User Guide

After Sales
Technical Documentation
System Module
THA–4S
Page 4–16
Original, 12/95
RF Circuit Description
Receiver
The receiver is a dual–conversion superheterodyne using two
intermediate frequencies, 86.85 MHz and 450 kHz.
The RF signal from the duplexer RX port is applied to the Low Noise
amplifier. The amplifier is realized with transistor V200. Amplifier stage
input matching is accomplished by C201 and L217. R200 and R201 are
used for biasing. Output matching is carried out by L216, R211 and C325.
Components C326 and C329 are used for RF bypassing. C200 and C202
are for amplifier stabilization. The Low Noise amplifier and the IF circuit
are connected in series with R202 so that the same supply current is
passed through both stages. The signal is then filtered with Z202.
The filter is followed by a single balanced diode mixer, realized with
C207,C208, Z205, Z204, Z203 and V201. Input matching from the Rx
SAW filter is realized by C205 and L201.
The signal is then filtered with a saw filter Z206. Input and output
impedances are 600 ohm // 3pF. The filter and the IF amplifier stage are
matched by C210 and L203.
The IF amplifier is realized with transistor V202; R205 and R206 are used
for biasing, R203 and C209 are for amplifier stabilization. The IF amplifier
stage and the second mixer are matched by R204 and C220.
The second mixer, the oscillator transistor, IF amplifier, LIM amplifier,
quadrature detector and audio amplifier are all integrated in the circuit
N201. The second LO frequency, 86.4 MHz, is realized by multiplying
VCTCXO (14.4MHz) six times. This multiplier is realized with transistor
V225, R226 and R303 being used for biasing. R299 and C324 are for
stabilization.
L215, C323 and C219 is the bandpass filter for 86.4 MHz. C265 and C219
are used for DC blocking. L204 and C215, are connected to OSCOUT.
After the mixer, the 450 kHz IF signal is fed to the multiplexer circuit N200.
Z207 is a narrow ceramic filter for NAMPS and Z208 is a wide ceramic
filter for AMPS. A multiplexer circuit is used to switch between these
filters. The LIM amplifier output signal is fed through C223 to a
quadrature detector. After this, the signal is phase shifted by resonance
circuit C225, R214 and L205.
Signal DAF is low pass filtered by R212, C222 and R213. The 2nd IF, DAF
and RSSI signal (450 kHz) is fed to the audio/logic unit.