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Strange amplitude (and phase) frequency response of the 4dsp FMC116
2 CommentsSorted by Oldest First
I agree, this is a strange response!
I have a few questions to help me better understand what you are seeing.
You are injecting two identical signals into Ch1 and Ch2, and are seeing a large difference in amplitude between the 2 channels correct?
You are concerned about what appears to be phase shifts/steps in the sine waves correct? (or is that an equipment artifact?)
Does the reference app show the same thing?
Can you please run the FMC116 reference app with a similar input signal and send me a screen shot of the Fmc116APP as well as (all 16) output files?
The lower amplitude on the second channel is suspicious, the factory test runs a tone thru all 16 channels, that low a level would clearly fail.
Is it possible that you are looking at an open channel and seeing cross-talk and not the real signal? possibly data from Ch0 & Ch1 vs Ch1 &Ch 2?
This could be verified with the reference app which would store data from all 16 channels at once.
Have you verified your source waveform with an oscilloscope?
What is the DC offset of your signal as measured across a 50 ohm load? if it is near zero everything should be fine.
Have you tried swapping the cables and does the problem stay the same or track the signal source?
have you tried injecting a fixed frequency signal, and does the collected data look clean? on both channels?
Assuming the generators DC offset is near ground, The phase shifts in plots does not look line an analog problem to me, but more like
either missing data, (either words or bits) or a data processing error, unless its a generator artifact.
Respectfully
Richard Mountain
I have a few questions to help me better understand what you are seeing.
You are injecting two identical signals into Ch1 and Ch2, and are seeing a large difference in amplitude between the 2 channels correct?
You are concerned about what appears to be phase shifts/steps in the sine waves correct? (or is that an equipment artifact?)
Does the reference app show the same thing?
Can you please run the FMC116 reference app with a similar input signal and send me a screen shot of the Fmc116APP as well as (all 16) output files?
The lower amplitude on the second channel is suspicious, the factory test runs a tone thru all 16 channels, that low a level would clearly fail.
Is it possible that you are looking at an open channel and seeing cross-talk and not the real signal? possibly data from Ch0 & Ch1 vs Ch1 &Ch 2?
This could be verified with the reference app which would store data from all 16 channels at once.
Have you verified your source waveform with an oscilloscope?
What is the DC offset of your signal as measured across a 50 ohm load? if it is near zero everything should be fine.
Have you tried swapping the cables and does the problem stay the same or track the signal source?
have you tried injecting a fixed frequency signal, and does the collected data look clean? on both channels?
Assuming the generators DC offset is near ground, The phase shifts in plots does not look line an analog problem to me, but more like
either missing data, (either words or bits) or a data processing error, unless its a generator artifact.
Respectfully
Richard Mountain
Hi Richard,
Thank you for your quick response. I'm trying atm to install the reference app to double check this frequency response of the amplitude (and phase) of the different channels, but I'm waiting for the license file.
Just to clarify some points:
"You are injecting two identical signals into Ch1 and Ch2, and are seeing a large difference in amplitude between the 2 channels correct?"
That's exactly the issue.
"You are concerned about what appears to be phase shifts/steps in the sine waves correct? (or is that an equipment artefact?)"
Yes, with our system (after the ADC) we measure the phase difference between these two signals. The difference over frequency we expect would be an offset (static delay between the channels) or a divergent trend towards higher frequencies (due to low pass in both channels), instead we see a fluctuating change of phase(Ch1-Ch2) over frequency similar to the amplitude of ch2 (green line) for relatively high amplitudes (250 mV - 750 mV) in the figures attached. I linked that to this artificial amplitude differences between the channels. As a reminder, the change in amplitude happens for all channels checked but not for Ch1 (the blue line in the figures attached, that would be the "real amplitude").
"Have you verified your source waveform with an oscilloscope?
What is the DC offset of your signal as measured across a 50 ohm load? if it is near zero everything should be fine."
Thank you for your quick response. I'm trying atm to install the reference app to double check this frequency response of the amplitude (and phase) of the different channels, but I'm waiting for the license file.
Just to clarify some points:
"You are injecting two identical signals into Ch1 and Ch2, and are seeing a large difference in amplitude between the 2 channels correct?"
That's exactly the issue.
"You are concerned about what appears to be phase shifts/steps in the sine waves correct? (or is that an equipment artefact?)"
Yes, with our system (after the ADC) we measure the phase difference between these two signals. The difference over frequency we expect would be an offset (static delay between the channels) or a divergent trend towards higher frequencies (due to low pass in both channels), instead we see a fluctuating change of phase(Ch1-Ch2) over frequency similar to the amplitude of ch2 (green line) for relatively high amplitudes (250 mV - 750 mV) in the figures attached. I linked that to this artificial amplitude differences between the channels. As a reminder, the change in amplitude happens for all channels checked but not for Ch1 (the blue line in the figures attached, that would be the "real amplitude").
"Have you verified your source waveform with an oscilloscope?
What is the DC offset of your signal as measured across a 50 ohm load? if it is near zero everything should be fine."
Yes, the signals fed to chs 1 and 2 were checked in the oscilloscope and no apparent amplitude changes over frequency were visible there. I will check that again.
As soon as I have results from the reference app I will post them here.
Thanks,
Germán
Germán Fernández Barranco
I was hoping someone here could shed some light on a strange issue I'm having at the moment testing the FMC116. To put it in just one sentence, I'm observing different amplitude responses over frequency of sampled MHz
signals, when I feed them to different channels.
In the pictures attached, you can see the normalized amplitude over frequency
for channel 1 (blue) and channel 2 (green). The signals fed to the two
channels are supposed to be identical. A sweeping sinus from a Rhode&Schwarz
signal generator is split using two active buffers, whose input and output
impedances are matched to avoid reflections (50 ohms).
The different pictures correspond to different amplitudes of the input signal,
ranging from 75 mV to 750 mV. We see how the normalized amplitude of ch1
remains similar for the different input amplitudes, which does not happen for
ch2. Ch2 shows a stronger fluctuating behavior that dissapears using small
input signals (100 mV, 75 mV). This behavior seen in ch2 seems to be the one
present in all other channels. In other words, ch1 seems to be the exception.
And at the moment, using the information from the FMC116 user's manual I can't
find an explanation for this. Maybe a different layout or configuration of the
DC offset correction?
This is a big issue for us, since apparently this "artificial" variation of the
amplitude affects the phase difference between channels, which is critical for
our measurements.
If you are wondering about the jumpy amplitude in ch1 (and in ch2 for small
input signals) that seems to be a problem of the signal generator, the ADC is
not causing them.
Thanks a lot in advance.