HYT TC-500, Service Manual

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TC-500 Service Manual
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matches the preset value and controls the MUTE and AFCO and the speaker sound output
according to the squelch results.
PLL Frequency Synthesizer
PLL circuit generates the first local oscillator signal for reception and the RF signal for
transmission.
1. PLL Circuit
The step frequency of PLL circuit is 2.5KHz or 6.25KHz. A 12.8MHz reference oscillator
signal is divided at IC2 by a fixed counter to produce a 2.5 KHz or 6.25 KHz reference
frequency. Output signal from voltage control oscillator (VCO) passes through buffer
amplifier Q14 and is divided at IC2 by the dual -module programmable counter. The divided
signal is compared in the phase comparator IC2 with the 2.5 KHz or 6.25 KHz reference
signal. The output signal from phase comparator is filtered through a low -pass filter and
passed to the VCO to control the oscillator frequency. (See figure 4)
Figure 4 PLL circuit
2. VCO
The operating frequency is generated by Q16 in transmit mode and by Q8 in receive mode.
The operation frequency generates a voltage through the phase comparator to control the
varactor, so as to keep the oscillator frequency consistent with the preset frequency in
CPU (D2 and D3 in transmit mode and D7 and D8 in receive mode). T/R pin is set high in
receive mode causing Q6 to turn off Q16 and turn Q7 on. The T/R pin is set low in transmit mode.
The output from Q8 and Q16 is amplified by Q14 and sent to the buffer amplifier.