Other Circuitry - Icom IC-H6 Service Manual

3- 3 - 4 REFERENCE FREQUENCY GENERATOR CIRCUIT
Reference frequency generator IC203 consists of a crystal oscillator and a highspeed divider. X201 oscil-
lates at 5.12MHz, which is divided by 2048. The 2.5KHz reference frequency is fed to phase detector
IC202. This 2.5KHz reference frequency decides the variation step of the PLL output frequency.
3 - 3 - 5 PHASE DETECTOR AND LOOP FILTER CIRCUITS
Digital phase detector, IC202, detects the phase difference of the pulse signals of the 2.5KHz reference
frequency and the output signal of the programmable divider, and proportionately puts out pulse signals
at Pin 3, which becomes high impedance when the PLL is locked.
Pin 4 is for detecting the lock failures and changes to ground level according to the phase difference of
the two pulse signals. When the lock fails, the pulse signal from Pin 4 is integrated by R202 and C215.
When the integrated voltage exceeds the junction voltage of 0214's base, 0214 is turned ON and then
0107 in the MAIN boards is turned ON.
The collector of 0107 is connected to the base of 0108, so the base voltage of 0108 becomes ground
level, and 0108 and 0106 are shut off to prevent transmitting unwanted signals.
The loop filter, consisting of R204, R205, R206, C213 and C214, converts the pulse signal from Pin 3
into a DC voltage and decides the response time of the whole loop.
The output signals are fed to tuning diode D203 of the VCO circuit as the control voltage for the VCO
frequency set.
3- 3 - 6 VCO CIRCUIT
The VCO (Voltage-Controlled Oscilator) is a Colpitts circuit using 0201, and oscillates in 70 - 80MHz
range.
The oscillator frequency is controlled by a DC voltage which is supplied from the loop filter to varactor
diode D203.
In the receive mode, R+6V is applied to D204's anode through L201, and D204 is turned ON and shunts
C220. Thus the free-run frequency of the VCO is lowered.
In the transmit mode, T +6V is applied to D204's cathode through D205 and L202, D204 is turned OFF,
and C220 is inserted in the oscillator circuit in series. Thus the free-run frequency of the VCO is
increased. In the same time, the VCO signal is frequency modulated by the audio signals from the
microphone which are applied to the gate of 0201 and varies 0201 's mutual conductance.
3 - 4 OTHER CIRCUITRY
3 - 4 - 1 POWER SUPPLY CIRCUIT
The regulated 6V is supplied to the main circuits, so that the set operates under a stable condition with
as low power voltage as possible.
The power supply voltage is fed to the AF power amplifier through the squelch switching circuit and to
the 6V regulator consisting of 0117 - 0120 and zener diode D104. This regulated 6V is supplied to the
PLL circuit.
In the transmit mode, the base of 0123 is grounded through R 155, the microphone and the PTT switch,
and 0123 is turned ON. Thus 0106 and 0108 are turned ON and T +6V is actuated, and supplied to the
transmitter circuit. At the same time, T +6V turns 0112 ON, and the power supply voltage is applied to
the MIC amplifier circuit through 0112.
In the receive mode, 0123 is turned OFF and the bias voltage of 0109 ON. Thus the R+6V is actuated
and supplied to the PLL board to switch the local oscillator crystal and the driver transistors of the
transmitter circuit.
At the same time, R+6V turns ON the voltage boost circuit consisting of 0110 and 0111, and +6V is
supplied to the receiver circuit.
3-3