Step recovery diodes and SPICE

... I forgot the model... sorry!

• A simple pulse sharpener

.TRAN 1ps 20ns

VPLS 1 0 PULSE(0 10 5ns 1ns 1ns 20ns 50ns) Rg 1 2 50Ohm C1 2 3 1nF Lp 3 5 1.5nH Cp 0 5 0.2pF D1 0 5 D_840 C2 3 4 1pF Rl 4 0 50Ohm If 3 0 10mA

.MODEL D_840 D (IS=500.0E-15 N=1.3 RS=0.22 CJ0=0.545p VJ=0.5 M=0.235 XTI=3.0 EG=1.12 BV=60 IBV=10E-6 TT=10E-9)

.END

I'd doubt that spice modeling will be worth the effort. You'd do better to experiment and measure. Getting below 100 ps Tr is hard, because package and pcb parasitics start to really matter, and because the faster srd's are hard to drive.

Faster srd's don't store much charge, so the problem is to reverse bias them fast enough, to get enough reverse current through them, before they snap. The driver becomes non-trivial, and it's sometimes better to buy a "slower" diode just because you can slam so much more reverse current into it.

The other way to get edges like this are to start with a fast ecl or cml edge - 40 ps is easy these days - and run it through a cheap 5 or

10 GHz mmic.

John

Say John,

When you're designing this stuff, what's your technique for prototyping? "Real" circuit boards (full layout and etched)? Copper clad (blank) PCBs that you Dremel away the bits you don't need from? Some of the fancier "universal" PCBs specifically meant for prototyping?

Thanks,

---Joel

I just solder stuff to a piece of double-side copperclad, midair or onto crude traces cut with an xacto knife, all impedance matched of course. I usually use flanged sma connectors soldered to the board edge-mount style, or solder hardline coax straight onto the parts. There's tons of sma hardline cables on ebay.

For more complex stuff, we lay out a board and have AP Circuits or somebody make us a few boards.

The critical thing is to have a good, 12 GHz or so, sampling scope. An

11801+SD22 can be had on ebay for $2K or less. Older 7000 series stuff isn't as good, but is a lot cheaper.

John

I agree with you; I am waiting for a few samples to play with and I wanted to simulate a little them while I am waiting... I was thinking to drive the SRD with a logic gate such as a 74AC14, paralleling a few gates in order to obtain enough current. I hope it is fast enough...

I have had a look at some very nice SiGe ultrafast comparators. I will try them if I do not have good results with SRDs. I was thinking to prepare a few boards with matched 50Ohm striplines to prototyping a little. I have access to a nice 40 GHz scope, but it is 100km away from where I am working, and we have a 20GHz network analyzer in house.

That's one place where simulation or calculation is useful. But you have to know the approximate equivalent stored charge of the srd, to calculate what the reverse current will be at the instant of snap. If you drive the srd through a small inductor, fairly common, the risetime of the driver pumping the inductor is the thing that has to be integrated. You'll need roughly 60-70 mA at snap time.

An NC7NZ34, all sections in parallel, powered by +6, will put 5 volts into 50 ohms in about 600 ps.

You probably refer to the Analog Devices parts. One of them makes a nice 35 ps edge or some such. You can differentiate that with

0603-size r/l/c parts, into anybody's fast mmic, and get about a 100 ps blip. We're doing that to get a 90 ps, 6-volt fiducial pulse into a laser modulator, but we're using $180 Hittite distributed amps to get that much swing.

But for a few volts at 100 ps, the srd is probably easier.

Oh, try M-pulse too. Thay have some nice parts.

John

Thank you John, your posts are very informative; I have seen that that SRD stuff has been debated also a few months ago and I read the old posts.

If I have understood correctly, the transition time of the diode depends on the stored charge. Should I minimize that charge? The the current risetime is mainly fixed by the inductor, if the driver has a sufficiently small risetime. How should I choose the inductor then? I guess that the diode should snap when the current equals the supply voltage divided by the 50 Ohm load resistance. In this case, I should get a clean output pulse. The inductor should be sufficiently small to have a fast dI/dt, but big enough to isolate the generator when the diode snaps. I can then choose the forward current in order to make the SRD snap at the right time. Is that correct?

Thanks. I will have a look at that part. These tiny logic devices are truly amazing.

Yes, I do. Something like the ADCMP580.

Thanks a lot!

No, or at least only indirectly. Diodes that store a lot of charge tend to have slower transition times.

Yes. The output voltage will be a bit less than (inductor current * load resistance) at the instant of snap. The problem is to get that much current into the srd before you use up all the stored charge. If you lose that race, it snaps when the current is too low, and you get a wimpy step. It's easier to pump a lot of current into an srd that stores a lot if charge, but those are the same ones that tend to have slow transition times.

Email me your address and I'll send you some parts.

John

Hi everybody, I'm doing my study in Belgium and for my final work I have to do a pulse generator too.

First I've planned to use a transistor (2N3904) in avalanche area. But with my circuit, I need a very fast signal generator because the width of my pulse is proportionnal to the rise time of the signal generator and I want a 1 ns width pulse, so I need a 1 ns rise time for the generator...

So I would like to use a SRD instead or inside my circuit. Could you please tell more about your work? Do you have a particular circuit for the pulse generator? Please email me and we can share informations!!

Thanks. Johan

If you put a small transistor in the avalanche region, you should easily get a sub-nanosecond rise time. You can find a good example in Jim Williams' AN94, from Linear Technology:

I found very interesting the old AN918 from HP:

If you need a 1ns pulse rise time at a few volts, keep in mind that this can be obtained with a reasonably fast logic gate, such as the NC7NZ34 John suggested.

If the transistor avalanches, the output rise time is unrelated to the input trigger speed. It fires or it doesn't.

Zetex sells, as far as I know, the only transistors specified for avalanche use. They are an old diffused process, fairly low Ft, made in Russia. Newer epitaxials don't seem to avalanche.

John

@ Johan: I played a little with the transistors I have in my junkbox. I did not see avalanching on a 2n2222, but I tried a few 2n2907 (they are PNP transistors, so one needs to change the schematics accordingly), from several brands and they all worked nicely. Of course, they are not specified for avalanche use, but a lot of cheap transistors can be made avalanching well. I did not run them for a very long time, but they did not vanish in a puff of smoke. You have to try. Of course, you need at least a 250V max DC adjustable power supply. I used a flyback inverter I made a few years ago. Be careful, of course.

@ John: I built several SRD pulse stretcher, but unfortunately I have to wait until I have access to a fast scope to test them. Thank you again.

@ Darwin: That sounds great!! Which schematics did you use to try the

2n2222 and the 2n2907? What sort of input trigger did you use (and specially the input trigger rise time)?? Because in a book that I have, they say that it could work with a 2n918, 2n2222 and 2n3904. Here is the schematics I would like to use for the transistor in avalanche area: and this one is the one I would like to use with a SRD: . For you, which ones could give me the best result? I mean the smaller width impulse?

Thanks!!

Hello,

if I understand, I can put any rectangular trigger on my schematics' input, it will not influence the width of the output impulse (Even if the rise time is more than 5 ns)? In fact, I would like to have an impluse of maximum 1ns of duration... if it's smaller, it better!!

What do you mean by the fire or not? Does it mean that the transistor died? Here is the schematics I would like to use:

But I don't know what to put for Vbb ... Do you have any idea??

Johan

That looks OK, but the avalanche will kick a lot of pulse back into the trigger input. I like to use a small trigger transformer, with the secondary connected directly b-e on the transistor. A little RF ferrite core with a few turns on each side, bifalar maybe, works fine for a ttl-level drive.

You might also add a bit of inductance in series with the output 50 ohm resistor, or even reduce its value, to speed things up a tad. Keep all lead lengths zero, of course, and build it on a solid ground plane.

I've never damaged a transistor by avalanching it. Many generations of Tek sampling heads used avalanche transistors, to either drive the sampling gate directly or to drive an SRD. There must be billions of device-hours there, and as far as I nkow, it's not a particular failure mode. The dual-channel 7S14 uses one avalanche transistor to drive both sampling bridges. It's spec'd at 350 ps risetime but is typically about twice that fast.

Picosecond Pulse Labs and Kentec sell commercial avalanche-based pulsers. We may do one soon too, for a custom application.

John

will

Thank you for all these advices!! I've try to simulate the circuit with the avalanche transistor with PSpice but I think the simulation device don't know the avalanche mode... Do you think there is a way to simulate that? May be with ADS? I've got it but I've never use it.

I will use a 2N3904 because that's what we have in the laboratory. But to fixe -Vbb and Rb, I don't really know what to put there. Will it depend on my transistor breakdown voltage?if I put Rb = 1Mohm and -Vbb = -60V will it be ok?

Johan

will

Simulation is useless here. Use solder.

To test transistors for avalanche behavior, try...

+300 | | 100k | | +---------+--- } 50r coax to attanuator | gnd--> } and fast scope | | 50 | | | gnd

This will self-oscillate and tell you the avalanche voltage and risetime. Invert supply for PNP of course. Most transistors, especially modern ones, won't avalanche at all.

I've noticed that some transistors will dump essentially all of the cap voltage into the load, and some just a fraction. The Zetex parts seem to turn on hard.

John

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will

Just to be sure...

50r coax to attanuator gnd--> } and fast scope

are connected to b of the NPN? So I have b ---10k --- gnd and before gnd I connect fast scope?? Is that correct??

thx!

snipped-for-privacy@gmail.com a écrit :

will

Nope. The 10k base resistor, the 50R resistor and the 50R coax/scope are all connected to the transistor emitter.

--
Thanks,
Fred.