Sampler diodes with more barrier height?

Right, it's the second sampler I'm talking about. Normally getting a

In a fast sampler running inside a sampling loop (where the previous sample is added to the bias voltages), the diode conditions are kept pretty nearly constant from sample to sample.

Cheers

Phil Hobbs

Looks like I may have to live with single-diode and then there's only a few tens of nsec available. It might have to be discrete again since even what they call fast (ADG820 and similar) isn't that great in speed. At least in the reasonable price range.

Or one just lives with the delta-V without a loop. Could be averaged locally or digitally later since AD converters have become so cheap.

Assuming that one has a sufficiently accurate model of diode behaviour under dynamic conditions. Feedback is nice that way.

A BF908 has a reverse transfer capacitance (G1-D) of typically 0.03 pF. The G2-D capacitance would be more of a worry, but one more cascode stage should fix that.

I'll have to build one of those one of these days--I haven't used one in about three zillion years.

Cheers

Phil Hobbs

...and you either from what I can see...

Or run G2 from the buffer output, which would also fix it. Have to try that.

Cheers

Phil Hobbs

True, but if you need a buffer or a charge amp with a beefy output you might as well use a Schottky diode there, too :-)

I use sampling scopes all the time, and have a modest collection of antiques and manuals. I have an actual Lumatron box

(single point-contact diode sampler, avalanche transistor, hideous but historical)

and a Tek type N plugin (also single diode, but pretty) and a 1S1 and a 1S2

The 1S2 was a wonderful gadget; I learned a lot about picosecond electronics from that plugin.

A bunch of HP stuff, too

On my bench I have a Tek 11802 with a bunch of sampling heads. The SD24 head does 20 GHz sampling/TDR and, as Phil says, it makes a good picosecond pulse generator, too. I recently got a 50 GHz head (SD32?) from ebay. Seems fine.

I built a 2-diode sampler, similar to my sketch, and it worked fine. Tr was about 70 ps, which is around 5 GHz.

A 20 GHz sampler is feasible with surface-mount parts on FR4... I've seen it done. It would be great fun to go into the sampler business, but it's hard to compete with all the gear on ebay.

Anybody who wants to get good at picosecond electronics can do it for $1000 and some time.

This was probably the world's first sampling oscilloscope:

Here's Mark Kahr's superb paper on the history of sampling scopes:

One problem with the single-diode sampler is that variations in the sampling pulse height rip right through with the signal, which can get noisy. And it tends to be nonlinear. And there is a huge kickout of the sampling pulse to the input connector.

The old Lumatron boxes, and the Tektronix N, were single-diode samplers with avalanche transistor strobe generators. They were fairly nasty. LeCroy did a single-diode sampler a while back but discontinued it. LeCroy did a lot of weird things, like their untriggered DDS timebase for telecom eye diagrams.

It would be interesting to see what could be done with MiniCircuits parts (baluns and mixers) to make an open-loop bridge sampler.

Yes, those are generally the downsides. But the sampling pulse can be kept fairly constant and calculated out. The kickout is huge because the sampling pulse roars right through the diode capacitance. But in equivalent time samplers that is mostly just an EMC concern. Provided that the load does not go berserk upon being hit with the kickout.

Luckily my stuff doesn't need to be high precision but it has to be cheap and small. For samplers it's truly slim pickens. Just now looking for something useful for the post-sampler. All the commercial S&H chips I saw so far are slow as snails, even the pricey ones.

I mostly came away disappointed when canvassing the commercial offerings. That was the same during my masters project in the mid-80's. I was designing a CCD camera that would beat the daylights out of the camera the CCD manufacturer was offering (and it did). After a frustrating search I just wound my own bridge sampler transformers. Which I should have done in the first place, would have saved me time. For the diodes I used matched quads, I believe from HP. Four individual diodes per pouch.

Interesting paper, thanks. It's roughly in IEEE format--Was it ever published in a journal, or was he just using the LaTeX style?

Cheers

Phil Hobbs

Skyworks has a medium-barrier bridge quad, the SMS3930-021, and a high-barrier version, SMS3931. Both 0.3 to 0.5 pF per diode. Might work as an open-loop bridge sampler.

I think he did it for an MTTS conference and it was published in the proceedings.

You should meet Mark some day. He teaches EE at Pitt, and his wife is the dean of the CS department.

He has an 11801, a gift from a friend.

It only has to drive a 3-pF capacitance, and it's 3 pF to the input and not to the sample pulse. Plus when it's off, it's really off. But that part will run out of steam above a few hundred megahertz.

My original suggestion was a cascode made out of two pHEMTs, e.g. SKY65050s. It would have much the same virtues at much lower capacitance, and has gain up to 12 GHz or so. The Skyworks ones are much nicer than the Avago ones for this sort of job, because their output conductance is a lot lower, i.e. they have a really high Early voltage.

Cheers

Phil Hobbs

That's like saying I'm "involved with" computer technology because I use a PC...

That may be better than the SMS7621 which also comes in a bridge quad. I'll call Skyworks this afternoon to see whether their higher barrier types are better in leakage current at higher temps. The board has to perform at least to 60C and I want to keep it simple, ideally without much fancy post-sampling and loop stuff.

They are nice but most likely they'd have to be adjusted in production for the upper gate bias (where the to-be-sampled signals would go in). The datasheet mentions about 30% total tolrance in the pinchoff. I guess I could automate that. The other thing is whether it'll be stable with one riding on the drain of the other. With GHz devices that can be like trying to balance a garden hose on the tip of a finger.

Wish they had a SPICE model for the SKY65050, there's only ADS and S-Parameters on their site.

Bracewell's version, which he called the Fast Hartley Transform, is equivalent to the FFT for real-valued signals and saves half the arithmetic. That mattered more in 1983 than it does now, but it's still nice, once you get to the last couple of turns of the optimization crank.

Bracewell was one of my favourite professors--extremely lucid and down-to-earth, plus he cared a good deal more for his students than for his ego. His class was one of the two most useful ones I took in grad school. (The other one was a class in asymptotic methods, taught by an EE-turned-matho, Stefanos Venakides. He didn't get tenure and had to move to Brown, where I think he did fine.)

But you have to admit that Agoston's version is pure art.

Cheers

Phil Hobbs

If you look at Agoston's Tek samplers, it's mind-boggling to think about how many hours they had to have spent to get it as clean as it is. There are strange wirebonds all over the place, notched microstrips, bizarre shapes, scary stuff. It must have been a nightmare in production and test, too.

Agoston's original SD24-looking patent used a trench/slotline machined into a metal block as the sampling pulse shaper and transmission line. I wonder why they didn't do it that way.

He emailed me a couple years ago about my deconvolution thing, and I sent him the demo and the source code. Don't know if he ever used it.

Den tirsdag den 18. februar 2014 00.44.55 UTC+1 skrev John Larkin:

I visited Rohde & Schwarz in Munich many years ago and saw the place where they made stuff like that

the guy there had a German saying something like: the higher the frequency the longer the face

-Lasse