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other inputs are unshifted in phase). This network
consists of C410 and ceramic discriminator T401. The
detected audio signal is then amplified and fed out of
U401 on pin 9.
4.3.3 SQUELCH CIRCUIT (U401)
The audio output signal on pin 9 of U401
contains audio and also high-frequency noise. This
signal is fed to the receive audio and data processing
circuits on the logic board and also to the squelch
circuit which is part of U401. The amount of noise in
this signal tends to be inversely proportional to signal
strength (the higher the noise level, the lower the
signal strength and vice versa). Therefore, it can be
used for squelch control.
A bandpass filter that passes only noise frequen-
cies occurring above the audio band is formed by an
op amp in U401 and external components. The input
to this filter is pin 8 and the frequency determining
components are connected to pin 7 (R410-R412,
C415, C416).
The audio/noise input level to the filter is
adjusted by VR401 which sets the squelch threshold
level. The detected noise output is on pin 13, and it is
applied to pin 108 of the microprocessor in the logic
unit.
4.4 TRANSMITTER DESCRIPTION
NOTE: A block diagram of the RF and logic boards is
located on page 8-8.
4.4.1 DRIVER AMPLIFIER (Q101), POWER
AMPLIFIER MODULE (U101)
In the transmit mode, the output signal from the
transmit VCO is the transmit frequency. This signal is
buffered by cascode amplifier Q533/Q534 (see
Section 4.2.2) and then fed to driver Q101 which
provides the required input level to power amplifier
module U101. Impedance matching between Q101
and U101 is provided by L102 and C106.
Power amplifier module U101 provides up to 4
watts of RF power at the antenna jack. It contains two
internal amplifier stages, each of which has a separate
supply voltage (Vcc, Vbb Control). The supply
voltage to the first stage (Vbb Control) is from the
power control circuit, and the supply voltage to the last
stages (Vcc) is the unswitched battery supply.
4.4.2 ANTENNA SWITCH AND LOW-PASS
FILTER
The antenna is switched between the receiver and
transmitter by an antenna switch consisting of D101,
D102 and several other components. In the transmit
mode, the T5V supply is enabled and current flows
through R105, L106, D101, L112, and D102. Since
D101 is forward biased, the transmit signal has a low
impedance path to the antenna.
L106/C112/D101 and L112/D102 each form
discrete quarter-wave line that is AC grounded at one
end. When one end of a quarter-wave line is AC
grounded, the other end presents a high impedance to
the quarter-wave frequency. Therefore, L106/C112
isolate the 5-volt supply from transmitter RF, and the
other two circuits isolate the receiver. In the receive
mode, D101 and D102 are no longer forward biased so
the receive signal has a high impedance path into the
transmitter and a low impedance path into the receiver.
From the antenna switch the transmit signal is fed to a
low-pass filter consisting of C113-C116, C118, and
L107-L110. This filter attenuates harmonic frequen-
cies occurring above the transmit band.
4.4.3 POWER CONTROL
The power output circuit maintains a steady
power output level by controlling the supply voltage to
the first stage in power module U101 (Vbb Control).
The output power is monitored using a directional
coupler consisting of a microstrip two-line coupler,
C123, D103, C124 and several other components.
Reflected power is dissipated by R107 and forward
power is detected by C123 and D103. These compo-
nents produce a DC voltage that increases with
forward power.
The rectified DC level is buffered by voltage
follower U102A and fed through R115 to pin 6 of
U102B. This stage amplifies the difference between
the forward power signal on pin 6 and the reference
voltage on pin 5. This reference voltage controls the
power output, and it is set by the control logic through
potentiometer 2 of U147 on the logic board.
4-4
CIRCUIT DESCRIPTION
November 2001
Part No. 001-7240-001
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