Tx Audio Circuit; Figure 2-5. Tx Audio Block Diagram - Motorola CP185 Basic Service Manual

2-6
2.2.5

TX Audio Circuit

The TX audio circuit is comprised of microphones, LPF, Audio Processor IC, and TX Sub-tone
system.
4 Order LPF
(Fc=4KHz)
U501-C,D
TXA1
pin4
pin19
Tone IN
(from CPU)
The TX audio enters the radio via the internal MIC or external MIC. This TX Audio is filtered through
a 4th order 4 kHz Low-pass filter (U501-C & D) which prevents aliasing noise from ASIC. TX Audio
enters the Audio Processor IC which is then directed to an internal Amplifier (TX A1) for gain
adjustment of audio signal. A HPF (VR1) controls the input level of TX audio signal from -6.0 dB to
+4.5 dB in 1.5 dB steps. A Compressor (if ON) compresses the amplitude of TX audio signal by 0.5
dB. A Pre-emphasis circuit (if ON) emphasizes the high frequency component of TX audio signal to
improve Signal to Noise ratio before modulation. A shared High-pass filter (TX/RX HPF) eliminates
low-frequency components <250 Hz from TX audio signal. A Limiter is used to limit the signal
amplitude and suppress frequency deviation during modulation. VR2 controls the output level from
-9.6 dB to +3.0 dB in 0.2 dB steps. A Splatter (LPF) eliminates high-frequency components >3 kHz.
A Smoothing filter (SMF) eliminates high-frequency and clock components generated internally by
ASIC.
For sub-tone data from CPU, DTA1 amplifies the signal, sends it through a Sub-audio
Programmable LPF to eliminate components of DAT1 amplification, and finally the signal is
regulated by VR5 from
-6.0 dB to +6.0 dB in 0.5 dB steps. The final sub-tone data passes through a 2nd order LPF
(U502-A) before it is mixed with TX Audio for modulation.
The processed TX audio signal from Audio Processor IC is amplified by TX audio frequency amplifier
(U502-C) to increase limiting range and then adjusted to a proper level for modulation by U508. Final
TX Audio signal passes through a 6th order 3 kHz low pass filter (U501-A & B) before sent to VCO
for modulation.
Note: Retune the TX modulation if U508 is replaced. Refer Chapter 5.5: Transmitter Alignment
Options on page 5-3.
Mic
(Audio IN)
VR1 Com-
(HPF) pressor
-6 to +4.5dB /
1.5dB
Sub audio
Programmable
-6 to +6dB /
DTA1
LPF
0.5dB
VR5

Figure 2-5. TX Audio Block Diagram

Audio Processor IC (AK2347 )
TX/RX HPF
Pre-
Limiter
Emphasis
Fc = 300Hz
U102
2 Order LPF
(Fc=300Hz)
pin17
U502-A
To VCO &
VCTCXO
Theory Of Operation: Major Assemblies
VR2 Splatter
-9.6 to +3dB / Fc=2.55KHz/
0.2dB 3KHz
6 Order LPF
TX AF Amp.
(Fc=3KHz)
Mod. Adj,
U502-C
U508
U501-A,B
SMF
pin8