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

Don Latham djl at montana.com
Tue Nov 2 08:38:40 UTC 2010


Oh heck, double duh. Send the data back on the same optical link. Duh again.

----- Original Message ----- 
From: "Don Latham" <djl at montana.com>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Tuesday, November 02, 2010 2:36 AM
Subject: Re: [time-nuts] A real world project need for timing accuracy...


> Or, happy thought, simply use a pointer laser at the target end, triggered 
> by the target chrono, and received at the bench chronograph. Simple, and 
> the remote stop pulse easy to pick up with simple optics. Heck, use an old 
> scope. Shoulda thought of that first.
> Even a picaxe will give you sufficient accuracy, and the 18x has an IR 
> input.
> Don
>
>
> ----- Original Message ----- 
> From: "Don Latham" <djl at montana.com>
> To: "Discussion of precise time and frequency measurement" 
> <time-nuts at febo.com>
> Sent: Monday, November 01, 2010 11:34 PM
> Subject: Re: [time-nuts] A real world project need for timing accuracy...
>
>
>>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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>>
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