Motorola P100 Series, Detailed Service Manual, Page: 33 / 336

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.
Figure 4-1. RX Audio Circuit
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.
VR3
Scramb l er/
D escramb l er
D e -
emp h asis
Ex pan d er VR4 SM F
S VR
IN-
IN+
O UT -
O UT +
Aud io M u te
contro l
RX L P F T X / RX H P F
RXA 1
-4 to +3. 5 d B/
- 1 8, -4. 5 to +4. 5 d B/
0. 25 d B
Aud io Processor I C (AK 2 347)
Aud io IN
(f rom IF I C )
VR 5
S u b a ud io
Programmab l e
L P F
To CPU
( tone d etect )
pin2 4
pin1 8
pin21
Aud io A mp .
U1 0 2
U 60 1
V o l 1
-6 to +6d B/
0. 5 d B
IN T SP K.
EX T SP K.
J60 1
S u b a ud io
H P F
U1 0 5 -A, B
Comparator
U1 0 5 - C
0. 5 d B