Fast edges from cheap logic

For cheap klunky old parts, the AC's are fast.

This is the fastest edge I've seen from CMOS:

You might consider driving an SAV541 or two from Tiny Logic gates. Rds-on is about 2 ohms with 0.7 on the gate. Heck, you could drive it from ECL, like an EP89.

If you drive the SAV gates really hard, like a volt or so, they keep enhancing. Abs Max is for sissies.

That would be a win for a laser, which is pretty fast--you can put the quiescent bias just below threshold, and then bang on it. The 'simmer' makes its dynamic impedance much lower, and there's not a lot of delta-V on its capacitance, so it can turn off pretty well.

LEDs produce light more or less linearly with current, but it might be worth thinking about whether it would be better behaved if I didn't drop it all the way to zero current between pulses.

Cheers

Phil Hobbs

If you want to look at detector fall times, pulling current OUT on the drop will be beneficial; the alternative, letting the recombination time determine the light output, doesn't show the fast fall time of the driver, but rather of the (slower?) LED light-emission process that gradually uses up the minority carriers.

For an oscillator-driven LED, that just means a parallel R-C element in series with the current limit R, assuming single-supply drive with cathode on the negative rail.

Ya'd think so, but it generally does nothing useful.

Upper state lifetime in direct bandgap semiconductors is short, and the hole mobility is very low, so you can't actually make them drift out of the junction very fast.

Right, that's the speed-up cap I was talking about upthread. Helps a lot with BJT switching, generally not much with LEDs.

Cheers

Phil Hobbs

Am 10.03.23 um 18:56 schrieb Phil Hobbs:

I got rise = 481 ps trise and 323 ps tfall from just 2

No need to risk the 2V samplers on 5V logic that does not deserve it.

That was just a byproduct of a 1pps signal from my GPS-synced

74AC was OK when there was the old Fairchild around. But now there is 5V tolerant 74LVC on a contemporary process, 30 years later.

Gerhard

Seems slower though!

Cheers

Phil Hobbs

Yes, with 4 drivers into 500 Ohms, that's easy living. Not so easy if you need a presentable 1pps...

But just take a look at the quality of these stones... (Live of Brian) <

Gerhard

Monty Python dubbed in German is a bit surreal.

Yessir, a very nice writeup. Out of curiosity, why did you choose crystal notch filters, as opposed to an LC tuned amp?

Cheers

Phil Hobbs

When I have a VNA on the table, the inductors are easy, at least known ones. The plot has been done with Amidon-Red inductors. Those from the junk box probably won't fit.

Finding the series resonance printed on an xtal will usually work and if it doesn't, it will at least leave the phase stability of the 10 MHz alone.

Gerhard

Gotcha. How deep a null do you get? I normally think of fundamental crystals as having 20 ohms or so of series resistance.

Cheers

Phil Hobbs

depends on the impedance level. Maybe 20 dB.

I took what I had here. AFAIR there are 2 traps for the 5 MHz fundamental.

cheers, Gerhard

The other way to do that would be a PLL. There are some stunning but affordable VCXOs and OCXOs around now with fs jitter specs.

Am 11.03.23 um 18:53 schrieb John Larkin:

My GPS already has a 5 MHz double oven, MTI-260 . Without a heavy effort, I could not improve that.

Really, it has 2 MTI-260. The other one is a hot spare for redundancy. Built by HP for Lucent; I got them as unused spare parts, still factory sealed.

BTW most of the cheap oscillators count the jitter only from 12 KHz offset & UP. That's a telecom spec that gives friendly numbers.

Cheers, Gerhard

Absolutely. As a fellow junkbox-builder, I get the ethos.

Cheers

Phil Hobbs