[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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