hi jon,
there are a couple of possibilities that might explain
the frequency response you are seeing:
1) the ADC2x1000-8 board has poor termination.
(the input impedance is not 50 ohms at all frequencies).
this causes reflections, which depend on cable length
and frequency. you can reduce the reflections by
placing an attenuator on the ADC input.
(use an SMA attenuator, and connect it directly
to the SMA input of the ADC).
you can also reduce low frequency resonances by using a short cable.
2) as danny pointed out,
in your tests injecting a pure sine wave, you need to make sure
the frequencies you select are always exactly centered in the
middle of the PFB/FFT spectral bins. it's best to phase lock the
signal input synthesizer to the same synth that's driving the
adc sample clock. otherwise, you are sensitive to the scalloping
(there's a 3 dB drop if you are exactly between two spectral bins).
best wishes,
dan
On 6/25/2010 6:29 AM, 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?
