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 high.

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 filter:

Simulated frequency response:

Photo of the filter:

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 (the

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 well.

Conclusion*

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 estimations.

Maciej Fimiarz - September 2010