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is it true, oscilloscope must reach at least 4x observed freq?

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BillyO:
I wonder if the OPs question is well answered here?  I would be good if he could return and try to answer the outstanding issues with his first question.

He may not know the difference between BW and SR.

switchabl:

--- Quote from: robert.rozee on September 13, 2022, 11:38:08 am ---just for a laugh, why don't we bring in some empirical data? a novel approach, i must confess, but do humour me just for a moment.

980Hz (approx) square wave of about 8v p-p. sampled at various sampling rates:

--- End quote ---

This may be a nice illustration of Gibb's phenomenon but it is fortunately not what happens if you sample with a properly matched anti-aliasing filter. It is what happens if you just ignore the Nyquist criterion. And then apply sinc-based interpolation anyway. Did you have to turn that on manually or is that actually the default? If so, that would seem to be either a bug or a strange design choice indeed.

tggzzz:

--- Quote from: mawyatt on September 13, 2022, 01:44:36 pm ---
--- Quote from: tggzzz on September 13, 2022, 07:57:49 am ---Precisely.

A good practical example is the "Tayloe mixer" found in SDR receivers.

Those irritate me. 40 years ago  I built an 4kHz bandpass filter with a Q of ~4000 using 10% capacitors. It was based on the N-path filter concept I found in a 1950s BSTJ paper. The concepts were so counterintuitive that I had difficulty explaining them to analogue/RF engineers.

They continued to fascinate me, I felt sure they would be useful in other ways, but had no professional reason to use them. After retiring I was just gearing up to playing with them again, and found Tayloe had "beaten me to it". Rats.

--- End quote ---

Interesting, we employed the N-path filter concept back in ~1980 to pick out signaling "tones" displaced within multiple 4KHz bandwidth "sections" displaced at microwave frequencies, the filter had a bandwidth of 10Hz. These multiple (thousands) 4KHz sections were down-converted using Nyquist as our "friend" rather enemy, employing Microwave RF Downconversion by means of Nyquist sub-sampling where we knew exactly where each multiple displaced 4KHz section would be and where the tones should be.

So in a way Nyquist can be helpful but not in the DSO discussions here!!

It's amazing that it took another ~35 years to discover the N-Path Mixer, or Polyphase Mixer, which is fundamentally the N-Path Filter without the up-conversion back end section :o

Best,

--- End quote ---

Yes indeed. I'm irritated that I didn't have a chance to (re)invent the Tayloe mixer. I would have had a fighting chance if I'd been working in the right field. Oh well.

My use was simply to reduce thermal and environmental noise when using a large area BPW92 photodiode to measure the loss in early installed multi mode fbres. When asked why I didn't use a PLL, I said I couldn't predict how long it would take to lock up, which would have been a problem.

I still like to think I could find a use for ASP, I.e. analogue sampling/signal processing.

Fungus:

--- Quote from: robert.rozee on September 13, 2022, 01:03:28 pm ---i think your ears might need checking...

--- End quote ---

(Yoda voice)

So sure, you are.

PS: What you're saying is that you can hear 30kHz sine waves - square waves are the sum of sine waves.

Fungus:

--- Quote from: switchabl on September 13, 2022, 01:30:16 pm ---there are no pure sines in the real world anyway.

--- End quote ---

Sure there are. There's just a lot of harmonics mixed in so isolating them is difficult.

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