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rfi blankers

feb06

07feb06: radar multiplexor . calixto's box.

Bill sisk's radar blanker manual (.pdf)

Terms:

rf pulse length: duration of transmitted pulse from radar
ipp: Inter Pulse period. The time from the 1st rf pulse of a sequence to the 1st rf pulse of the next sequence. During this time the rf pulse goes out and all of the echoes are returned.

Intro:

    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.

Testing:

07feb06: radar multiplexor . calixto's box.

The iridium blank output from the iridium supplied box is synced with the uplink cycle. You must pass the /blank to the multiplexor since we want to blank the downlink (and positive is blank).
The front panel test point for the multiplexor can not drive 50 ohms. If you plug a 50 ohm load into it, the blanking signals to the wapp and the interim correlator will fail. This needs to be buffered and a 50 ohm driver added. Someone should also check the drivers to the wapp and the interim correlator.
The "modified iridium" mode is supposed to change the duty cycle so that more than 50% of the time is blanked. Currently the device is making the "positive" pulse narrower. This makes the blanking period shorter, not longer.

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