Building a pulse generator: puzzled by PCB trace impedance matching

[iMo]

MIC5205 - should not the BYP pin capacitor wired to its output (like 100pF for example) make the MIC5205's response actually faster?
Also you may try to impedance match the 12ohm (or whats ever) outputs of the CMOS drivers directly to the 50ohm connector with a pcb traces microstrip design as "the impedance transformer", without the output resistors there (the resistors have some parasitic L and C)..

[shapirus]

MIC5205 - should not the BYP pin capacitor wired to its output (like 100pF for example) make the MIC5205's response actually faster?

Hm, maybe. I didn't try this, just followed the datasheet circuit, knowing that I can, as usual, fix stability issues with a fat electrolytic cap, if necessary :).

Also you may try to impedance match the 12ohm (or whats ever) outputs of the CMOS drivers directly to the 50ohm connector with a pcb traces microstrip design as "the impedance transformer", without the output resistors there (the resistors have some parasitic L and C)..

That's an interesting idea, but I need to do my research, and that'll be a different project, maybe as an exercise in designing a 4-layer board and ordering it at a fab house. This one is done with a straightforward approach of using series resistors.

[shabaz]

Nice-looking board! And that selectable frequency is handy.
Even though I don't use the pulse generator much, it still comes in useful occasionally, as an extra signal source. I used it to test a DIY H-field probe by feeding it into a test microstrip. Also the edges are so sharp, you can use like the 101st harmonic etc., for crudely testing VHF receivers.

[shapirus]

Nice-looking board!

Thanks. And that's a handmade board! The solder mask really improves the looks of it.

And that selectable frequency is handy.
Even though I don't use the pulse generator much, it still comes in useful occasionally, as an extra signal source.

Yeah, that was the idea: make a small reasonably functional device for both learning and practical applications (which are still mostly for learning, as it's a hobby).



p.s. It emits sound at 1 kHz, 50% duty cycle output. Must be the microphonic (or rather the opposite) effect of the caps and maybe the board itself (I did VCC copper pour on the top layer). Ouch. And some instability at output high still remains, but I think I don't really care at this point. There's still room to add another 820 uF/6.3V cap, if I want to.

[shapirus]

I'm going to try to use a spectrum analyzer to estimate the edge speed of this pulser.

@tggzzz, what source of noise did you use in https://entertaininghacks.wordpress.com/2015/08/11/measuring-digital-signal-edge-rates-without-an-oscilloscope/ ? What shoud its frequency spectrum be: does it need to have uniform power distribution all the way into RF, or we only care about up to tens of MHz?

[tggzzz]

I'm going to try to use a spectrum analyzer to estimate the edge speed of this pulser.

@tggzzz, what source of noise did you use in https://entertaininghacks.wordpress.com/2015/08/11/measuring-digital-signal-edge-rates-without-an-oscilloscope/ ? What shoud its frequency spectrum be: does it need to have uniform power distribution all the way into RF, or we only care about up to tens of MHz?

Something cheap and nasty from fleabay.

The knee frequency and risetime are related as indicated. The noise source needs to have power above the knee frequency.

It is an approximate technique, and any vaguely competent noise source ought to be flat enough.

Make sure you don't overload the a spectrum analyser input; you may find a 200MHz high pass filter useful.

[shapirus]

Something cheap and nasty from fleabay.

The knee frequency and risetime are related as indicated. The noise source needs to have power above the knee frequency.

It is an approximate technique, and any vaguely competent noise source ought to be flat enough.

Yes I understand that this technique is approximate, that's why I used the word "estimate" rather than "measure".

Was wondering if my JDS2800 signal generator was suitable (it has noise as one of possible output waveforms), but apparently it won't do: its noise power nicely rolls off after 60 MHz, which, not so surprisingly, matches its max output frequency.

I'll try to find that Johnson's book for a more detailed explanation.

This is a rabbit hole indeed (but a welcome one). Since the most difficult part of creating a white noise generator seems to be finding a suitable high-frequency op amp, and it looks like I can obtain some of those locally, now I, naturally, want to try building one myself, especially now that I have an SA with which I can test its performance, rather than just buy a ready made one from Ali (even if they are sold for just $20-$25). That will likely turn out not as easy as it seems, but that's okay.

[shapirus]

Actually... aren't we just testing the max switching frequency of the IC in question this way rather than the edges speed? They are of course related, in a way, but not quite directly.

[tggzzz]

Not opamps; use RF MMIC amps.

They seem to be ~£12 on fleabay. Bg7tbl design, e.g. https://www.ebay.co.uk/itm/296127208776

[shapirus]

Not opamps; use RF MMIC amps.

Yes, figured that out already. Can probably be done with discrete RF/MW transistors as well. I think I have some, but this is something that I'll rather build using ICs. For example, found one BGA616, seems to be suitable and can be had locally for about $1.5 apiece.

They seem to be ~£12 on fleabay. Bg7tbl design, e.g. https://www.ebay.co.uk/itm/296127208776

Ebay doesn't ship to Ukraine now, but that same stuff can be bought on Aliexpress. But price is not the point. $25 vs £12 or whatever makes no difference: I want to build the thing myself now, as it seems to be a perfect little project for experimenting with: cheap, simple in theory, low component count, easily measurable performance.