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

[Erik Kaashoek]

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.