User Guide

PAMS
Technical Documentation
NSB–1
System Module
Page 3 – 31
Original 06/98
Transmitter
Transmitter chain consists of IQ–modulator, upconversion mixer, TX filter,
TX buffer and a power amplifier.
The differential I and Q signals are generated by COBBA_GJ and are fil-
tered by an external RC network (R501, R504, C525 and C526,
fc=200kHz) before being fed into the IQ modulator in PLUSSA (N401).
The modulator generates a TX IF of 400 MHz which is derived from the
VHF synthesizer output (divide by two). Inside PLUSSA the 400 MHz is
amplified and then fed to an external filter before being up–converted in
CRFU2a. The up–converter in CRFU2a is a double balanced image re-
jection mixer. The local oscillator signal for the upconversion is generated
by the UHF synthesizer. Following CRFU2a is a TX SAW Z503 filter
which attenuates the image frequency, LO leakage and wideband noise.
After the bandpass filter is a buffer V510 with 12dB gain, then a 3 pole ce-
ramic bandpass filter (Z505) to further suppress spurious from the up–
converter.
After filtering, the signal goes to the final amplifier, which is a MMIC PA
(N500) with an input impedance of 50 ohms. The MMIC contains three
amplifier stages with interstage matching. The first amplifier stage is vari-
able and is control by the TX power control circuity. An external driver is
required to supply the necessary current to the TX power control circuitry.
The PA has over 45 dB power gain and is capable of producing an output
of 32.5 dBm with an input of +3 dBm. Harmonics generated by the non-
linear PA (class AB) are attenuated with the output external matching net-
work and the lowpass/bandstop filtering in the duplexer (Z401).
Power control circuitry consists of a directional coupler power detector
and an error amplifier in PLUSSA. The directional coupler is situated be-
tween the duplex filter and the external RF connector. With this configu-
ration, variations in the IL of the duplexer are compensated by the control
loop. The directional coupler converts the forward going power with a
certain ratio into a signal which is rectified by a Schottky diode and a filter
to create a DC voltage. This DC voltage is fed to the error amplifier in
PLUSSA
The error amplifier in PLUSSA compares the detected voltage and the
TXC voltage, which is generated by a D/A converter in COBBA_GJ. This
creates a closed control loop.