Interesting discussion.
See this link this should help some better understand what's happening:
https://youtu.be/MP7m5OjXWUg
Thanks for the interesting post
Nice finding, that's a very clean explanation, indeed!
The same video from the 'ISSCC Videos' channel was also posted on the 'icdutwentenl' channel
https://youtu.be/tYgc5mbdXMwIt seems like it is/was a workshop for the N-Path filter (video from Sept 2020), but I couldn't find the workshop:
https://youtu.be/OTxGakNCeeA
The N-Path filter is a very clever circuit. While it feels like most of the aspects are understood by now, this looks like the kind of circuit full of surprises, and this is before even starting to experiment with it.
What intrigues me the most at the moment is the fact that I can change the cutting frequency of an RC filter without varying the R or the C value!
Not sure if this is a well known thing, my guess is I am probably rediscovering the hot water here, but this alone can have a lot of applications. Maybe it's just me who missed this obvious aspect, but as close as two weeks ago I would have bet my head on the validity of the fc = 1/(2*pi*R*C). Would have never thought this can be changed with a switch.
My first prediction was a switched signal should have affect only the amplitude, and not the fc.
That is why I was saying last time that a switch is very different from a PWM source.
In fact, when I read in the pdf thesis linked in former posts that "
The short exposure to the input signal divides the time constant by the duty cycle" and therefore it will change the fc, it was so obvious yet so unexpected that I thought it's a mistake, or maybe I am reading it out of context. I had to make a time domain LTspice simulation to convince myself:
The 4 colors are the frequency response for 50%, 25%, 12.5% and 6,125% on time for the switch:
What exactly I was missing all this time? What part of EE do I need to study more so to not be surprised about this change in fc? Asking because the variable fc, or the change of tau in an RC cell, is stated in all the papers, yet no author seems to make a big deal out of it.
Another thing I don't understand is why the fc is much lower in that simulation? I see about 400kHz (first blue plot), for the switch with 50% Dwell time, but my calculation for half of 1/(2*pi*R*C) with 1k and 100pF is 796kHz, about 2 times bigger than measured.
Letting aside for a moment why does the formula and the simulation show different fc, the main question is why I don't see this switching principle exploited more often? Or is it exploited, but I didn't noticed it?
What other related applications are out there that tune by fast switching instead of changing analog values?