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
Gain Adjust
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
Mic Audio
Gate
Tx
Data
Gate
U302B,
U302C
Amplifier
Splatter Filter
> 3 kHz
U303A
Rx Audio
Gate
Speaker
RECEIVE AUDIO
RECEIVE DATA
SQUELCH
TRANSMIT AUDIO
TRANSMIT DATA
Rx Audio/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
Mic Audio Gate Control
Tx Audio Gate Control
Data
Analog Transmit Data
Tx
Audio
Squelch Signal
Digital Receive Data
Audio Mute Control
Supervisory Tones
Rx Audio 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)
