Sampler diodes with more barrier height?

I was browsing schottkies the other day and came across BAR18, highish voltage (70V), CJO max 2pF, stupidly high Vf (2V at only 40mA!). How much leakage and CJO do you require?

Tim

-- Seven Transistor Labs Electrical Engineering Consultation Website:

That's too big a diode, I need

BZX84C75 ?

Cheers

Joke?

Cheers, Jeroen Belleman

The classic 2-diode feedback sampler doesn't much care about leakage. Within about a microsecond after the sampling pulse, the delta-v glitch is amplified in a charge amp, usually back up to 100% sampling efficiency, and dumped into a slow s/h. That voltage, with the diode back-biases, is applied back onto the diodes for the next shot. That makes the overall sampling process ultra-linear and allows it to work down to very low trigger rates.

There are higher-barrier diodes in the Skyworks family. They have a great sample kit.

What sort of risetime did you get? Are you doing an open-loop sampler? What the sampling pulse generator like?

Am 15.02.2014 22:54, schrieb Joerg:

Sth. with a larger band gap like GaAs? Abuse a MGF1302 gate that you may have in your junk box?

regards, Gerhard

If push comes to shove I'll have to use a 2-diode sampler and figure out a balun for driving. I could also use a follow-up S&H on my single-diode sampler but it's a real estate and cost issue. Essentially my range gate will be around 300psec and the sampling cannot happen more than about once per usec. So around 3000:1. Delta-V is a few tens to hundreds of uV.

I think I'll have to talk to the guys there. Things like Schottky barrier height aren't in the datasheets but their FAEs should know.

I haven't tried the diodes in a sampler yet because the sampler only exists on paper so far.

The sim shows about 120psec rise time. Open loop sampler. Essentially an RF BJT driven hard by a fast logic chip and then the collector signal is capacitively coupled into the sampler (fractions of a pF).

One of the challenges with a post-S&H is that there seem to be no ICs for that. The SMP04 still needs 4usec, too long, and it costs an arm and a leg. It's a pity they don't sell the fast samplers from ADCs sans the ADC. "I'd like to have burger and fries but without the burger".

I'll call Skyworks and see what else they can offer. Maybe something that isn't so mainstream at distributros but longterm available.

It needs to be something that's not unobtanium :-)

Like the mountain bike frame I was eyeing and now it's out of stock. Hurumph!

MGF1302 & friends have been workhorses for ham EME for decades... Someone still makes GaAsFETs, no????

:-) Gerhard

Schottky, like Germanium diodes are inherently leaky (very similar that way) because of the low Vf. A standard plain silicon diode is far better and has a higher barrier height; thousands to choose from, and a fair number of fast ones at decent prices.

I did it with discretes; a low charge injection analog switch would work in the downstream s/h.

Data sheet:

"Where to find MGF-1302's"

...and of course, there's eBay:

I use one in an antenna mounted preamp for my 1090 MHz ADS-B receiver. About 0.5dB NF and 20dB gain. Kinda overkill using a 4-8GHz GaAsFET at 1GHz, but they were cheap and easy to ummm... "borrow".

A mere 1N4148 for instance is already in the same ballpark as others (i.e., 1-2pF CJO, or something like that), while being a massively better conductor (Vf ~ 0.7V up to, what, a few 100mA, as compared to that BAR18 I mentioned which goes resistive in the 10s of mA). But do PN junction diodes exist with short enough forward and reverse recovery times to actually handle a 200ps pulse? Could one made of GaAs or whatever be better? ...Could a very small LED be suitable?

Tim

There are lots of nice analogue muxes with very low charge injection, and the dual-gate MOSFET trick works even better.

Cheers

Phil Hobbs

Pretty well has to be Schottky, because PN junctions of whatever sort have diffusion delays that slow things down very badly.

You can't do picosecond sampling with a 1N4148 because it doesn't turn on fast enough.

Cheers

Phil Hobbs

Too bad the terahertz detector methods couldn't be utilized downward abit. There seems to be more available with detecting that spectrum.

Devices like the Golay cell, bolometer, pyroelectric effect, the photo rectification (acoustic which I don't think would work any how)..

As for pulsing lasers onto a semi for a detector, I would think the semi would need to be of a large area for the Ghz, if it would work at all?

Jamie

etc..

Yeah, that's the point pretty much. I already know a 1N4148 takes a nanosecond or so to do anything, besides blowing the CJO spec by about 5x. But I don't know if there's anything with faster diffusion (GaAs?) or thinner junctions that could do better.

Only other thing that comes to mind, then, would be a really small thermionic diode, which doesn't depend on diffusion, more or less (transport is ballistic). The best I have is a few of these,

But really, even the engineers from the '40s knew the limits of their technology, and though there may be some planar diodes (with flange or coax terminals for mounting in waveguides or cavities), early "hot carrier" (schottky) diodes, e.g., cartridge style, were pretty much the way to go.

And hey, there's some on eBay right now:

Tim

Be wary of the BJT response. I've pulsed all sorts of BJTs, including some of the 45 GHz SiGe parts, and have never seen especially fast output edges. Phemts are different; they can switch fast.

Sampling pulses are usually created with SRDs and clipping lines. The really fast samplers use NLTL shock lines.

SRD steps are usually pretty-fast steps that are made faster and cleaner with steering diodes. See Agoston's patents. People worked very hard to build samplers and step generators that had clean picosecond response.

I've done a little playing with software deconvolution for TDR. The idea is to build a fast but ugly sampling or TDR system and clean up its step response with a software FIR filter, the problem being how to train the filter.

I have an algorithm that seems to work well, although I've never really understood it. It just seemed like a reasonable thing to do.

Not at picosecond speeds, there aren't.

Cheers

Phil Hobbs