Service manual

August 30, 2002 99R85452E01-O
7-6
VHF Band Information
: Theory of Operation
7.3.6.3 Power Control
Output power is controlled by the CPU (Q101), the D/A converter (Q121), the dual Op-Amp (Q419),
which is used as a differential amplifier and comparator. Current is sensed by the voltage drop
across R445. When the radio is in high power mode, this voltage is compared to the
D/A converter voltage at 5-watt. When the radio is in low power mode, this voltage is compared to
the D/A converter voltage at 1-watt. The power output is then reduced or increased by varying the
applied voltage to the gate of the power amplifier Q411 and Q412.
7.3.6.4 Transmitter Audio Circuits
The transmitter audio circuits consist of the audio processing circuits, the CTCSS circuits and the
DCS circuits.
7.3.6.4.1 Audio Processing
Transmit speech audio is provided by either the internal electric microphone M101 or the external
microphone. The audio is pre-emphasized by 6dB per octave by C615 and R655, and then its signal
is amplified. The microphone audio is directed to amplifier Q602B, Q602A and low-pass filter Q602C
and Q602D. Q602A has limiting function. The modulation adjustment is done by Pot RV601, and
directed to a four-pole active low-pass filter. The resulting signal is then limited, and is directed to a
low pass filter (18dB per octave roll-off above 3KHz). The audio is then directed through the 25KHz/
12.5KHz channel spacing SW Q601 to varactor diode Q329 in transmit VCO. By varying the voltage
on the varactor diode Q329 at an audio rate, VCO output is frequency-modulated.
7.3.6.4.2 CTCSS Tone Encoder / Digital Code Squelch (DCS) Encoder
CTCSS signals and DCS signals are synthesized by CPU Q101 and appear as pulse waveform on I/
O line Pin 7. This I/O line is connected to a digital-to-analog converter network (consisting of C136,
C173, R150, R177 and R182), which produces a pseudo-sine wave at its output. The waveform is
smoothed by low pass filters Q610B to produce an acceptable sine wave output. The CTCSS tone
signal is adjusted to the proper level by RV602.
7.3.7 Receiver Section
7.3.7.1 Receiver Front End
In the receive mode, the RF signal enters thorough the antenna, then through the low-pass filter
C439-C445, and L414-L416. The diodes Q409 and Q410 are biased off, so that the output of the
low-pass filter is coupled (C437, C438, L413 and R418) to the first band-pass filter C401-C406,
C408-C410, L401, L402, PCB_L1, Q401, R401, and to the Front End RF overload protection diode
pair Q403. The signal from the band-pass filter is directed to the input of the RF amplifier Q404. The
output of the RF amplifier is sent to three stages of band-pass filters, consisting of C415-C429,
C486, L404-L406, Q405 and R410. The output from the band-pass filter is directed to the mixer’s
Q407/L407.
7.3.7.2 Local Oscillator (LO)
The Receive VCO (C344-C351, L302, L305, L306, L312-L314, Q311, and Q324) provides the LO
signal. The VCO is running at 45.1 MHz above the desired receive frequency and is applied to output
Buffer Q308/Q416. The output of the buffer is sent through the low-pass filter C430, C431, L409, and
to the mixer Q407/L408.