Well put Dan.
fyi.
IEEE 1241-2010 describes a few ways to test ADCs.
https://ieeexplore.ieee.org/document/5692956
Online searches find the -2010 version at
http://class.ece.iastate.edu/djchen/ee509/2018/IEEE1241-2011.pdf
The older version, IEEE 1241-2000, is also easy to find on the WWW.
CASPER is definitely not the only group that use high speed ADCs.
https://ohwr.org/projects/adc-testing/wiki/adc-characterization-project
Matt
On Thu, 8 Apr 2021, Dan Werthimer wrote:
Date: Thu, 8 Apr 2021 15:22:11 -0700
From: Dan Werthimer <d...@ssl.berkeley.edu>
Reply-To: casper@lists.berkeley.edu
To: CASPER Mailing List <casper@lists.berkeley.edu>
Subject: Re: [casper] ROACH-2: Identifying Spurs in FFT of Noise Spectrum
hi ben,
to test for ADC spurs, it's best to use a noise source, and inject wide band noise into your ADC,
setting the noise to a similar power level that that which you will use in your experiment.
you'll probably want to run your experiment with a power level (and noise
source) that feeds the adc with an RMS of
around 40 ADC units..
(assuming an ADC range from -128 to +127, and assuming you have no strong RFI
getting into your experiment)
(eg: assuming you are processing a gaussian noise like signal).
you can not test for ADC spurs by terminating the ADC input, because the ADC will chatter between two levels.
(eg: it will usually chatter between 0 and -1, or 0 and +1, depending on the DC offset of the particular ADC chip,
which depends on temperature...).
if the ADC's DC offset is right in between two steps on the ADC staircase, only a few microvolts of RFI will cause
gigantic spurs. if the DC offset is exactly between two ADC staircase steps,
then the RFI will need to be larger to get really big spurs.
in any case, the spurs you see from a terminated ADC won't tell you very much about the spurs you'll
see when you send a noise source or send the signal from your noise dominated experimental source.
i think what you care about is spur to noise ratio, and that is best tested by injecting wide band noise.
best wishes,
dan
Dan Werthimer
Marilyn and Watson Alberts Chair
Astronomy Dept and Space Sciences Lab
University of California, Berkeley
On Thu, Apr 8, 2021 at 1:42 PM 'Benjamin' via casper@lists.berkeley.edu
<casper@lists.berkeley.edu> wrote:
Hi everyone,
This question is a little out of left field, but I know there's lots of
experience in this regime. I'm
working on an experiment looking for a Q~10^6 signal embedded in broad-spectrum
noise. To this end, we are
using a ROACH-2 board with a katADC, to sample the incoming signal, and then
offloading the data to a PC where
a GPU performs a (large) FFT. Any candidate signal will look like excess power
in just a single bin.
(In part) Because of this, we're very concerned with identifying the source
of any spurs that we see in our
FFT spectra. For the past little bit I've been doing some very simple tests
where I terminate the input to the
ADC, take data, do an FFT, and look at the resulting spectrum. An example is
shown below:
AttenTest_0_EnableOff_ClosedTop_2GHz_FFT_Zoom_4-4-21.png
2^26-point FFT sampling at 2 GHz (interleaved mode) zoomed in between 490-510
MHz. For this test, ADC input was
terminated and enable register was set to 0.
When I do this, I get results like the above, where I notice a couple of
things: Firstly, there is a large
clock spur at 500.0 MHz. Ultimately, this will have to be dealt with, but the
source of it is known. Secondly,
there are a bunch of much smaller spurs. These are all separated by ~923 kHz
from each other, and they are seen
across the entire spectral range (from 0 all the way to 1 GHz). Actually, at
lower frequencies you see spurs
every 462-463 kHz.
One further thing I've noticed is that if I set the programmable attenuator to
it's maximum (31 dB), the spurs
disappear but the main clock spur remains at its original amplitude. With all
that in mind, this leads me to a
few questions:
* Has anyone dealt with these spurs before?
* Do you have any hypotheses what their origin may be? I've been concerned
about switching power supplies
(namely the ATX and the DC-to-DC converters) but those are on the ROACH not
on the katADC. Plus, from the
schematics, it looks like there's a lot of filtering happening both on the
ROACH and the katADC.
* Do you have any suggestions for getting rid of these spurs?
Any insight is greatly appreciated.
Many thanks,
Ben
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