Icom IC-F21S service manual PLL Circuit, General, VCO Circuit

4-3 PLL CIRCUIT

4-3-1 GENERAL

A PLL circuit provides stable oscillation of the transmit fre- quency and receive 1st LO frequency. The PLL output com- pares the phase of the divided VCO frequency to the refer- ence frequency. The PLL output frequency is controlled by a crystal oscillator and the divided ratio (N-data) of a pro- grammable divider.

The PLL circuit, using a one chip PLL IC (IC1), directly gen- erates the transmit frequency and divided ratio based on serial data from the CPU and compares the phases of VCO signals with the reference oscillator frequency. The PLL IC detects the out-of-step phase and output from pin 5.The ref- erence frequency (15.3 MHz) is oscillated by X1.

4-3-2 TX LOOP

The generated signal at the TX-VCO (Q11, D7) enters the PLL IC (IC1, pin 8) and is divided at the programmable divider section and is then applied to the phase detector section.

The phase detector compares the input signal with a refer- ence frequency, and then outputs the out-of-phase signal (pulse-type signal) from pin 5.

The pulse-type signal is converted into DC voltage (lock voltage) at the loop filter (R40–R42, C75–C77), and then applied to varactor diodes (D6) of the TX-VCO to stabilize the oscillated frequency.

4-3-3 RX LOOP

The generated signal at the RX-VCO (Q10, D5) enters the PLL IC (IC1, pin 8) and is divided at the programmable divider section and is then applied to the phase detector section.

The phase detector compares the input signal with a refer- ence frequency, and then outputs the out-of-phase signal (pulse-type signal) from pin 5.

The pulse-type signal is converted into DC voltage (lock voltage) at the loop filter (R40–R42, C74–C77), and then applied to varactor diodes (D5) of the RX-VCO to stabilize the oscillated frequency. The lock voltage is also used for the receiver circuit for the bandpass filter center frequency. The lock voltage from the loop filter is amplified at the buffer amplifier (Q13) and then applied to the CPU (IC7, pin 60). The signal is analyzed at the CPU, and then applied to the D/A convertor (IC9). The D/A convertor outputs “T1”, “T2”, “T3”, “T4” signals to RF bandpass filters D12–D15 to sup- press harmonic components.

4-3-4 VCO CIRCUIT

The VCO outputs from Q11 (TX) and Q10 (RX) are buffer amplified at Q7 and Q6, and are then sent to the T/R switch (D3, D4). The receive LO signal is applied to the 1st mixer circuit (Q19) through an attenuator, and the tramsmit signal is applied to the YGR amplifier (Q4). A portion of the VCO output is reapplied to the PLL IC (IC1, pin 8) via the buffer amplifier (Q6) and low-pass filter (L18, R53, C89–C91).

PLL CIRCUIT

Loop

filter

45.9 MHz signal Q18 to the FM IF IC

⋅3

X1

15.2 MHz

TX VCO	Buffer	D3	to transmitter circuit
	Q5
Buffer			to 1st mixer circuit
Q11, D7		D4
Q7

RX VCO

Buffer

Q6

	Q10, D5
					IC1 (PLL IC)		LPF
5		Phase	Programmable		Prescaler	8
		detector	counter
2	Crystal	Programmable				11	P.STB
				Shift register		9
							CK
	oscillator	divider				10
							DATA
1

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