[time-nuts] ADCs for phase noise measurement (was: windows for FFT measurements of phase noise)

Attila Kinali attila at kinali.ch
Sat Jun 11 09:33:30 EDT 2016

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...

On Fri, 10 Jun 2016 17:15:22 -0700
"Richard (Rick) Karlquist" <richard at karlquist.com> wrote:

> I am now putting together a modern homebrew phase noise system.
> I am planning to use a ZRPD-1 phase detector driven by AMC-123
> amplifiers.  The phase detector will drive a SpectraDAQ-200 digitizer
> with SpectraPlus-RT FFT software.  

>From the specs of the SpectraDAQ-200 it looks like they just employ some
audio codecs for the sampling. This has two consequences major consequeces:
1) The performance below 10-20Hz will be bad (yes, they spec the cut of
frequency as 2Hz, but the audio ADCs are usually not spec'ed below 20Hz).
2) Due to the way these ADCs are built, the noise above 20kHz will be
a lot higher than below (aka noise shaping).

If this is ok for you and you actually want to use an audio ADC,
but then I would recommend using one of the USB "soundcards" out there.
I doubt that the SpectraDAQ-200 is anthing more than an implementation
of the evalboard circuit with BNC connectors.
You can get very good soundcards already for 30€. 100€ should already buy
you one that is about as good as these things can get. The only thing
you need to have an eye on is, if you need two inputs that the microphone
input is stereo, which should be the case, but is not necessarily given.

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.

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

Malek's Law:
        Any simple idea will be worded in the most complicated way.

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