[time-nuts] ADCs for phase noise measurement

Richard (Rick) Karlquist richard at karlquist.com
Sat Jun 11 14:53:22 EDT 2016

On 6/11/2016 6:33 AM, Attila Kinali wrote:
> Hoi Rick,
> I don't know which phase noise measurement sturcture you are going
> to use exactly or what your goals are, so I am guessing here a little bit...

I'm interested in at least 10-100 Hz offset, 1-1000 Hz offset
would be a bonus.  Thus audio is fine on the high end (even 20 kHz
is overkill) but the low end is an issue.

> Alternatively, I would suggest using one of the modern sigma-delta
> or SAR ADCs, which can deliver increadibly high ENOB and SNR at
> astonishing sampling rates. Good candidates might be:
> AD7982, 18bit 1Msps
> AD7984, 18bit 1.33Msps
> LTC2378-20, 20bit 1Msps
> LTC2368-24, 24bit 1Msps
> Eval boards for these are available (between 100 and 200€) and interface
> with SPI. You can either use an USB SPI dongle (between 5 and 50€) or
> use a small uC board to interface with the PC. Saving the samples in
> a wav file and using one of the many FFT tools shouldn't be a problem.

Good call on the LTC2368-24.  The eval board for it has considerably
better support (like p-scope) than do the Analog Devices eval boards.
I think this might work for me.  What are the tradeoffs between SAR
and sigma-delta ADC?  Can you point me to a few of the "many" FFT
tools?  Maybe I don't have to buy spectrum analyzer software.



> If you don't mind designing your own board, then you can go for something
> like the LTC2386-18 which offers 18bit at 10Msps (and still 96dB SNR)
> and enjoy a very wide frequency range. But the requirement of LVDS input
> might be a killer.
> 			Attila Kinali

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