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Timing / Re: Pulse collision avoidance
« on: June 13, 2018, 11:34:54 AM »
Hello Daniel,
sorry for the late reply - I just saw your questions now ....
In Graz, we have tried to optimize this overlap avoidance procedures / minimize the rep rate losses, using the FPGA on our home-made PC card:
- This PC card creates the laser firing pulses, and triggers the HiQ laser
- The FPGA stores all future return event times of just fired laser pulses (sseveral 100 events for up to GEO satellites) in a FIFO
- On each laser start pulse event, the FPGA checks the time difference between this laser firing epoch, and the next return epoch time
- If the next return epoch time is closer than 100 µs (can be adjusted...), one additional 100 µs delay is inserted before the next laser fire command,
followed then again with laser firing commands in the usual 500 µs intervals (then referred to this first delayed one)
- When the echo of this first delayed pulse arrives, and the overlap conditions is still alive, once again a 100 µs delay is inserted .... etc....
as long as the overlap conditions is valid
- This procedure reduces the rep rate only during overlap, and only by a very small percentage (e.g. from 2 kHz to 1960 kHz or so; depends on satellite)
This overlap avoidance procedure can be enabled / disabled by setting a control bit via PC.
Works perfect for 2 kHz (and a bit higher); for > 10 kHz, the 50% operation might be necessary, but this is a rather high price ... it might be better to just neglect overlaps, which could be acceptable in view of much lower energy per pulse (i.e. less backscatter).
Georg
sorry for the late reply - I just saw your questions now ....
In Graz, we have tried to optimize this overlap avoidance procedures / minimize the rep rate losses, using the FPGA on our home-made PC card:
- This PC card creates the laser firing pulses, and triggers the HiQ laser
- The FPGA stores all future return event times of just fired laser pulses (sseveral 100 events for up to GEO satellites) in a FIFO
- On each laser start pulse event, the FPGA checks the time difference between this laser firing epoch, and the next return epoch time
- If the next return epoch time is closer than 100 µs (can be adjusted...), one additional 100 µs delay is inserted before the next laser fire command,
followed then again with laser firing commands in the usual 500 µs intervals (then referred to this first delayed one)
- When the echo of this first delayed pulse arrives, and the overlap conditions is still alive, once again a 100 µs delay is inserted .... etc....
as long as the overlap conditions is valid
- This procedure reduces the rep rate only during overlap, and only by a very small percentage (e.g. from 2 kHz to 1960 kHz or so; depends on satellite)
This overlap avoidance procedure can be enabled / disabled by setting a control bit via PC.
Works perfect for 2 kHz (and a bit higher); for > 10 kHz, the 50% operation might be necessary, but this is a rather high price ... it might be better to just neglect overlaps, which could be acceptable in view of much lower energy per pulse (i.e. less backscatter).
Georg