[time-nuts] Looking for RTC or some combination of MPU/crystal for 0.1ppm accuracy.

WarrenS warrensjmail-one at yahoo.com
Sun Sep 13 00:30:23 UTC 2009

It seems like you have given plenty of information for me to give you two possible suggestion.

Here is an almost no extra cost solution, (just takes care and helps if you have done it before)
Take a pretty standard xtral osc with EFC,  buffer its output real good, 
give it good clean power from preferable its own regulator, 
put a little insulted house around it so that it is not effected by direct air flow and quick temperature changes, 
Add a processor controlled PLL a driving a Dac to control it freq, 
Update the PLL loop with the time beacon, (assuming it is good to better than 1us) to correct the Phase offset error, 
and it is pretty easy to get much better than 0.1 PPM with this.
I did about the same thing using the 1PPS of my GPS and could get around 1e-8 accuracy. 
The most important things are if using a cheap $5 osc like I was is to isolate real well it from the effects of PS, 
load and temperature, and have of low noise reference voltage to provide a settable coarse freq tuning and use the Dac just for fine adj.

Or of course you could just buy a better osc TCXO and save a lot of development time, but that's not as much fun.
Depends how may you want to build, Anything below say 50 units the TCXO is going to be the cheaper solution if time is money.


to answer you question, which is 
----- Original Message ----- 
From: "Hal Murray" <hmurray at megapathdsl.net>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Saturday, September 12, 2009 2:55 PM
Subject: Re: [time-nuts] Looking for RTC or some combination of MPU/crystal for 0.1ppm accuracy.

> rickharold at gmail.com said:
>> I need to find a solution for a RTC which provides 0.1 ppm accuracy in
>> the form of a RTC/MPU or some other combination, oscillator etc.. I'm
>> creating some sensors which all need to do timed measurements and need
>> to be at the same time.  They can get a time beacon every 10 seconds
>> but then need to accurately perform independently 20x / sec.  They
>> cannot communicate with each other and other than the time beacon are
>> unconnected.  After a period of time they will send their time-stamped
>> data out.
> What do you mean by "0.1 ppm accuracy"?  I'm assuming you mean over 
> temperature changes.
> How accurate is your beacon?  What sort of accuracy do you need?
> Assuming that you don't have wild temperature swings...
> Just calibrate whatever crystal you would normally use.  It will be off by 
> many ppm, but at the ppm level, it's stable at a given temperature.  Using 
> your 10 second beacons, you can measure the actual frequency and correct in 
> software.  It will change with temperature, but that will be slow enough so 
> you can track it.
> -- 
> These are my opinions, not necessarily my employer's.  I hate spam.

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