[sdiy] Reality versus simulation?

Tom Wiltshire tom at electricdruid.net
Fri Nov 27 17:32:45 CET 2015


Sorry about the lack of images. I was trying to get 'scope shots off the scope last night, but the non-standard USB connection and software to do it is terrible and I couldn't get it to work. Why they couldn't just get the scope to appear as an external drive filled with however many saved images it'll store is beyond me…but I digress.

I tried swapping the filter stages around as per Florian's suggestion. No change.
I tried altering the values as per Phil's suggestion (from 560R/10n to 5K6/1n), and that gave a marked improvement. So increasing the impedance of the circuit seems to help.
On that basis, I'd expect that Sarah's suggestion of buffers between the stages as a test will give a still-better result, but I haven't done that experiment yet.

The scope's FFT Y-axis is dB, so that's the same as I've got in LTSpice. Changing FFT window (Blackman, Hanning, etc) doesn't alter the result. Scope probes are set to x1.


On 27 Nov 2015, at 15:50, Walker Shurlds <walkershurlds at gmail.com> wrote:

> Probe capacitance is the first thing I thought also. In addition, the relatively low impedance of the circuit could be shifting the oscope's pole(s) into a lower frequency. 
> 
> Another factor could just be windowing selection from the fft, or maybe the axes aren't logrithmic but are in spice, this only affects whether the response *appears* flat, of course. A few extra pictures might help with diagnosis.
> 
> Walker
> 
> 
> 
> On Fri, Nov 27, 2015 at 9:09 AM, Sarah Thompson <plodger at gmail.com> wrote:
> 
> Another possibility would be stray capacitance from the breadboard itself causing loss at higher frequencies. Also, are you using a X1 or a X10 scope probe? If you are using X1, this might be the culprit, though 20kHz is a bit low to start seeing much difference there.
> 
> As an experiment, try putting opamps before, after and between the stages just configured as voltage followers. This will greatly reduce the distance any high impedance signals need to travel, and tends to make filter weirdness go away. I'm not necessarily suggesting this for a production version, but sometimes going for overkill then backing off is less pain. 
> 
> Sarah
> 
> Sent from my iPad
> 
> > On Nov 27, 2015, at 5:30 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
> > 
> > As per schematic, +/-10% for the caps, +/-1% for the resistors.
> > 
> > Values wobbling about would shift the cutoff points a bit, but shouldn't give the significant downhill slope I'm actually seeing. Should it?
> > 
> > For example, RC variations give me a lowpass cutoff of supposedly 28.4KHz, but actually 25.6 kHz – 31.9 kHz.
> > 
> > 
> >> On 27 Nov 2015, at 11:52, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> >> 
> >> Tom,
> >> 
> >>> So what's going on with the real-life filters? Can I improve their performance?
> >> 
> >> What are the actual values of the capacitors and resistors in your circuit?
> >> 
> >> Neil
> > 
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