pretty good frequency counter

Am 02.04.20 um 06:40 schrieb snipped-for-privacy@highlandsniptechnology.com: cheers, Gerhard

The Stanford and the HP5370 use comparators, not agc/log. They rely on exact trigger levels. You can even measure rise/fall time.

The HP type number was wrong.

So it has good performance with high or low impedance inputs, with or without attenuation, tolerates a variety of buffers/filters/amplifiers if you want to process the input.

Comparators have high output but also limited slew rates make 'em a pain at 250 MHz.

Yep, you can build a front-end for almost anything. For taking Hsync from a video signal, I made a self-biased one-transistor amp that forced the duty cycle but let the input DC level float (because that's useful for composite video). It woudn't be optimal, however, for general purpose use (the forced duty cycle wasn't 50% and would have caused unnecessary jitter).

Built-to-order front ends just don't belong inside the box, where switches and pots are a signal-path nightmare when they get dirty.

It was the HP5386 I was looking at. I'll see if I can find the section again.

The comparator approach is fine until you have a mix of signals. What should a freq counter do if you have a strong fundamental but a spurious signal introduces an extra zero-crossing every 3rd or 4th cycle?

The HP5386 just seemed rock-solid, where the Chinese hobby clone tended to be just vague. Well duh... but looking at its input schematic I couldn't really see why it wasn't better.

Clifford Heath

I found the SR620 schematics. After some FET+BJT buffering, the signal goes to a AD96685 comparator (ECL differential output), chosen for its low delay dispersion, which the data sheet defines as "a measure of the difference in propagation delay under differing overdrive conditions."

Only the tiniest amount of hysteresis though: 1/50th of the ECL swing (is that 10mV?), with an extra 1pF (about 200R, tau~=10ns). Hysteresis is applied from the negative output to the negative input, so it doesn't feed into the signal path.

It's a nice thing. Thanks for mentioning it Gerhard.

Clifford Heath.

Clean limiting amps help a lot in messier situations, e.g. radio links with fading. AGC is bad news in a counter because it's slow, so that fast fading causes artifacts.

Cascaded limiters do give a logarithmic response, but that's not the essential point--clean limiting is the key.

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

Am 02.04.20 um 13:24 schrieb Phil Hobbs:

< >

Googling for "Oliver Collins zero crossing" delivers pointers to more work, most of it on the time nuts list.

Gerhard

We used to use a bunch of 5370s here. It had 20 ps single-shot resolution and 30 ps RMS jitter for time measurements. The user interface was wonderful. Somehow it did all its math with a single

8-bit 6800 (not 68K) CPU, which took 2 us to execute a no-op instruction and had no multiply.

They eventually died, so we use a Keysight box with about the same specs, 20 ps LSB and better jitter, 15 maybe. But a brain-damaged user interface.

The 5370 has a custom IC as the trigger discriminator, and if it gets zapped it can't be replaced, I think.

Does anybody use the SRS box?

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard