Pulse collision avoidance

[danielhampf]

Hello everybody,
I am currently re-working our trigger hard- and software and I was wondering about pulse collision avoidance schemes at high rep rates. Currently, we use either:
1) Standard continuous triggering, ignoring pulse collisions. Works okay up to ~5 kHz in our configuration.
2) Burst mode: One ToF fire, one ToF quiet/receive. Works well at all rep rates, but you always loose half of the time.
I suppose for the really high rep rates, burst mode is about the best we can do. But what about 1 to 10 kHz? Do you have any recommendations of pulse collision avoidance schemes? I suppose we could trigger at, say, 5 kHz and reduce to 4.5 kHz every time the ToF come close to N x LI (laser interval). But is that the best we can do? What do you use? What would be an ideal scheme?
Thanks,
Daniel

[Georg]

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