[sdiy] Low Cost Comb Filter Methods

[Andrew Simper]

I've had my best go at doing an ideal norton op-amp, measuring the current
at each input terminal (and sending them to ground), and then generating an
infinite gain voltage of the difference between the input currents at the
output, 1/G is used so I can set that entry to zero for infinite gain:

https://cytomic.com/files/dsp/ideal-norton-mna.pdf

In the previous version I had to trim the resonance pot rq to not get
infinite gain, but the norton version is fine without trimming.

The bias voltage isn't needed since the devices are ideal, but I added it
in anyway and set it to zero volts, in the ideal case all it does is offset
the output, which isn't useful here. Here are the plots:

https://cytomic.com/files/dsp/comb-filter-resonator-norton.png

And the python code for both:

https://cytomic.com/files/dsp/comb-filter-resonator-norton.py

Cheers,

Andy

On Fri, 15 Jun 2018 at 10:06, Andrew Simper <andy at cytomic.com> wrote:

> Hi Neil,
>
> Thanks for pointing that out, I didn't take a loop at the part number and
> there was no regular symbol of the current arrow at the inputs like there
> is meant to be on the op-amp inputs to differential it from a regular
> op-amp as shown in figure 4 part (b) here:
> http://www.ti.com/lit/an/snoa653/snoa653.pdf .
>
> Can you please give me the theory of operation of an ideal "Norton
> op-amp"? Once I have those equations I can add this as a new device to my
> solver and re-generate the code. As an example the defining equations for
> an ideal op-amp are here:
>
> http://qucs.sourceforge.net/tech/node67.html
>
> Thanks!
>
> Andy
>
> On Fri, 15 Jun 2018 at 00:26, Neil Johnson <neil.johnson71 at gmail.com>
> wrote:
>
>> Hi,
>>
>> Andy wrote:
>> >> If anyone is interested in the linear case then here are the equations
>> to solve the circuit:
>> >> https://cytomic.com/files/dsp/comb-filter-resonator-equations.pdf
>> >>
>> >> and here is an implementation in python generator by my circuit solver:
>> >> https://cytomic.com/files/dsp/comb-filter-resonator.py
>> >>
>> >> and a plot of the filtering of a sawtooth given one set of component
>> values:
>> >> https://cytomic.com/files/dsp/comb-filter-resonator.png
>>
>> Brian wrote:
>> > There’s one error in your reproduction of the schematic, though. rp is
>> not supposed to be grounded, but should be connected to the +V supply. How
>> much does that change the equations? It should reverse the current through
>> the resistor, at the very least.
>>
>> And also note that the op-amp used is a Norton op-amp, so it draws
>> current on its inputs.  That will change the equations.
>>
>> Neil
>> --
>> http://www.njohnson.co.uk
>>
>
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