[time-nuts] Phase Noise and ADCs

[jimlux]

On 9/26/20 9:56 AM, Bob kb8tq wrote:
> Hi
> 
> In addition, the input to the ADC has it’s own noise issues. If you have a really
> clean clock (or a poor ADC), the noise floor of the input may dominate the noise floor.
> 
> Bob
> 

Yes - and typically, this spec is not given in a useful fashion for 
time-nuttery.  You have to test it and see what it is.

for instance, the AD9650 has a specified aperture uncertainty specified 
as jitter of, say, 0.08 picosecond, rms.

And they don't specify what the clock input bandwidth is.




>> On Sep 26, 2020, at 12:28 PM, jimlux <jimlux at earthlink.net> wrote:
>>
>> 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
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at lists.febo.com
>>> To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
>>> and follow the instructions there.
>>
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at lists.febo.com
>> To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
>> and follow the instructions there.
> 
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.
>