I support what Dan and Danny have said here. It's likely that I've used
PlanAhead more than anyone else in CASPER and have never experienced this
problem ("Use of planahead reduces system dynamic range"). Here are things
I *have* seen:
1. slow designs don't meet timing on some ADC input bits -> when signals
with enough input power toggle those bits errors occur
2. Some ADC (katadc I think?) has a SPI interface to control various
settings. I couldn't make it clock faster than ~280MHz (even though the
rest of the design was at 325). Since ADC configuration usually seems to
survive an FPGA reprogram, we had to configure the ADC with a slower design
and then reprogram to the faster one
3. Design didn't meet timing. ADC samples came out right, but started
seeing errors when I cranked up the input power. I have a theory involving
propogation delays into upper bits which describes this problem, but it's
not going to fit into this email
I would suggest adding snap blocks to the ADCs for the two designs, so that
you can get raw adc samples. Then pick the "working" design, and make sure
you can produce adc sample waveforms which look right (full dynamic range
exercised, waveform is of correct period). Then now that you have the snap
decoding scheme correct, go to the non-working design and check to see if
the raw samples coming in are good as well. I'm betting that you'll find
your problem there.
Cheers
--Ryan
On Tue, Sep 16, 2014 at 10:44 AM, Raul Sapunar Opazo <[email protected]>
wrote:
> Danny,
>
> I am actually using that ADC on a Roach 2 rev 2 board... but Iam using the
> same adc calibration for all my spectrometers.. and as I said before, for
> the one that planahead was not needed, the dynamic range is fine..
>
> Regards,
> Raul
>
> 2014-09-16 14:13 GMT-03:00 Danny Price <[email protected]>:
>
> Hi Raul
>>
>> As Dan mentioned, planahead shouldn't make a difference to things like
>> dynamic range. If you're using an ADC that interleaves several cores (eg
>> 5GSPS ADC), this could be something to do with poor ADC calibration. I'd
>> suggest looking into that as a possible cause?
>>
>> Regards
>> Danny
>>
>>> Raul Sapunar Opazo <mailto:[email protected]>
>>> September 16, 2014 at 10:21 AM
>>>
>>> Hello everyone,
>>>
>>> I used planahead to fix the timing problems of a 1.8Ghz 32k channel
>>> spectrometer in roach 2. I did it without troubles, using 38% of the DSP
>>> and less of the 50% of RAMs..
>>>
>>> After I compiled the model with the new constrains/placement and I
>>> tested the bof, I found out that the numeric noise increased a lot as the
>>> power of the harmonics...
>>>
>>> This issue results in a big decrease of the dinamic range of the
>>> spectrometer in the whole BW (around 25 db against 40-45 db of a 1.8Ghz 16
>>> k ch spectrometer that didn't need planahead)
>>>
>>> I encountered this problem with an 1.8GHz 8k ch aswell.. so the issue is
>>> not related with the increase of the number of channels..
>>>
>>> Anyone encountered this kind of problems when using Planahead? As I
>>> understand planahead shouldn't change the performance because it doesn't
>>> make any change in the designs...
>>>
>>> Any help will be appreciated!
>>>
>>> Best Regards,
>>> Raul Sapunar
>>>
>>
>
