rfi  blankers


07feb06: radar multiplexor . calixto's box.

Bill sisk's radar blanker manual (.pdf)



    Radars are pulsed signals with low duty cycles. An rf pulse is transmitted and then the radar waits for the return echo. At the start of the next ipp another rf pulse is transmitted. The rf pulse and the echo's cause rfi in our receiver system.
    A radar blanker synchronizes with the ipp of a  radar and generates a blanking pulse that is high while the radar rfi is present in our system. This duration depends on how much multi path scattering and echoes there are.
    The observatory has a radar blanker for the airport "FAA" radar and for the iridium satellites (they can be considered radars since they have an 50% uplink, downlink duty cycle).  The blanking signals are sent to the interim correlator and to the wapps where it temporarily stops the lag accumulators.

Iridium blanker:

    The iridium signal   has a 50% uplink/downlink duty cycle (45 ms downlink, 45 ms uplink). Since there are no iridium phones near the observatory, only the downlink cycle causes rfi. Iridium has supplied us with a radar blanker that is synchronized with this 90 ms period. It generates a square output pulse.
    The "blank" signal is currently the uplink signal. You must send the /blank signal to the blanker mux.

Radar blanker multiplexor:

    This takes the iridium and faa blanking signals and multiplexes one of them to the interim correlator and wapp.
It is currently under test. It also contains the circuitry to extend the iridium blanking signal.


07feb06: radar multiplexor . calixto's box.

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