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4. CIRCUIT DESCRIPTIONS
1) PHASE-LOCK LOOP (PLL) CIRCUIT
* REFERENCE OSCILLATOR
The reference oscillator consists of X401 in U401 with a frequency of 12.8MHz. The reference oscillator
frequency is stabilized by the thermistor R403 and drives a divider to produce a comparison frequency.
This comparison frequency is selected by decoding the first three bits of the data input from microcomputer.
* PROGRAMMABLE DIVIDER
The programmable divider in U401 consists of a two-modulus prescaler with a 7bit control register followed by a
11-bit internal programmable divider. The overall division ratio is selected by a single 19-bit world located on the
serial data bus.
* PHASE COMPARATOR
A digital-type phase comparator in U401 with output (pin 6) and an open drain lock detect output (pin 10)
compares divided VCO frequency with the comparison frequency.
* VCO CIRCUIT
The transmit/receive frequency is directly generated by the Colpitts oscillation circuit contains
Q402, Q405. The oscillation frequency is variable by applying the VCO control voltage to variable
to variable capacitors D401, D402, D403, D404 and D405, 406, 407, 408.
2) TRANSMITTER
* MIC AMP CIRCUIT
Voice signal from the microphone are applied to microphone amplifier U8 through MK2.
U8 contains a low-pass filter that has a 6dB/oct response between 300Hz and 3 kHz and eliminates hamonics
above 3 kHz. The Pre-emphasized audio signal is applied U8 pin 22 to adjust maximum frequency deviation by
software.
* VCO AND AMPLIFIER
The VCO signal output is amplified by Q101 and then fed to power module U405.
* POWER AMPLIFIER CIRCUIT
U405 is provided approximately 7.5V DC power source.
RF power output is adjusted by software (DA-TX power) to 1W~5W.
Signals from U405 is supplied through antenna switch D101 to a low-pass filter made up of L101, L102, L103 and
C101-C106, then applied to Antenna Jack.
3) RECEIVER
* ANT SWITCHING CIRCUIT
Signals from antenna connector fed to the antenna switching circuit through the low pass filter consisting of
L101~106. In receive mode, D101 is turned off, isolates the antenna from the transmitter circuit and matching
circuitry, so that the incoming signals are fed to the RF amplifier through L105.
* RF AMPIFIER CIRCUIT
The signals from the switching circuit are fed to the RF amplifier Q201 through a band pass filter made up of
spring coil, vvc diode and capacitor.
* FIRST MIXER CIRCUIT
The amplified signals are fed to Gate 1 of the first mixer Q202 through C230. First local oscillator signal is
supplied to Gate 2 of Q202 form the PLL circuit through C258 to convert the RF signals into 21.4MHz first IF
signal.
* IF CIRCUIT
The first IF signals from Q202 are fed to the matched pair crystal filter XFL201, then IF signals are amplified in
Q203. And those signals are fed to U201, which is composed of the second local oscillator, second mixer, limiter
