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

Don Latham djl at montana.com
Tue Nov 2 05:34:41 UTC 2010


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?
Don
----- Original Message ----- 
From: "Hal Murray" <hmurray at megapathdsl.net>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Monday, November 01, 2010 10:10 PM
Subject: Re: [time-nuts] A real world project need for timing accuracy...


>
> mpb45 at clanbaker.org said:
>>    However, time-of-flight info (via an RF link requires
>>    that I sync the 10MHz clocks at both ends together.
>>    Use of GPS receivers seems to be the most likely way
>>    to do this but how do I keep the cost and complexity
>>    down?
>
> What level of accuracy do you need?
>
> I think GPS will do what you want.  You can get one of the 
> mouse/hockey-puck
> units for well under $100.   I think you need one at each end so that 
> doubles
> the cost.  Be sure to avoid the USB units because they don't have PPS 
> signals.
>
> Option 2:  How long can you coast?  (holdover mode)  If your crystal is 
> off
> by 1E-6 and you coast for 1000 seconds, you will be off by 1 ms.  Is that
> good enough?  You can get better accuracy with better hardware and/or 
> better
> software.  By software I mean 1) correcting for the initial calibration
> error, and 2) correcting for the temperature.  [It would be fun to see 
> what
> you could do starting with an inexpensive oscillator package.]  You can
> double-check and/or get more data for correction if you bring the unit 
> back
> to the base station at the end of the run and see how far it has drifted.
>
> Option 3:  Use a bi-directional RF link.  Send a "PPS" signal to the gear 
> at
> the target.  I think you will have to calibrate out the delays through the 
> RF
> links but that should be simple if you assume symmetry.
>
> What sort of RF gear are you considering?  Are the normal units both 
> transmit
> and receive, or do you have to buy another pair for the other direction?
>
>
> Option 4: Send the PPS signal upstream and compensate for the clock drift 
> at
> the upstream end.  Again, I think you may have to calibrate for the delays
> through the RF link which may vary slightly with distance and antenna 
> setup.
>
>
>
> PS: How accurately do you measure the distance?
>
>
> -- 
> These are my opinions, not necessarily my employer's.  I hate spam.
>
>
>
>
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