[sdiy] Reality versus simulation?

Florian Anwander fanwander at mnet-online.de
Fri Nov 27 12:57:02 CET 2015


Having no electro-mathematical background at all:

What happens if one has the HPF as first stage and the LPF as second stage?

Floran

Am 27.11.2015 um 12:05 schrieb Tom Wiltshire:
> Hi All,
>
> I've been playing with the following circuit. It's a couple of simple passive filters to follow a digital noise generator. The LPF is set around 25KHz to smooth the digital pulses, and then the HPF acts as a DC blocker at about 15Hz.
>
> http://electricdruid.net/images/PentaNoiseFilter.pdf
>
> Now, the interesting thing is that in simulation, this works fine, and I have a flat frequency response between the two filters. However, in reality, there's a pronounced downward slope to the frequency response (high frequency rolloff) which amounts to about -10dB at 25KHz. This is measured by feeding the noise into my oscilloscope's FFT function. I know the rolloff isn't coming from the 'scope or the FFT, since feeding the digital noise in directly gives me a flat response out to 100KHz or so.
>
> So what's going on with the real-life filters? Can I improve their performance?
>
> Thanks,
> Tom
>
>
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