[time-nuts] Phase Noise and ADCs

[jimlux]

On 9/26/20 8:10 AM, 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?

To a first order, the ADC is like an ideal multiplier/mixer - phase 
noise on the clock contributes to phase noise on the sampled data by 
reciprocal mixing, just like a mixer.



> 
> 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?
> 
> Thanks!
> 
> John
> 
> 
> 
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