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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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As you can see, the second harmonics is only 55 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 observation that made us suspicious
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was the harmonics level that did not differ due to the all changes shown
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above.
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same, so the conclusion was to focus on the FmcAdc card. We did some
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additional measurements, for different frequencies, input voltage
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ranges, and signal amplitude, to find something that could help us in
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the investigation. The observation that made us suspicious was the
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harmonics level that did not differ due to the all changes shown above.
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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 is truly clean.
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... | ... | @@ -31,12 +31,12 @@ signal. |
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![](/uploads/40183edda830c26609c83dfec4912084/P1040720.JPG)
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Marker at the second harmonics. Filter's attenuation at this frequency
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is
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44dB.
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is 46 dB (-3 dB for the base signal
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frequency).
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![](/uploads/0c8296bbd8c6e74dc2cc9daabb13448f/P1040721.JPG)
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SUCCESS\!\!\! FFT of the filtred signal indicates the second harmonics
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level of -90dB and third harmonics of -80dB. I don't have to add, that
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level of -90 dB and third harmonics of -80 dB. I don't have to add, that
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these values are much closer to our
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estimations.
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![](/uploads/a52614e977c3f6023551f99645724c06/sine_filter.jpg)
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