User's Manual

FILE: IS 400 Service manual.doc 2005.11.29
E- TECH Co., Ltd 17/16
2) VCO circuit
The transmit frequency is directly generated by the Colpitts oscillation circuit contains Q402,Q403.
3) POWER AMP
Signals from Q202, Q206 is supplied through antenna switch Q203,Q204 to a low-pass filter made up
and then applied to Antenna Jack.
5-2 Receiver
1) LOWPASS FILTER and Antenna switching circuit.
Signals from antenna connector fed to the antenna switching circuit through the low pass filter
consisting of L212~214. In receiving mode, D204, D202 is turned off, isolates the antenna from the
transmitter circuit and matching circuitry, so that the incoming signals are fed to the RF amplifier
through L211.
2) RF AMP Circuit
The signals from the switching circuit are fed to the RF amplifier Q101 through a band pass filter made
up of molded coil, vvc diode and capacitor.
3) MIXER Circuit
The amplified signals are fed to Gate 1 of the first mixer Q102. First local oscillator signal is supplied
to Gate 2 of Q102 from the PLL circuit to convert the RF signals into 21.4MHz first IF signal.
4) IF Circuit
The first signals from Q102 are fed to the matched pair crystal filter FL101, then IF signals are amplified
in Q103. And those signals are fed to U101 which is composed of the second local oscillator, second
local oscillator, second mixer, limiter amplifier, quadrature detector and active filter circuit. The second
local oscillator at 20.945MHz with X 101 and is fed to the second mixer with the first IF signals to
convert into 455kHz second IF signals.
5) Audio and squelch Circuit
The detected audio signals are put through a 6dB/oct de-emphasis circuit made up of Q602.
The signal is then applied to audio power amplifier U603 to obtain enough power to driver the speaker.
Part of the recovered noise signal is fed to the integrated operational amplifier inside U101 which
makes up an low pass filter. The sensitivity of squelch is adjusted by VR 101.
5-3 VCO Circuit
The transmit / receive frequency is directly generated by the Colpitts oscillation circuit contains Q402,
Q405.