Ah! Now I understand what you were saying in the other message, I misunderstood your idea of "switching" (which in my mind was more like redirecting the current). 100ns turn off time is way too much for what I am after.
I am not after your ps range pulses stuff, never been there (yet). It is ns range current switching, I had found some diodes (I think again after something you had posted) which are good but the voltages between which I have to switch the current are inconvenient, doable though. Will keep on looking, no rush after 30 years of waiting :-). Meantime my pulser is still a mercury wetted relay based, about
30 years old.Am 20.10.21 um 19:52 schrieb John Larkin:
There is capacitance vs. backward voltage and resistance vs. forward current. On page 1. What else could one want? Getting 20 mA forward current through a xyzzy diode is for beginners.
Gerhard
What's so special?
A simple re-sort of smd marking files by function, voltage and current, pulls up quite a few similar or superior devices.
Voltage drop vs DC current. Diodes usually specify that, unless they are "RF" parts.
Beginners with big power supplies.
Treat it like a tiny relay.
Phemts can be truly fast switches, parts like SAV541.
The tiny EPC GaN fets can switch 10s of volts in under 1 ns.
MiniCircuits SAV-series parts are cool, to about 10 volts.
Laser drivers are fun, like the Leo Bodnar thing.
The amps and volts numbers appear to be abs max, forward current and reverse voltage. That tells nothing about the conduction curve.
RF!
I get it now that Jeroen said about the 100ns turn off time, I am not an RF person so when I hear of a fast diode I think fast switching, not HF insulation capability with slow switching, did not even know this was on offer :-). I have looked at various diodes, the one in the .asc file now is some SMS7630 which looks good (last time I have checked it was about a year ago). If this diode sounds familiar to you chances are I have seen it in one of your posts. This pulser has been in the state "this interesting part hit me, perhaps I could use it for it" for decades now...
Just to make sure we clear out any misunderstandings then, such diodes are optimized to switch RF signals. With a fair amount of forward current, the dynamic series resistance of the diode is an ohm or two, so an RF signal can pass through. With a reverse voltage of appropriate magnitude, the series impedance becomes very high, and is dominated by a small capacitance, so the RF signal can not pass.
From the low impedance state, reverse recovery typically takes about 100ns, so that down to 10MHz or so, even signals big enough to drive the diode momentarily in reverse still see a low impedance.
PIN diodes aren't necessarily switches. They are also used in continuously variable attenuators.
PIN diodes exist from tiny SMD devices handling mW signals or less, up to hefty power diodes switching multi-kW signals.
The key to using PIN diodes as RF switches is how to separate the switching current from the RF signal current, because both must pass superimposed through the diode.
Some rectifier diodes, although also called PIN diodes, are completely different beasts.
Jeroen Belleman
They may work as power RF switches too, if the capacitance isn't a killer.
Some higher-voltage rectifiers are a PIN structure that can make a cool DSRD (drift step-recovery/Grehkov) diode.
RF switches typically need to stay turned on for the whole cycle. If you want to pass 10mA of RF current, you need 20mA of DC, or more, depending on linearity requirements.
Not very familiar with class-A, are you John? RF is almost all class-A.
CH
They achieve low capacitance by sweeping (slowly) all the charge carriers out of the intrinsic region, leaving a large gap between the conductors. Damn physics gets in the way of low-C and rapid switching.
CH
There is plenty of excitement around the NanoVNA and the TinySA. It's time to do a TinyTDR. Bet you'd sell many thousands. If you do the pulse gen and the sampler I'll do the rest.
Clifford Heath
There is no way around the 20mA. It is needed that the serial resistance goes below 2 Ohms.
The ds says that the instrinsic zone is 19 um thick. That is more than the copper thickness of my multilayers. That is the reason for the low capacitance. It is also the reason for the slow switching. The 19 um must first be filled with carriers before the onset of conductance. And when switching off, the carriers must be removed from the I zone. When it is empty, the resistance rises suddenly. Snap-off diodes are optimized that the carriers are all gone at the same time.
At low frequencies (few MHz) these PIN-Ds act like normal diodes. Then they do NOT behave like resistors. PIN attenuators create IMD at low frequencies. That is not their habit at uwave frequencies. There are some diodes usable down to 1 MHz, but not cheap. Growing the long I zone probably costs a lot.
cheers, Gerhard
sure about that? if the frequency is low and/or you are using regular silicon diodes, sure but for pin diodes at high frequencies?
You mean the 'R' values . . . ?
I'd assumed you were praising C and R specs. You can sort for those. They're a start. For a nominal V/I plot, you'd have to get the datasheet.
Nominal plots aren't much use in design, where every part used has to work.
RL
One virtue of PIN diodes is that a very little DC current can switch a lot of RF current.
Not very familiar with semiconductors, are you?
A pin diode doesn't quit conducting when the current reverses. That's the point of the thick I region with a long charge recombination lifetime. A little DC current can switch a lot of RF current.
The Macom part that I cited is under 2 ohms RF resistance at 100 MHz and 10 mA DC. The RF current is limited by power dissipation. Which, of sourse, they don't specify.
Without useful data and without Spice models, I suppose people design around these parts by fiddling.
I have around here somewhere a proto PCB that has a pretty fast, reasonably cheap TDR circuit. It seemed to work but I haven't had time to really play with it.
It would need a DAC and an ADC and a lot of code to make a sellable instrument.
I also have a deconvolution algorithm that makes an ugly ringy TDR into a beautiful TDR. That's key to doing TDR easily with Digikey parts on FR4.
You can't see the green target waveform because the white processed one perfectly overlays it.
My code computes the required FIR filter. This is the ill-posed deconvolution problem.
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