Service manual
CIRCUIT DESCRIPTION
5-8
April 2001
Part No. 001-9800-203
Figure 5-3 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)
translation from 5-volt logic levels of U111 to the 8-
volt logic levels of U307B.
From U301B the receive audio/data signal is fed
to audio, data, and squelch circuits. Refer to the
following descriptions for more information.
5.4.2 RECEIVE AUDIO PROCESSING
Bandpass Filter (U301C, U301D)
U301C and U301D form a bandpass filter which
passes frequencies in the 300-3000 Hz range. This
attenuates frequencies below 300 Hz such as LTR data
and Call Guard signaling, and frequencies above 3
kHz such as noise. These stages also provide 6 dB per
octave de-emphasis to remove the pre-emphasis that
was added to the signal when it was transmitted.
Mute Gate (U307C), Summing Amplifier (U301A)
The receive audio signal is then routed via the
option wireouts to mute gate U307C. This gate mutes
the signal when no carrier is being received or if the
message is intended for someone else. It is controlled
by the Q5 output (pin 14) of latch U110. When the
AUDIO/LOGIC DESCRIPTION (ALL MODELS)