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POWER446 Service Manual
Q652 performs switching between transmission and reception under the control of T/R: in TX mode, T/R
is set to low level and Q653 is turned on, making Q100 operate; in RX mode, T/R is set to high level and
Q653 is cut off, making Q101 operate. The output from Q100 and Q101 is amplified by Q102 and then
feeds to the buffer amplifier for further processing.
When the PLL is unlocked, the LD pin of U202 outputs low level. If the microprocessor detects such
situation, TX/RX operation is prohibited, and an alarm sounds.
2. RF Power Amplifier Circuit (TX Section)
Figure 2
To obtain the desired RF power, the RF signal from VCO goes to the buffer Q104 and to the driver
amplifiers Q400 and Q401 in sequence. Then the amplified RF signal goes to Q402, which pre-amplifies
power of the input signal to drive the final-stage power amplifier Q403. The input RF signal is then further
amplified by Q403, and goes to the LC low-pass filter (LPF) through the TX/RX switching diode D401.
Finally, it is transmitted from the antenna after high-order harmonics are removed by LPF
3. RX Amplifier and Mixer Circuit (RX Section)
1) Low-noise Amplifier and Bandpass Filter Circuit
Figure 3
After the RX signals are received by the antenna, the undesired out-of-band signals are filtered out at the
electrically tunable bandpass circuit (D503, D504, L501, L502, C503, C505 and 507). Then the signals
are amplified by the low-noise amplifier (LNA) Q500 to reach a certain level for receiving. Afterwards, the
amplified signals pass through a two-stage bandpass circuit (mainly composed of D505, D507 and
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