User Guide

PAMS
Technical Documentation
NSE–8/9
System Module
Page 2– 80
Issue 1 07/99
Next stage in the receiver chain is an AGC-amplifier. It is integrated into
the SUMMA. The AGC gain control is analog. The control voltage for the
AGC is generated with a DA-converter in the COBBA in baseband. The
AGC-stage provides an accurate gain control range (min. 57 dB) for the
receiver. After the AGC-stage, the 71MHz IF-signal is mixed down to
13MHz. The needed 58MHz LO signal is generated in the SUMMA by
dividing the VHF-synthesizer output (464 MHz) by eight.
The following IF-filter is a ceramic bandpass filter. It attenuates the signals
in the adjacent channels, except for those separated +/- 200 kHz relative
to the carrier. Very little attenuation is achieved for those signals in the
filter, but they are filtered digitally by the baseband. Because of this the
RX DACs has to be so good, that there is enough dynamic range for the
faded 200 kHz interferers. The whole RX has to be able to handle signal
levels in a linear way too. After the 13 MHz filter there is a buffer for the
IF-signal, which also converts and amplifies the single–ended signal from
filter to a balanced signal for the buffer and AD-converters in the COBBA.
The Buffer in the SUMMA has a voltage gain of 36 dB and the buffer gain
setting in the COBBA is 0 dB. It is possible to set the gainstep (95 dB) in
the COBBA via the control bus, if needed.