How to make a super fast sampling head?

Usually a transistor that avalanches nicely, a chunk of good coax and blazingly fast diodes, preferable a matched quad.

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Regards, Joerg

http://www.analogconsultants.com/

On a sunny day (Thu, 10 Jan 2008 17:51:19 GMT) it happened Joerg wrote in :

Any suggestion for diodes?

I have a stash from Microwave Associates which has become M/A COM. You can find the really hot stuff here but you might want to sit down when the quote comes in:

--
Regards, Joerg

http://www.analogconsultants.com/

I did one with a step-recovery diode impulse generator, feeding a

2-diode sampler. Got 70 ps risetime, roughly 5 GHz, and apparently good waveform fidelity. The parts cost is tiny, but it's labor intensive. The timebase and trigger stuff is more work than the sampler.

The older Tek sampling stuff, the S-series heads and their 7000-series plugins, are dirt cheap on ebay. But not nearly as nice and quantitative as the 11801-series stuff.

I doubt you could make a head that would work in an 11801 frame; the interface is undefined.

It would be fun to do a sampling scope/TDR as a USB dongle.

John

I just got John Mulvey's book, "Sampling Oscilloscope Circuits", that goes through the operation of the 1969-vintage Tek samplers in truly gory detail. If you can find a copy, that would be a great place to start. For instance, they used a tunnel diode for the triggered sweep--one diode controlled both the trigger and the reset. It also covers the importance of getting the sampling loop right--you feed back

100.0% of the previous sample to the sampling head. This helps with linearity and (interestingly) speed variations.

Cheers,

Phil Hobbs

On a sunny day (Thu, 10 Jan 2008 12:22:19 -0800) it happened John Larkin wrote in :

I had some idea (not had much sleep, and then barriers do not function on my ideas) to make a small PIC scope, say one PIC, one graphics display (found one dirt cheap), and because PIC will not digitise faster then a few kHz perhaps, add a sampling head just for fun. I think the main problem is in the timebase, to do it digitally, and have it set so it actually produces a steady picture on a repetitive signal, could be very difficult. And there a is sample memory size issue too... It is just an idea, I have several other wish projects, but seems fun, and would give me some more experience with high frequencies (I top out at about a GHz atm). Do I see it right that you have to adjust sample frequency gradually to get a good 'view', is this perhaps done with some PLL?

On a sunny day (Thu, 10 Jan 2008 20:03:29 GMT) it happened Joerg wrote in :

mm 80Ghz! wow.

On a sunny day (Thu, 10 Jan 2008 15:37:33 -0500) it happened Phil Hobbs wrote in :

Thank you Phil.

Good stuff. But check you bank account before ordering :-)

--
Regards, Joerg

http://www.analogconsultants.com/

A lot in this field is a trade secret or barely defined. And then old Leroy goes into a nursing home and one day takes that knowledge with him into the grave.

Same on my main turf (medical ultrasound). Lots of stuff is never disclosed in detail other than what's required for biocomp testing. To avoid potential risks some isn't even written down. So if someone would knock me over the head and walk away with the whole office contents it wouldn't do them any good.

Sure would be fun. But I assume the market size would be rather paltry. Especially in view of the ever smaller number of young lads who would know what to actually do with such gear.

--
Regards, Joerg

http://www.analogconsultants.com/

Many ways to do it. Hint: You can build delay lines where the delay can be set in almost infinitely fine steps. And yes, from a processor if you want to and you can also servo that so you don't have to spend hours aligning and calibrating. 'nuff said ;-)

--
Regards, Joerg

http://www.analogconsultants.com/

Got it, and most of the other Tek Concept Series books. Publishing stuff like that would be unheard of nowadays.

I loved tunnel diodes; too bad they're gone. The fabrication process was insane. I bet an amateur could make them.

Do you have Mark Kahr's paper on the history of samplers? The technology dates back to the 1800's.

John

If it did TDR, you could sell bundles of them to PCB houses, and to engineers and QC people who care about trace impedances. Especially if it were cheap and interfaced to a PC, so it could document pcb test coupons.

I'd design the fast stuff if somebody else handles the USB part and the PC software. I have a slick deconvolution algorithm that would run on the pc side and really beautify the TDR response.

John

Skyworks SMS7621-079, 0.25 pF schottky. I have a reel of them, 23 cents each.

John

Neat! That's a great deal.

Jan, in Europe you can get this brand via BFI Optilas. In case you want to look these up:

--
Regards, Joerg

http://www.analogconsultants.com/

Really? We did lots of RF things in the companies I worked for but never used much TDR. Ok, it wasn't GHz gear.

Hey, you always want to do the fun stuff ... ;-)

--
Regards, Joerg

http://www.analogconsultants.com/

Another part you might want to look at are those blazingly fast transistors from your companies over there, Infineon and NXP. For example the BFP620. Ok, it's a bipolar transistor but it sports a whopping 65GHz ft and is very cheap. Just don't push it much past 2V.

--
Regards, Joerg

http://www.analogconsultants.com/

No, I didn't, but I'll download it, thanks. The 19th century example is the dynamo waveform plotter, I expect. Nice piece of work.

Looking down the list of references, I know a couple of those guys: Mark Rodwell and I were grad students together--he's now at UCSB trying to make 1-THz InP bipolars; Dave Bloom was on my PhD committee, and has retired to Montana after making a pile in the telecom boom.

One missing person is Sadeg Faris, the guy who started Hypres--they had a SQUID-based sampling scope with (iirc) 70 GHz bandwidth back in the late 80s. It was a cool gizmo--the input card was about 2 inches long, and went from room temperature at one end to liquid helium at the other. Another pair are Dan Grishkowsky and Jean-Marc Halbout, who pioneered picosecond plasma-optical sampling back in the mid-80s.

Cheers,

Phil Hobbs

un.

I've done it twice - once for real, with an 800MHz clock and GaAs counters for the coarse intervals and a built in time to voltage converter to work out when the trigger arrived within the the 1.25nsec clock period. My 800MHz clock was poor and we had about 60psec of jitter on the sampling edges.

A few years later I redesigned the delay-generating part of the circuit around a 500MHz clock (which would have been crystal controlled with less than a picosecond of jitter) and ECLinPS counters for the digital part of the delay, and I was planning on using the MC100E195 for the fine delays - it offered 2nsec of delay range and

20psec resolution. The MC100EP195 looks even nicer.

The delays through the MC100E195 are temperature dependent, and the design did depend on recalibrating these delays against the crystal controlled clock every few minutes, by using the system to set up a pulse-width modulated waveform and digitising the DC level to find what each delay really was. We should have been able to run through all 128 discrete delays within a millisecond or so.

The schematics had been passed on for printed circuit layout when the customer backed out.

The MC100E196 offers infinite resolution, but would need even more calibration.

-- Bill Sloman, Nijmegen