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# Distorion level in the FFT of the sine wave was to high.
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# Distorion level in the FFT of the sine wave was to high\!
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Following picture shows the FFT of the sine wave attached to the input.
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Doing the measurement of the board we noticed, that harmonics level in
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the sine wave spectrum was to
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high.
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![](/uploads/140e7ddf0fadb587fb9ae88714f54335/sine_no_filter.jpg)
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As you can see, the second harmonics is only 50 dB below the first
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harmonic level. The sine wave source was the Rhode & Schwartz SML01
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generator - expensive and solid looking device. To verify, if the input
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signal is free of the harmonics, we've connected another generator from
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the Agilent network analyser. Unfortunately, spectrum was looking the
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same. We did some additional measurements, for different frequencies,
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input voltage ranges, and signal amplitude, to find something that could
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help us in the investigation. The thing that made us suspicious was the
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harmonics level that did not differ due to the all shown above changes.
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Next step was to build filter that would suppress the harmonics, to make
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sure that input signal if truly clean.
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