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

[Erik Kaashoek]

Corby,

I'm struggling to understand.
Suppose the single internal reference of a DMTD is 10MHz and 0.1Hz high 
due to phase noise
Now take two inputs, one at 10MHz, 0.3Hz high and one at 20MHz, 0.4Hz high
For simplicity assume the internal IF is zero Hz so we can have negative 
frequencies.
The 10MHz input gets mixed with the internal reference producing 
negative 0.2 Hz
The 20Mhz input gets mixed with two times the single internal reference, 
resulting in 2 * 10Mhz at 2 * 0.1Hz to high being 20MHz at 0.2Hz high 
and after mixing this results also in a negative 0.2Hz.
The measured difference between the two input signals is now zero Hz.
How can I remove the offset of the internal reference to get to the 
actual 0.1Hz difference when the phase noise is multiplied with 
different factors because the need to match the different frequencies of 
the two inputs?
Erik.

On 7-10-2022 18:00, Corby Dawson via time-nuts wrote:
> Erik,
>
> When using a common clock into a DMTD it is fairly easy to make sure your
> DUT and REF are close to in phase at the start of a measurement.
> This allows the cancelling of noise from the offset oscillator.
> However if you use separate clocks of different frequencies you will lose
> that advantage.
> For measurements that don't need ADEV better than parts in the -11 or -12
> range it would probably work, but a trial would tell the tale.
>
> Cheers,
>
> Corby
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