[time-nuts] Re: Do time nuts only use and measure 10MHz clocks?

[Erik Kaashoek]

Yes, I took inspiration from Andrew's design, although using substantial 
less HW power.
After implementing the ability to have two different input clocks I 
discovered (yes, I could have known before implementing) that as soon as 
you use two different frequencies for the two LO's, used for down mixing 
to the ADC input , you loose coherence, even if they are derived from 
the same clock, because they mix different frequencies into a common IF 
frequency so the difference in LO frequencies causes different impact of 
the phase noise into the IF frequency.
So, different input frequencies for ref and DUT are possible but the 
resolution is destroyed by the phase noise of the internal reference 
TCXO. Maybe I could use the ref input as reference for the internal PLL 
to eliminate the TCXO, something to test.
Erik.

On 7-10-2022 12:00, zfe via time-nuts wrote:
> Do you know Andrew Holme's phase noise project:
> http://www.aholme.co.uk/PhaseNoise/Main.htm
> There it is possible that the DUT and references have different
> frequencies up to a factor 4. Even differnt frequencies for the two
> references would be possible, but that is not yet implemented (it would
> need a second NCO in the FPGA - I did ask for that in a future version
> of the software).
>
> I measured clocks for audio puposes (24.576MHz, resp 22.5792Mhz and
> multiples). My OCXO with best close-in phase noise have 16.384MHz. So
> this freedom in choice of frequencies is very helpfull.
>