[time-nuts] Re: Testing frequency pulling on a DYI counter

[Magnus Danielson]

Erik,

Which algorithm of linear regression do you use? Describe the 
cancellation details.

I just want to be sure we talk the same language here.

Have you seen this? https://arxiv.org/abs/1604.01004

Cheers,
Magnus

On 8/4/22 20:04, Erik Kaashoek wrote:
> Bob, Magnus,
>
> Using a second counter (my famous Picotest U6200A) locked to the 
> reference output of the DIY counter and measuring the output of the 
> signal generator and also set to gate of 10 s it is confirmed that the 
> frequency pulling (if any) is below 1E-11 (not more digits on the 
> display of the U6200A)
> Generator is set to 10.000,000,000,2 MHz and is measured as such by 
> the U6200A
> As there seems to be no frequency pulling I went back to the 
> simulation of the linear regression algorithm and discovered that when 
> there is a integer  divide/multiply relation between the internal 
> reference and the measured frequency the regression looses some accuracy.
> For sure if the reference is close to an integer multiple of the 
> measured frequency (10 Mhz measured -> 200 MHz reference) the 
> regression collapses completely in accuracy. I hoped that by creating 
> a fractional relation this collapse would not happen at 10 MHz but is 
> still there, although much smaller. For this test I'm using a "div 3 
> times 64 e.g. 213.333,333,333,333... MHz" internal reference frequency 
> derived from the external 10MHz reference. Ton van Baak warned me 
> against using fractional relations in a counter but otherwise it is 
> impossible to measure a 10 MHz input signal with any accuracy without 
> a HW time to digital as the interpolation no longer works. I can 
> switch dynamically to 200 MHz or 245 MHz reference and these produce 
> much much worse results.
> I realize this test only measures if the TCXO used as reference in the 
> DIY counter does not show frequency pulling but it does not show if 
> the PLL used to convert the 10MHz to 213.333333333... MHz for the 
> internal counters shows any frequency pulling.
> NOt
> Erik.