CIRCUIT DESCRIPTION

3-8 February 2001

Part No. 001-9800-001

Figure 3-3 Audio and Data Processing Circuitry Block Diagram

Bandpass Filter/
De-Emphasis
300-3000 Hz
U301C,
U301D
U300B,
U300C
U304A
U303C
U304A
U300D,
U300A
U304B
Amplifier
Microphone
Amplifier
U301B
U303B
U307B
Q300
U305
U307C
Q301
Q307
GainAdjust
U301A
Summing
Amplifier Audio
Amplifier
U306
8V
Front Panel
Volume Control
(9842)
Q302
Low-Pass Filter
< 140/211 Hz
Filter Bandwidth Control
Data
Detector
Bandpass Filter
High Pass Filter
> 300 Hz
Low Pass Filter
< 140/211 Hz
Comparator
Digital
Squelch
Control
Microphone
U303D
Pre-Emphasis/
Limiter
MicAudio
Gate
Tx
Data
Gate
U302B,
U302C
Amplifier Splatter Filter
> 3 kHz
U303A
RxAudio
Gate
Speaker
RECEIVEAUDIO
RECEIVE DATA
SQUELCH
TRANSMITAUDIO
TRANSMITDATA
RxAudio/Data
From
Receiver
From
Logic
From
Logic
From
Logic
From
Logic
From
Logic
To Synthesizer
To Synthesizer
To Logic
From Logic
From Logic
To Logic
From
Logic
Serial Data
MicAudio Gate Control
TxAudio Gate Control
Data AnalogTransmit Data
Tx
Audio
Squelch Signal
Digital Receive Data
Audio Mute Control
SupervisoryTones
RxAudio Gate Control
U308B
U309
Digital Volume
Control
(98x1)
Pin 5 is the input of an internal gain control stage. Gain increases in proportion to increases in the DC voltage on this pin. With the low tier models, volume control is provided by a D/A converter formed by shift register U309 and several resistors. The six-bit output controls the volume in 64 steps. U309 is programmed by the SPI bus described in Section 3.3.1. With the mid and high tier models, the front panel volume control is part of a voltage divider which includes R356 and R426. If the voltage on pin 5 falls below approximately 0.4 volt DC, the output is muted. Speaker muting is controlled by the Q7 output (pin 12) of latch U110. When this output goes high, inverter/level translator Q302 turns on which grounds the volume control input of U306 and mutes the speaker.

3.4.3 RECEIVE DATA PROCESSING

The receive audio/data signal from amplifier U301B is applied to a low-pass filter formed by U300B and U300C. This filter attenuates voice and harmonic frequencies occurring above the data band.

AUDIO/LOGIC DESCRIPTION (ALL MODELS)