[time-nuts] Re: What phase variations to expect in a DMTD due to temperature fluctuations?

[Erik Kaashoek]

Hi Carsten,
In my DMTD I use a PLL to lock the LO of the analog down mixers to the 
reference input. In the digital domain the I/Q mixer has a fixed LO 
converting to an fb of zero Hz (kept at zero with the PLL) and the 
output of the mixer is averaged as decimation.
This should (I hope) also avoid spectral leakage, just like you did with 
the NCO. I've tested this by adding/removing a window function and this 
did not make any difference, as long as the fb was kept at zero Hz. The 
further processing is done as you describe in the second part of your 
reaction.
The problem with my DMTD seems to be not spectral leakage but the 
limited isolation between the two inputs (only about 80dB isolation) so 
the DUT input leaks into the REF input and because the frequency 
difference is small the downconverted DUT leakage is not filtered out by 
the averaging. Adding/removing the windowing did not make any difference 
for this DUT leakage.
But as the leakage is linear it may be possible to model the leakage and 
compensate for it.
Erik.


On 25-10-2022 9:43, Carsten Andrich wrote:
>
> Hi Erik,
>
> spectral leakage only occurs with the DFT due to its implicit 
> rectangular window applied to the input samples. The appeal of the 
> digital down conversion (DDC) implemented with the NCO is that 
> coherence does not matter and spectral leakage does not occur. It's 
> simply a perfect, digital implementation of the down mixing you 
> perform in the analog domain. Its purpose is to shift the frequency of 
> the signal to enable reduction of the sample rate to reasonable levels 
> (a few kSa/s instead of >= 25 MSa/s) via decimation. The decimation 
> can also be seen as averaging, so this approach also uses every 
> available sample.
>
> Additionally, the use of complex down-conversion enables 
> straightforward phase estimates. Relative to the sampling clock by 
> taking the phase angle of the complex samples of a single channel. 
> Between two channels by taking the phase angle of fraction of two 
> channels' complex samples. If you unwrap the phase angles and apply 
> linear regression, you can compute the average frequency difference.
>
> Best regards,
> Carsten
>
>