User's Manual

4.2.3 HIGH IF (48.5 MHz)

The IF passes through Crystal Filter XF1, IF Amplifier Q3, IF Transformer T3, Crystal Filter XF2, Amplifier

Q4 and on to the input of IC1 -20. IC1 uses crystal X1 to generate 45.045 MHz to convert the 1st IF of

48.5 MHz to the 2nd IF of 455 kHz.

4.2.4 LOW IF (455 kHz)

The 455 kHz IF is filtered by Ceramic Filters CF1 & CF2. IC1 also limits, detects the audio and provides a

squelch (BUSY) signal, an RSSI signal (Receive Signal Strength Indication) for use in other circuits of the

radio. Discriminator audio leaves IC1-11 to the Audio Processor IC8-40 (see Audio Processing).

4.2.5 SQUELCH

The high frequency noise from the audio is passed through FVR1 and C34 to IC1-13. FVR1 provides an

internal adjustment of the squelch range. The high frequency noise present in the audio is reduced when

a signal is received. IC1 amplifies and detects the noise and compares it to the squelch threshold set by

the operator to produce a squelch logic (BUSY) signal at pin 17. This is buffered by IC2 and used to

control the Squelch Gate IC9 which breaks the audio path when no signal is present.

Squelch control data originates in microprocessor, IC203 on the logic unit. Under control of impulses from

the channel/squelch switch on the control unit, squelch data is generated by IC203 and appears on pins

41, 42, 43. These are combined to provide eight steps of squelch threshold vo1tage (SQ-0 thru SQ-7)

which enters the main unit on CN6-13, is buffered by IC4 and goes on to the squelch circuit R26, R24, D7

to IC1-15. This voltage is compared to the detected noise level to generate the squelch logic (BUSY)

signal mentioned in the last paragraph.

In the transmit mode, the voltage is removed from the receiver which causes a loss of voltage on the

anode of D7. This immediately mutes the receiver.

4.2.6 RECEIVER MUTE

Operating voltage for the receiver circuits described above comes from IC12, 5 Volt Regulator. Q12

switches the voltage off during PTT to mute the receiver. Also, all voltage is removed upon activation of

the power-off timer.

4.2.7 AUDIO PROCESSING

Discriminator audio from IC1-11 goes to the Audio Processor IC8-40. IC8, contains the audio bandpass

filter,300 to 3000 Hz, de-emphasis @ 6 dB/octave, voice inversion scrambler and CTCSS. The audio

leaves on pin 12 and passes to Volume Control chip IC9-6. The volume control on the control unit

changes the gain of IC9 by changing the bias on pins 3 and 5. Audio leaves IC9-7 and goes through the

Squelch Gate Q10 which is controlled by the BUSY signal from the squelch circuit, the CTCSS decode

from the audio processor and the DECODE signal from the logic unit. From the Squelch Gate, the audio

goes to the Audio PA IC11-1. Operating voltage for the Audio PA is filtered by Q24 to minimize noise

from the automobile 12 volt system. Speaker audio leaves IC11, pin 4 and is fed to CN7-3 and to the rear

connector CN10-3. An external speaker may be connected between pins 3 and 1 (ground). To use the

internal speaker, a jumper plug connects pins 3 and 4 allowing the audio to pass to CN4-2 & 3 and on to

the Control Unit speaker through R414 and CN401 to the speaker. R414 limits the power applied to the

internal speaker. Audio is also supplied to the microphone connector through R415 and CN402-12. This

permits the use of a handset or another audio accessory. R415 reduces the audio level applied to the

external accessory to match the volume of the speaker.