Mic And External Mic; 4-Order 4 Khz Low-Pass Filter (U501-C, D); Audio Processor Ic (U102); 4.2.3.1 Tx Audio Processing - Motorola P100 Series Detailed Service Manual

4-4
4.2.1

MIC and External MIC

The TX audio enters the radio via the internal MIC or the external MIC jack. When using
the internal MIC, the audio passes through the external jack then rest of the circuits. When using the
external jack, the audio from the internal MIC is cut-off from the rest of the circuits and the external
MIC audio is passed to the rest of the circuits.
4.2.2

4-order 4 kHz Low-Pass Filter (U501-C, D)

4-order 4 kHz Low-pass filter to prevent aliasing noise of the ASIC switching cut-off frequency.
4.2.3

Audio processor IC (U102)

4.2.3.1 TX Audio Processing

Tx Audio from Pin 14 of U501-D enters the ASIC at Pin 3, 4 (Internal TX AMP).
• TX A1: Operational amplifier for gain adjustment of TX audio signal. R102, R103, C102, C103
are used to set the gain.
• VR1 (HPF): This circuit controls the volume for adjusting the input level of TX audio signal.
Adjustment range: -6.0dB, +4.5dB in 1.5dB steps
• Compressor: This circuit compresses the amplitude of transmit audio signal by 1/2 in dB scale.
• Pre-emphasis: This circuit emphasizes the high-pass frequency component of TX audio signal
to improve the S/N ratio of the modulation signal.
• TX/RX HPF: High-pass filter to eliminate low-frequency components lower than 250 Hz which
are included in transmitter audio signal. This circuit is turned on and off by control register of
Audio Processor IC.
• Scrambler: This circuit inverts the spectrum distribution of transmitter audio signals with
respect to the carrier frequency. The carrier frequency is 3.388 kHz or 3.290 kHz.
• Limiter: Amplitude limiting circuit to suppress frequency deviation in the modulation signal. The
limit level can be adjusted by applying a DC voltage the LIMLV Pin 7. When the pin is left open,
the level predetermined within the device is output.
• VR2: This circuit controls the volume for adjusting the output level on the MOD Pin 8.
Adjustment range: -9.6dB to +3.0dB in 0.2dB steps.
• Splatter: Low-pass filter to eliminate high-frequency components higher than 3 kHz which are
included in the limiter output signal. The cut-off frequency is 3 kHz.
• SMF: Smoothing filter to eliminate the high-frequency and clock components generated in the
ASIC.

4.2.3.2 Tx Tone PL/Digital PL Encode Filtering

The encode circuit of CTCSS and CDCSS mixes signals from 4 ports of microprocessor. This mixed
signal passes through ASIC Pin 19, 20 (U102).
• DTA1: Amplifier for gain adjustment of sub-audio signal which generated from CPU. Use
external resistor and capacitor to set the gain.
• Sub-audio Programmable LPF: Low-pass Filter to eliminate components of DAT1 signal in
the transmitter. This circuit is controlled cut-off frequency by the internal ASIC registers.
• VR5: This circuit controls the volume for adjusting the output level from the Sub-Audio LPF
signal. Adjustment range: -6.0dB to +6.0dB in 0.5 steps.
All manuals and user guides at all-guides.com
Controller Theory of Operation: TX Audio Circuit