User Guide

PAMS
System Module DB6
NHB–3
Technical Documentation
Original 26/97
Page 4–44
TRANSMITTER
Modulator Circuit, TX part of CRFRT
The modulator is a quadrature modulator contained in Tx–section of CRFRT
IC. The I– and Q– inputs generated by RFI interface are DC–coupled and fed
via buffers to the modulator. The local signal is divided by two to get accurate
90 degrees phase shifted signals to the I/Q mixers. After mixing the signals are
combined and amplified with temperature compensated controlled gain amplifi-
er (TCGA). Gain is controlled with power control signal (TXC). The output of the
TCGA is amplified and the maximum output level is –10 dBm, typically.
Upconversion mixer
The upconversion mixer is a single balanced passive diode mixer. The local
signal is balanced by a printed circuit transformer. The mixer upconverts the
modulated IF signal coming from quadrature modulator to RF signal.
TX Interstage Filters
The TX filters reject the spurious signals generated in the upconversion mixer.
They reject the local, image and IF signal leakage and RX band noise, too.
1st TX Buffer
The TX buffer is a bipolar transistor amplifier. It amplifies the TX signal coming
from the upconversion mixer.
2nd TX Buffer
The TX buffer is a bipolar transistor amplifier. It amplifies the TX signal coming
from the first interstage filter.
3rd TX Buffer
The TX buffer is a bipolar transistor amplifier. It amplifies the TX signal coming
from the second interstage filter.
Power Amplifier
The power amplifier is a two stage discrete amplifier. It amplifies the 13 dBm TX
signal to the desired output level. It has been specified for 6 volts operation.
Power Control Circuitry
The power control loop consists of a power detector and a differential control
circuit. The power detector is a combination of a directional coupler and a
diode rectifier. The differential control circuit compares the detected voltage