Distortion level in the FFT of the sine wave was too high!
Doing the measurement of the board, we noticed, that harmonics level in
the sine wave spectrum was to
As you can see, the second harmonics is only 55 dB below the first
harmonic level. The sine wave source was the Rhode & Schwartz SML01
generator - expensive and solid looking device. To verify, if the input
signal is free of the harmonics, we've connected another generator from
the Agilent network analyzer. Unfortunately, spectrum was looking the
same, so the conclusion was to focus on the FmcAdc card. We did some
additional measurements, for different frequencies, input voltage
ranges, and signal amplitude, to find something that could help us in
the investigation. The observation that made us suspicious was the
harmonics level that did not differ due to the all changes shown above.
Next step was to build filter that would suppress the harmonics, to make
sure that input signal is truly clean.
We don't care of the in-band nonflatness in frequency response, so it
was decided to build Chebyshev filter,
where edge sharpness is good.
Schematic of the
Photo of the
Measured frequency response - marker at the frequency that is
subharmonic of the sampling frequency (no "signal leakage" in FFT).
This is the frequency of our desired input signal
Marker at the second harmonics. Filter's attenuation at this frequency
is 46 dB (-4 dB for the base signal frequency).
It matches simulation results very
SUCCESS!!!* FFT of the filtred signal indicates the second
harmonics level of -90 dB and third harmonics of -80 dB. I don't have to
add, that these values are much closer to our