Icom IC-T90A manual VCO Divider Circuit VCO and RF Units, PLL Circuit

• 430 MHz VCO CIRCUIT

The oscillated signal at the 430 MHz VCO circuit (Q4, Q5, D3) is amplified at two buffer amplifiers (Q7, Q11), and is then applied to the PLL IC (IC201, pin 19). The signal is divid- ed by serial data from the CPU (LOGIC unit; IC1) and phase- detected with the divided reference frequency (5 kHz). The phase difference is output from pin 5 as pulses.

The output signals from the PLL IC (IC201, pin 5) are con- verted to DC voltages (lock voltage) by the loop filter, and are then fed back to the 144 MHz and 430 MHz VCO circuits to stabilize the VCO frequency.

When the signal bypasses the divider circuit (IC1), it passes through the divider switch (D8, D10).

The VCO signal is applied to the LO amplifiers (Q16, Q212), and then passed through the transmit/receive switch (D21, D202). The signal is applied to the buffer amplifier (RF unit; IC51, pin 1) for the TX LO frequency, or applied to the 1st mixer circuit (IC351, pin 3) for the RX 1st LO frequency as “LO” signal via or bypass the doubler circuit (Q354).

4-3-3 VCO DIVIDER CIRCUIT (VCO AND RF UNITS)

The PLL circuit employs the two VCO circuits (144 MHz and 430 MHz) and VCO divider (IC1) to transmit on 3 bands and receive wide band.

The oscillated signal at the 144 MHz or 430 MHz VCO circuit is amplified at the buffer amplifier (Q7), and is then passed through the divider switch (D7, D8).

When the signal is applied to the divider circuit (IC1, pin 2), the circuit divides the VCO signal into the ratio of 1/2. The divided signal passes through the low-pass filter (L12, L13, C48–C51) and divider switch (D9).

• PLL CIRCUIT

D7	LPF	D9
1/2		Q16	Q212
430 MHz VCO
Q1, Q2,		LO	LO
D1
Q7		D10
D8
Buff.

		50, 144MHz VCO	Q11
		Q3, Q4,	Buff.
		Q5,
Q206,	Loop	D3, D4
Q207	filter		IC201 (PLL IC)

	13	Phase	Programmable		Prescaler	19
		detector	counter
		Programmable				3	PLSTB
				Shift register		4
Q211		divider					CLK
Amp.	15			PLL lock detector		7	UNLK

Q1	Q2
Buff.	Amp.	to FM IF IC (IC151, pin 2)

X1	AF UNIT
13.8 MHz

TX/RX	RF UNIT
switch
D51	to transmitter circuit
D201	to 1st mixer circuit
	(IC351, pin 3)

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