User's Manual

2) First Mixer

The signal from RF amplifier is mixed with the first local oscillator signal from PLL

frequency synthesizer in the first mixer Q15 to generate a 21.7 MHz first IF signal. The

first IF signal is then fed through the 21.7MHz crystal filter F2 to remove spurious signals

from adjacent channel.

3) IF Amplifier

The first IF signal is amplified at Q6 and then enter the IF process chip IC3. The signal is

mixed with the second local oscillator signal to generate a 450KHz second IF signal. The

second IF signal is then fed to a 450 KHz ceramic filter F3 to eliminate unwanted signals

before it is amplified and detected at IC3.

4) AF Amplifier

The AF signal obtained from IC3 is filtered in IC2-A, and then amplified in Q18. Then

resulting AF signal passes through an AF squelch switch IC10 and a volume control circuit

and then is amplified in an AF power amplifier IC5 to drive the speaker.

5) Squelch

Part of the AF signal from IC3 enters IC3 again and the noise component is amplified by a

filter and an amplifier, and then enters Q21 to amplify the noise further. After rectified and

filtered by D7 and C94, the DC signals goes to the analog port IC8 of the microprocessor.

IC8 determines whether to output sounds from the speaker by detecting whether the input

voltage is higher or lower than the preset value.

To output sounds from the speaker, IC8 sends a mute and an AF control signal APA-EN to

IC5. (See figure 3)

Fig. 3 AF amplifier and squelch circuit