Chapter 4 Controller Theory Of Operation; Rx Audio Circuit; Audio Processor Ic (U102); Rx Audio Processing - Motorola EP350 Service Manual

Chapter 4 Controller Theory of Operation

4.1

RX Audio Circuit

The RX audio circuit consists of Audio Processor IC, Audio Amplifier, Internal (INT)/External (EXT)
speaker and Sub-Audio Tones System.
RXA1
Audio IN
(from IF IC)
pin24
-4 to +3.5dB/
0.5dB
Programmable
4.1.1

Audio processor IC (U102)

The RX audio from Pin 9 of U201 enters to Pin 24 of Audio processor IC.
• RX A: An operational amplifier used for gain adjustment of the receive demodulation signal
from Pin 9 of U201. The gain is unity and it acts as a buffer amplifier.

4.1.1.1 RX Audio Processing

• VR3: This circuit controls the volume for adjusting the input level of receive demodulation
signal. Adjustment range: -4.0dB to +3.5dB in 0.5dB steps.
• RX LPF: Low-pass filter to eliminate high-frequency components higher than 3 kHz which are
included in the receive demodulation signal.
• TX/RX HPF: High-pass filter to eliminate low-frequency components lower than 250 Hz which
are included in receive audio signal. This circuit is turned on and off by control register of Audio
processor IC.
• Descrambler: This circuit inverts the spectrum distribution of receive audio signals with respect
to the carrier frequency. The carrier frequency is 3.388 kHz or 3.290 kHz.
RX LPF TX/RX HPF
VR3
Sub audio
-6 to +6dB/
LPF
0.5dB
VR5
pin18
Sub audio
U105-A,B
HPF
To CPU
Comparator
(tone detect)
U105-C

Figure 4-1. RX Audio Circuit

Audio Processor IC (AK2347)
Scrambler/ De-
Descrambler emphasis
U102
INT SPK. J601
EXT SPK.
Expander VR4
-18, -4.5 to +4.5dB/
0.25dB
Audio Amp .
IN-
OUT-
IN+
OUT+
SVR
U601
Audio Mute
control
SMF
pin21
Vol1