Hi Josh,
Are you looking at the time domain data (i.e. through a adc snap
block), or at the frequency domain data (i.e. final output)?
If you're looking at the frequency spectrum output, the PFB visibly
scallops the data coming out of that roach spectrometer tutorial - so
if you're choosing frequencies that don't lie in the middle of a bin
you'll see that they're significantly lower. This confused me for a
while at least ;)
I've got a script and spectrometer bof file which I was using for
calibration but never quite finished. I was controlling a signal
generator through GPIB and pyVISA, then getting the spectrum from the
ROACH. Let me know if you're interested and I'll see if I can dig them
up.
Cheers
Danny
On 25 Jun 2010, at 14:29, Jon Losh wrote:
Hi,
I'm working with a group under Professor Tegmark at MIT. We've been
messing around with the roach as a spectrometer (tutorial 3) and
we're trying to get the unit conversion from the strange power-like
units the roach spits out to dBm. I looked at the datasheet for the
ADC, and it seems like the ADC accepts inputs from [-1, 1] V and
digitizes them. We then retrieve this data off the roach, and it
seems like it's scaled, but we can't figure how.
Also, the input voltage at which the ADC clips seems to depend on
input frequency, as seen in the attached plot. To make it, for each
frequency, we ramped up the power of the input sinusoid until the
ADC clipped. I guess there might be some resonances where the input
voltage is low?
<clipping.pdf>
