Service manual
CIRCUIT DESCRIPTION
5-10
April 2001
Part No. 001-9800-203
The receive audio, data, and noise signal from
amplifier U301B is applied to a bandpass filter and
amplifier formed by U304A and other components.
This stage attenuates voice frequencies and LTR and
Call Guard signaling so that only noise frequencies in
the range of approximately 7-8 kHz are passed. The
output signal of U304A is applied across a resistor
network which sets the input level to a rectifier. Ther-
mistor R318 provides temperature compensation of
this network.
C304 charges through the forward biased diode in
CR300 and discharges through R306. C303 discharges
through the other diode in CR300. When the voltage
on pin 6 of comparator U304B rises above the refer-
ence on pin 5, the output goes low. For example, when
received signal strength is low, more noise is rectified
which causes the voltage on pin 6 of U304B to
increase and the output on pin 7 to go low (squelched
condition). A voltage divider formed by R416 and
R417 reduces the 8-volt output of U304B to the 5-volt
level required by the microcontroller.
The squelch threshold level is set by a D/A
converter formed by shift register U305 and several
resistors. The eight-bit output of U305 can control the
reference voltage applied to U304B in 256 steps. U305
is programmed by the SPI serial port described in
Section 5.3.1. R310 provides hysteresis to the
threshold level to prevent intermittent squelching
when receiving a weak or fading signal.
5.5 TRANSMIT AUDIO/DATA PROCESSING
NOTE: A block diagram of the audio and data
processing circuitry is shown in Figure 5-3.
5.5.1 MICROPHONE AMPLIFIER (U303B),
HIGH-PASS FILTER (U303C)
The microphone audio signal is coupled by C349
to amplifier U303B which provides a gain of approxi-
mately two. R410, R414, and C354 provide a bias
voltage of approximately 3.2 volts on the nonin-
verting input. An 8-volt supply voltage to the micro-
phone amplifier is provided by R401, C348, and R406.
From U303B the microphone signal is coupled by
C350 to a high-pass filter formed by U303C and
several other components. This filter attenuates
frequencies below 300 Hz that could cause interfer-
ence with LTR data and Call Guard signals.
Gate U308B blocks the microphone signal when
microphone audio is not transmitted such as during the
data handshake to set up the call. A high signal on pin
5 closes the gate and passes the signal. This gate is
controlled by the Q6 output (pin 13) of latch U110.
Transistor Q307 functions as an inverter and level
translator.
5.5.2 SUMMING AMPLIFIER (U303D), PRE-
EMPHASIS/LIMITER (U303A)
U303D amplifies the transmit audio signal and
also the transmit data signal from an optional modem
if one is used. Jumper R398 is installed to route the
modem signal to U303D and the filtering circuitry, or
jumper R399 is installed to route a wideband signal
directly to the synthesizer so that it bypasses the
filtering circuitry. U308A closes when wideband (25
kHz) channels are selected. This bypasses R403 which
provides a higher deviation level with those channels.
NOTE: If the wideband data input is used, the external
device must provide FCC-approved modulation
limiting and splatter filter circuitry and a stable DC
level.
The output signal from U303D is then routed via
the option wireouts to U303A which provides limiting
and 6 dB per octave pre-emphasis. This stage is an
amplifier which limits by saturating. Limiting prevents
over-modulation caused by high-level input signals.
R370 and R378 set the input level to the next stage,
and C334 provides DC blocking.
5.5.3 SPLATTER FILTER (U302B, U302C)
U302B and U302C form a five-pole, low-pass
splatter filter which attenuates frequencies above
3 kHz. This prevents adjacent channel interference.
Frequencies over 3 kHz may be produced if limiting
occurs in the limiter stage just described. The signal is
then fed to digital potentiometer U802 on the RF
board which sets the deviation level. Refer to Section
5.7.4 for more information.
AUDIO/LOGIC DESCRIPTION (ALL MODELS)