Sampler diodes with more barrier height?

Probably the leakage is very low to begin with. Unlike the Schottky diodes with low barriers.

John: Just to to Skyworks and the are sending me samplers of their DMJ2824. Around 0.1pF, amazing. Seems those are the only ones with high Schottky barrier and thus their Vf is more like a silicon diode. Big downside is that they cost a ton.

I've also asked Avagotech but there you have to fill out a form so that takes time. Their HSMS286x series is low in Vf so I assume their temp drift in leakage current is pretty bad.

Skyworks confirmed this afternoon that their is indeed no SPICE model :-(

In this case I could live with that.

Maybe on the next project. I need to press on with this one and get that part finalized at least in SPICE.

Really, the EL/EP52 flipflop 1-bit sampler looks like a good bet here.

Or like an Italian sports car where the boss in the shop said: "Oh, only Giuseppe can do those".

Yes, it sure is tempting. But I have to wrap up soon and differential stuff with both inputs close together can cause nasty oscillation. Then I'd be in hot doodoo because we essentially have to commit straight from SPICE to board. In some of the fast comparators that's almost guaranteed.

It's like standing on that huge tower at an olympic pool. I did finally jump but there was a trainer down there who could fish me out if I screwed up.

Thanks! I didn't really understand what you guys were talking about.

So the diodes are kept "almost on" by the feedback? Then with the sampling pulse they (the diodes) are like switches and transmit the input voltage to the caps? Hmm well not a voltage. A current proportional to the input voltage? "Scratch scratch" I've printed a copy, I'll stare at it over lunch. (I guess I should think about currents. (?))

George H.

No, they are back-biased by a volt or so. The bias tracks the last-sampled value.

The signal level info is actually stored in C1 and C2. If the signal (at the sampling instant) is equal to the feedback voltage, C1 and C2 receive equal amounts of charge and there is no net signal into the amps. If not, one of the caps charges more than the other, and you get a signed glitch to work with. The voltage glitch at A may be just a few per cent of the difference between the signal level and the output... low "sampling efficiency." The feedback loop boosts the signal and makes everything linear.

Joerg wants to use a full-bridge sampler with no feedback.

Probably just single-diode, the full bridge needs too much "stuff" for this design and there is no negative supply. Linearity isn't a concern and, provided each delta-V step is at least several millivolts, there isn't much to be gained in SNR by a charge accumulation method versus just averaging or lowpass-filtering these delta-Vs into an RC or something. Of course, then the sampling pulse must come from a very clean source.

Now I just need a high Schottky barrier (or at least middle barrier) diode that does not leak so much and won't cost an arm and a leg. The latter seems to be a problem. Worst case I'll have to live with a solution where nearly all the charge leaks out during each cycle and I average the resulting "sawteeth". Lowers the SNR but not by much.

I wonder how they do that on the cheap cable testers.

Hi, John -

Are those transmission lines where it looks like the pulse generator output is shorted to ground?

Thanks, John S

Yes. Like this:

That was intended to be a dual-channel sampler, but I never built the one on the right. There's a little ceramic pill SRD in the center of the pattern, fed by the twisted pair from below.

Thanks for the description... I clearly have no clue how it works :^) George H.

The Tektronix Concepts sampling book is a really good read. S2 sampler vintage rather than SD-24, but oh well--there's still a lot of good stuff there, including an excellent description of how sampling loops work.

Cheers

Phil Hobbs

Ask them, or RFMW (or both!) for the Skyworks diode sample kit. Varicaps, PINs, schottkies. No quads.

I also have a dusty loosleaf binder, HP RF/uW Diode Designers Kit, full of data sheets and samples.

I could loan you both if you promise to not use *all* of the samples.

Thanks much, John.

Cheers, John S

Oh, I would promise. But I don't need a whole kit right now, essentially I just have to qualify one good diode type and design that in. From the discussion with their engineer yesterday it pretty much boils down to the DMJ series. There seem to be no more high barrier diodes available in coach class these days. Unless they are at other mfgs such as Avago, which unfortunately is not as responsive as Skyworks.

That looks nice... clipped from page 3

"The fundamental reason why sampling scopes can respond to faster-changing signal voltages than conventional scopes is that with certain semiconductor devices we can generate much narrower pulses than we can amplify. With vacuum tubes this was not so."

I still have to read the sampling pdf that John L linked to.

George H.

It turns out that a lot of Skyworks's MMICs are basically just a pHEMT cascode pair. Might be interesting to try one out as a sampler.

Cheers

Phil Hobbs

That would be nice. Though the ones I scanned only have input, output and ground piped out, not the 2nd gate.

The Vbias pin is G2, with an internal bypass cap. See e.g.

Cheers

Phil Hobbs

Thanks, Phil, very interesting. They say they built it on an SOI process but did it actually make it into production later? Of course, the 10pF would put a serious crimp in there for a sampling app. At 5GHz that's almost dead short and the upper gate is where the signal would have to go in.