Service manual

CIRCUIT DESCRIPTION
5-9
April 2001
Part No. 001-9800-203
audio signal is muted, this output is high. This signal is
then inverted by Q301 resulting in a low signal on the
control input (pin 6) of gate U307C.
U301A is a summing amplifier which combines
the supervisory tone signal from microcontroller pin
36 (PA6) with the receive audio signal. Supervisory
tones include the busy and intercept tones and other
beeps that are heard by the user. C320 provides addi-
tional feedback of the higher frequencies present in the
square-wave output of the microcontroller. C323 and
R352 also provide shaping of this signal.
Audio Power Amplifier (U306)
The output signal from U301A is fed to the
microphone connector through C321 and to audio
power amplifier U306. This is a 5-watt (with an 8-ohm
load) bridge-type amplifier. Therefore, both outputs
are connected directly to the speaker and neither
speaker terminal is grounded. This device is internally
protected from damage resulting from shorting either
output to ground or B+, or shorting across the outputs.
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 5.3.1. 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.
5.4.3 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.
The passband of this filter is controlled by the Q1
output (pin 18) of latch U111. When LTR or digital
Call Guard data or low-frequency Call Guard tones are
received, this output goes high which turns Q300 on.
This switches additional capacitance into the circuit
and the filter cut-off frequency decreases to approxi-
mately 150 Hz. Then when high-frequency Call Guard
tones are received, the output goes low and turns Q300
off. This increases the cut-off frequency to approxi-
mately 220 Hz.
From U300C the data signal is fed to a DC resto-
ration circuit formed by U300D and U300A. This
circuit converts it from an analog signal floating at half
supply to a digital signal at 0 and 5 volt levels that can
be detected by the microcontroller. U300D is a stan-
dard noninverting amplifier with a gain set by R308,
R316, and R317 (R317 is AC grounded by C309).
Two attenuation levels are selected by gate
U307D similar to gate U307B described in Section
5.4.1. When wideband (25 kHz) channels are selected,
the control input (pin 12) of U307D is high and R317
is effectively shorted which increases gain. The gain
of U300D is approximately 3 with wideband channels
and 2 with narrow-band channels. This compensates
for excess amplification of the data signal in the
narrowband mode by U301B.
The CR301 diodes charge and discharge C309 to
establish a DC reference on pin 2 of comparator
U300A. This reference voltage is the average of the
positive and negative alternations of the data signal.
When pin 3 of U300A rises above the reference on pin
2, the output goes high (8 volts) and vice versa.
Voltage divider R304/R311 provides the 5-volt level
required by the microcontroller.
5.4.4 SQUELCH CIRCUIT (U304A, U304B)
The microcontroller uses the output from the
squelch circuit and also the RSSI output of the limiter/
detector (see Section 5.8.4) to determine when to mute
and unmute the receive audio and also when valid data
may be present. The squelch circuit is controlled by
the amount of noise present in the receive audio signal.
When no signal or a weak signal is being received,
there is a large amount of noise present. Conversely,
when a strong signal is received, there is very little
noise present.
AUDIO/LOGIC DESCRIPTION (ALL MODELS)