[time-nuts] Question about frequency counter testing

Oleg Skydan olegskydan at gmail.com
Tue May 15 07:47:59 EDT 2018


From: "Bob kb8tq" <kb8tq at n1k.org>
> What I’m suggesting is that if the hardware is very simple and very cheap, 
> simply put two chips on the board.
> One runs at Clock A and the other runs at Clock B. At some point in the 
> process you move the decimated data
> from B over to A and finish out all the math there ….

The hardware is simple and cheap cause it is all digital, requires no 
calibrations and the same HW is capable of driving TFT, UI handling, 
providing all control functionalities for input conditioning circuits, GPS 
module and etc. It also  provides USB interface for data exchange or remote 
control. So doubling it is not a way to go if I want to keep things simple 
and relatively cheap.

I think I will stay with the current plans for HW and try to handle some 
troubles to GPS timing in software. I have to make initial variant of HW, so 
I will be able to move on with the SW part towards useful counter. Then I 
will see how well it performs and will decide if it satisfies the 
requirements or I need to change something.

>> BTW, after quick check of the GPS module specs and OCXO's one it looks 
>> like a very simple algorithm can be used for frequency correction. OCXO 
>> frequency can be measured against GPS for a long enough period (some 
>> thousands of seconds, LR algorithm can be used here also) and we have got 
>> a correction coefficient. It can be updated at a rate of one second 
>> (probably we do not need to do it as fast). I do not believe it can be as 
>> simple. I feel I missed something :)…
> That is one way it is done. A lot depends on the accuracy of the GPS PPS 
> on your module.

The module is uBlox NEO-6M, I know there is better suited for my needs 
NEO-6T, but the first one was easy to get and insanely cheap. It should be 
enough to start.

> More or less, with a thousand second observation time you will likely get 
> below parts in 10^-10, but maybe not to the 1x10^-11 level.

1e-10 should satisfy my requirements. More sophisticated algorithm can be 
developed and used later, if needed.


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