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

[zfe]

I did, up to now, not experience a loss in measurement resolution  using
different frequencies for DUT and REF.

The first picture is a 10MHz DUT vs two 16.384MHz REFs. The picture
using 10MHz Refs looks alike (not attached).
The second is a 22.5792MHz NZ2520SDA as DUT vs two 10MHz REFs (with PN
of -150dBc for higher frequencies). It looks quite close data in the
data sheet. The differences are surely more due my not very perfect
measurement setup.

The sampling clock for the ADC comes from a free running SI5345
evaluation board. Its phase noise is decent for high frequencies, but
not for the frequencies measured with Andrew's setup (<1MHz).


Am 07.10.22 um 18:26 schrieb Erik Kaashoek via time-nuts:
> 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.
>>
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