User Guide

PAMS
System Module DB6
NHB–3
Technical Documentation
Original 26/97
Page 4–42
Transmitter
The TX intermediate frequency is modulated by an I/Q modulator contained on
transmitter section of CRFRT IC. The TX I and Q signals are generated in the
RFI interface circuit and they are fed differentially to the modulator.
Modulated intermediate signal is amplified or attenuated in temperature com-
pensated controlled gain amplifier (TCGA). The output of the TCGA is amplified
and the output level is typically –10dBm.
The output signal from CRFRT is low–pass filtered to reduce harmonics and
the final TX signal is achieved by mixing the UHF VCO signal and the modu-
lated TX intermediate signal with passive mixer. After mixing the TX signal is
amplified and filtered by two amplifiers and filters. These filters are dielectric fil-
ters. After these stages the level of the signal is typically 1 mW (0 dBm).
The discrete power amplifier amplifies the TX signal to the desired power level.
The maximum output level is typically 0.8 ... 1.0 W.
The power control loop controls the output level of the power amplifier. The
power detector consists of a directional coupler and a diode rectifier. Trans-
mitted power is controlled with controlled gain amplifier (TCGA) on TX–path of
CRFRT. Power is controlled with TXC and TXP signals. The power control sig-
nal (TXC), which has a raised cosine form, comes from the RF interface circuit,
RFI.
RF Characteristics
Duplex Filter
The duplex filter consists of two functional parts; RX and TX filters. The TX fil-
ter rejects the noise power at the RX frequency band and TX harmonic signals.
The RX filter rejects blocking and spurious signals coming from the antenna.
Pre–amplifier
The bipolar pre–amplifier amplifies the received signal coming from the anten-
na. In the strong field conditions the gain of the amplifier is reduced 39 dB, typ-
ically.
RX Interstage Filter
The RX interstage filter is a three pole ceramic filter. The filter rejects spurious
and blocking signals coming from the antenna. It rejects the local oscillator
signal leakage, too.