[time-nuts] Phase Noise and ADCs

[Bob kb8tq]

Hi



> On Sep 26, 2020, at 7:22 PM, Magnus Danielson <magnus at rubidium.se> wrote:
> 
> Hi John,
> 
> On 2020-09-26 17:10, John Ackermann N8UR wrote:
>> We know that phase noise scales with frequency, so if you multiply
>> frequency by 10 you get a 20 dB increase in noise.
>> 
>> What I don't fully understand is how that relationship works with
>> other than simple multiplication/division.
>> 
>> For example (and my real life concern), if I have an analog to digital
>> converter that is clocked at 122.88 MHz and know the phase noise of
>> that clock signal, what do I know about the effective phase noise when
>> the ADC is receiving a signal at, e.g., 12.288 MHz?
>> 
>> In other words, if I were to measure the phase noise at the output of
>> the ADC when fed a high-enough quality 12.288 MHz signal, would I see
>> something like the 122.88 MHz phase noise, or something better due to
>> the scaling by 10?
> 
> In this case, your 12.288 MHz phase-noise will be augmented with the
> scaled-down version of the 122,88 MHz phase-noise. The trick being used
> is to actually let the sampling clock of the ADC be a transfer clock
> such that it samples a reference also, at which time one subtracts the
> phase-data from the reference, most of the transfer clocks noise cancels
> out, and it does so fairly well as it have common integration time
> between the channels, so you avoid the decorrelation that DMTD normally
> suffers from. The second trick being used is to use a second pair of
> ADCs to make ADC deficiensies cancel out too, as the DUT and REF is
> common mode and the individual ADC noise contributors can be averaged out.
> 

If you are going to do the “cancel out” process, a dual ADC with a common 
clock interface is a really good idea. Not all dual ADC devices have that
feature……

Bob


> Now, as you decimate data etc. you still have the issue of ADC
> resolution, as that has a tendency to loose weak side-bands. The
> non-linearity is kind of peculiar actually.
> 
> I recommend you to dig up Sam Stein's papers on his phase noise
> development. I think you will find them a good read and help you with
> your understanding.
> 
> Cheers,
> Magnus
> 
> 
> 
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