EFJohnson 764X - Manual (Page 51)

UHF (7640) CIRCUIT DESCRIPTION
5B-8
November 1998
Part No. 001-7600-001
Figure 5-4 Second IF System IC1 (UHF)
Figure 5-5 Audio Switching and Amplifier Circuit (UHF)
and applied to the limiter amplifier and quadrature
detector. The quadrature detector demodulates the
audio signal contained in the second IF signal. Com-
ponents connected to pins 10 and 11 including X1
are part of the quadrature detector. The audio signal is
fed out on pin 9 (IC1) and applied to the AF amplifier
circuit.
5.5.5 AUDIO AMPLIFIER CIRCUIT
The AF (audio frequency) amplifier circuit pro-
vides drive to the speaker (see block diagram in Figure
5-5). The audio signal from IC1, pin 9 is applied to AF
amplifier IC7b and then fed to high-pass filter IC6c/
IC6d. The cut-off frequency of this filter is controlled
by the “AFHPF” line. When “AFHPF” is high, the
cut-off frequency is shifted higher to attenuate the Call
Guard (CTCSS or DTCS) signals.
The filtered output signal on IC6d, pin 14 is
applied to pin 8 of AF switching circuit IC11. The
switched output signal on IC11, pin 9 is then applied
to pin 16 of level controller IC12. This device allows
the CPU to control the speaker volume level. The vol-
ume controlled output on pin 15 is then applied to pin
2 of AF switching circuit IC11. The switched output
on pin 1 is applied to pin 4 of de-emphasis circuit
IC4a. The signal is then amplified by IC9 to provide
drive to the speaker.
5.5.6 RECEIVER MUTE CIRCUIT
Noise Squelch
The noise squelch circuit turns off the audio sig-
nal when no RF signal is being received. The squelch
circuit is controlled by the amount of noise present in
the audio signal.