Hi all, so spad games again. Here's my simple reset circuit. The positive voltage will be bigger with the 'real' spad. But at the moment I'm using a green led as spad... and it can't handle more than a few volts of over-voltage.
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Here's a 'scope shot with and without the B-clamp Schottky. (Brown trace is w/o B-clamp)
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So here's my question. I (mostly) understand faster reset. by why do I get a deeper discharge without the B-clamp?
(Aside, I tried this first with a fet as reset switch.. and I had similar type problems.. too deep a reset.)
Yeah, I think so. Using some sort of digital drive would be nice. (pulses are born with sharp edges.) but the 'real' spad get's ~10-20V of over voltage.
I should be able to capacitively couple a quench pulse in too, and then piglet sent me some that use a pulsed transformer. (transformers are somewhat magical to me... I'm not sure how to pick one.)
Right, it depends on the over voltage.. in theory the quench has to be a little bigger than the over-voltage.
10-20 ns for the whole thing would be wonderful I think I'd settle for 80-100 ns.
Yeah I made my own from an 74hc14 ~10-100 ns pulse width.
OK thanks. Hey I've got the thing working with the real spad... but it's a bit weird and delayed. (At times like this a fast four chan. 'scope would be nice!) Here,
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The single orange trace is the compartor out, Brown is the base drive (width ~50 ns) And green is the anode of the spad...via a buffer opamp (I understand nothing of that trace.)
I'll be careful.
Here's an unrelated statistics thing. I was checking the dead time by setting the 'scope to 4 sec. persistence. I get pulses right after the first so dead time looks small. But I hardly get any pulses at all before the one that triggers the 'scope. That strikes me as weird... I naively expect random pulses to be random in time. I think I can hand wave my way out but...
Pic worth 1k words
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This is all to slow, and I can't get above 5V... but at least it works :^)
Good suggestion. I seem to recall goal is 20-30V swing as fast as possible, current is minimal. I suggested a 5V logic line driver stepped up but your gate driver even better as needs less or no step up. Heck, even if step up needed a charge doubler might do.
The saturation voltage of the BJT is lower than V_BE - V_f of the diode.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com
OK as soon as we order 10 'real' spads I'm sending you one.
Re saturation: Most likely, I guess I don't understand it very well. It went below 'my' expected c-e voltage... but there's ~ -100V on the other side... I'm pulling both sides down.(?) I tried first with a fet, and that also went deeper than expected.
I look at my 'scope trace and I figure, there must be stuff happening faster than I can see.
I should think of diodes as low C switches, when biased properly.
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