PmWiki | Systems / DFB

Input requirements

There are four input ports (A,B,C,D). Normally A & B are the two orthogonal polarisations from the astronomy receiver. C & D may be used for an additional pair of astronomy inputs or for reference signals for RFI mitigation.

The input signals must be band limited to between m*fn and (m+1)*fn, where fn is the Nyquist frequency for the sampling and m is an integer 0, 1, 2 , 3 .... The input bandpass should be at least 15 dB down at these frequencies. Ideally, the bandpass should be level within +/- 1 db across 90% of the band, but differences of up to 10 db are OK provided data calibration is done carefully. The digitiser is set up so that the Nyquist frequency is equal to the chosen bandwidth. Except for the 1 GHz bandwidth (for which m must be zero), usually it is best to choose m = 1.

For normal operating, the input level is -20 dBm. The amplifiers should have a saturation level about 10 db higher than this, ie -10 dBm or so. It shouldn't be too much higher as the digitisers can be damaged by excessive input levels.