Maintenance Manual
8
of capacitor C431, C434, C432 and inductor L75. The output signal fed into the synthesizer IC and the
feedback signal level is –15dBm.
Transmitter modulation signal from Audio/Logic section is applied to R452, D77 and C413.
Reference Oscillator (U303)
Reference Oscillator U303 provides a reference frequency of 12.8
㎒
at
±
2.5ppm (-30
°
C~+60
°
C) to
Phase- Locked-Loop circuit. The oscillator has modulation capability. Frequency is adjusted by the
FRE_CON signal from the D/A converter section of U101. The voltage is controlled by PC Tune program.
Charge Pump and Loop Filter
Synthesizer circuit outputs phase error signal between the desired frequency and feedback frequency from
the VCO section. This phase error signal fed through the external charge pump circuit. Charge pump
circuit provides a control voltage of 0 ~ 10Vdc which is control the frequency of the VCO. The Voltage
converter (U52) converts 5Vdc to 10Vdc to provide the necessary voltage range.
The low pass filter is used for Loop filter circuit to reduce the side band noise of reference frequency and
obtain a high signal-to-noise.
TRANSMITTER CIRCUIT
The transmitter section consists of a driver amplifier, final amplifier, power control circuit and transmitter
switch.
Driver Amplifier
The driver amplifier consists of driver buffer amplifier (Q74, Q71) and drive amplifier (Q72). Transmit
RF signal form the VCO feed to driver amplifier and amplified up to 37dB by the driver amplifier section.
The output signal fed through the final amplifier.
Final Amplifier
The final amplifier (Q73) can amplify the power output up to more than 5W. The Final Amplifier (Q73)
amplifies to make final power output of more than 37dBm. The drain of final amplifier is connected into
the battery voltage in order to obtain maximum power. The output signal is fed through D61 and low-pass
Filter section.
