Maintenance Manual
9
temperature and load. To do this, the Power Control circuit maintains a steady power output level by APC.
The transmit output power is controlled by varying the power control voltage (DA_TXPOW). The output
transmit output power is monitored by D80 and produce a DC voltage. The Amplifier (U305) amplifies
only the difference between the forward power level and DA_TXPOW level. This signal play a role to
gate voltage driver amplifier (Q72) and final amplifier (Q73).
TX Switches
The antenna switch part separates TX from RX and when transmitting, it emits only the carrier through
antenna and removes the Spurious and Harmonic. TX and RX switching is operated by pin diodes D60,
D61 and D62. Regarding TX operation, if TX5V2V is supplied to D60, D61 and D62, the diodes will be
turn on and the carrier is emitted to Antenna. The RX direction is blocked by D62, and it protects the RX
part. When RX operation, TX5V2 will be OFF and D60 & D61 are blocked. The RX signal is entered into
the RX direction.
The output signal passes through the low pass filter (L55, L56, C57, C58, C501, C502, C503, C504,
C505, C506, C507, and C508) to remove of Spurious and Harmonic.
RECEIVER CIRCUIT
The dual conversion receive circuit consists of a receiver front end, a 45.3
㎒
1
st
IF (UHF model), 21.4
㎒
1
st
IF (VHF model) and 455KHz 2
nd
IF and an FM detector. All audio processing and squelch
functions are accomplished on the Audio/Logic section.
Front End
The front-end circuit consists of low pass filter, tunable band-pass filter and low noise amplifier. A
desired RF is coupled from antenna to the main board through antenna connector. The 400 ~ 470
㎒
RF
receive frequency is passed through a low-pass filter and passed the transmit switch and tunable band-
pass filter (L61, L62. L67, L68, D64, D65, D68, D69). The frequency characteristic of tunable filter plays
a role in making rejection of low frequency by DA_BPF voltage.
RF Amplifier
RF amplifier Q200 provides typ15dB of gain to the RF signal. The output of the RF amplifier is
coupled through tunable band-pass filter (U202). The center frequency of tunable band-pass filter is
controlled by DA_BPF. The output of band-pass filter connected to the input of mixer circuit (Q203).
