[sdiy] Reality versus simulation?

Tom Wiltshire tom at electricdruid.net
Fri Nov 27 14:24:10 CET 2015


Interesting idea. I'll swap it around on the breadboard and let you know.

On 27 Nov 2015, at 11:57, Florian Anwander <fanwander at mnet-online.de> wrote:

> 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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