TTI Freequency TCB-770, Руководство по обслуживанию

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4-3. Descriptions of Each Block
4-3-1. Introduction
The synthesizer is implemented with the following components;
PLL IC (IC301)
X-TAL (X301)
VCO, VARICAP DIODE (D301)
IC1301 is CMOS LSI that includes most of PLL block.
The Q301, L301, C311, C312, C313, C314, variable capacitance diode D301 are clapping
oscillator circuit to operate as a VCO of the IC301. Q305 is a switching transistor to connect or
disconnect the tuning capacitor in the VCO oscillator tank circuit for transmitter or receiver.
Q302 works as a buffer amplifier for RX local frequencies (
≒
16MHz) and TX carrier
generating frequencies(
≒
13MHz)
4-3-2. Reference Frequency
The crystal X301(4.5MHz) and other components at Pin 1 and 24 of IC301 can make a reference
frequency oscillator with internal amplifier.
4-3-3. VCO
Q301 and surrounding parts are consisting a clapping oscillator works as a VCO of IC301. With
appropriate control voltage on D301 the VCO can be oscillate over the required range of 13.4825MHz to
16.710MHz.
4-3-4. Programmable Divider and its Control
The programmable input for each channel are stetted by the PLL Clock(Pin 11), PLL Data(Pin
12), PLL Enable(Pin 13) of IC801. Each input signal to control the PLL IC is done with provide key input
Pin 18, 19, 25, 26, 27, 28, 29, 32, 33, 34, 35, 37. For each key input, an internal code converts EEPROM
appropriate control to the programmable divider for that channel.
Since the change transmit and receive, and additional bit is required at Pin 8 of IC801 to allow
the ROM to recognize the status TX or RX.
During transmit the push to talk switch makes Pin 8 ground with BRT(Q802), PLL IC works
under transmit status.
The programmable divider output fed to the phase detector for compare with the 2,5KHz
reference frequency IC301. See table 1 for actual input and divide ratio on all channels.
4-3-5. Phase Detector and VCO Control
The phase detector is a digital phase comparator witch compares the phase of the reference
signal with programmable divider output square waves and develops a series of pulses whose dc level
depends on the phase error of each signal.
The phase detector pulse output is fed to an active low pass filter and RC low pass filter output
signal of IC301 is filtered and fed to varicap D301 control the VCO frequency.
4-3-6. Transmitter / Receiver Buffer AMP
Output signal of Q301 is fed into the buffer amplifier Q302, L302 to generate TX carrier
frequency and 1’st local frequencies.