voltage controls FET power.