[time-nuts] A real world project need for timing accuracy...

[Hal Murray]

djl at montana.com said:
> I guess i don't understand. If you start a counter with the start pulse from
>  the local chrono, and send back a stop pulse from the start pulse of the
> distant chron over the zigbee link using an interrupt on the distant
> controller, the response time can be subtracted. Simple calibration of the
> electronics delay should be measureable? 

I was assuming a dumb/simple RF link, either on-off keying or FSK, with low 
power and low baud rate and that 800 m would be pushing the operating range.

I was assuming that the filters would take longer to respond at (very) low 
power levels so you couldn't calibrate the delay when the transmitter was 
next to the receiver and expect to get the same answer when they were 800 
meters apart.

This may not be significant.  Ballpark worst case delay would be the baud 
rate, 1 ms at 1200 and 100 microsec at 9600.  Maybe more for the initial 
carrier turn-on until the AGC settles in.


Has anybody measured the delay through the Xbee units?  They are totally 
different type of operation with lots of opportunity for delays due to 
retransmission and such (I think).  [I guess I'll add that to my list.]

The Xbee Pro units claim to have a range of a mile.  Has anybody tried them?

I just took at look at round trip times using WiFi.  Lots of hits around 3 ms 
with obvious noise up to 5 ms, then a band at 10 ms and another fuzzy band at 
20 ms.  That's from a PC over Ethernet to an AP, then WiFi to a laptop.  I 
assume some of the noise is ARP and such.



-- 
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