Detecting tiny pulses after a very large one

How about a Nuvistor? Sounds like a job for a tube, for sure.

Cheers

Phil Hobbs

One other possibility would be to use Schottky diodes to shunt the big pulse, and a second identical winding wired the other way round, to cancel the circulating current in the first one and so turn off the Schottky in time for the little pulse.

You'd obviously need to delay the signal from the second winding, e.g. with a bit of coax.

Cheers

Phil Hobbs

Haha! Here I was, looking for depletion mode MOSFETs to see if they could be of any use here... Thanks for the hint. I'll check it out.

The beam transformer is a 'wall current monitor'. It doesn't really have anything that looks like a winding.

Cheers, Jeroen Belleman

No wisdom, just 'silly' ideas. If you can't shunt the big pulse from the low level input, can you turn-on a low level channel after the big pulse goes by... (Thinking of a box-car averager but maybe without the average function.) I guess you'd need a second detection path.

George H.

Mark

Mmmh. I tried a Mini-Circuits VLM-33-S+ limiter, which I presume, uses this sort of diodes. It should be fine to protect a receiver front-end from gross RF overload, but for my purpose, it was awful, useless.

That is not to say that some other circuit incorporating these diodes cannot be made to work. I have to look into that too. I looked at Schottky diodes first because those recover so much faster.

Jeroen Belleman

We've done that recently. We are making a controller for a modelock/pulsepick/pumped-fiber laser, and tiny pre and post pulses, before and after the main optical pulse, are bad news. We built a photodiode amp with 1x and 200x outputs, where the 200x clips hard and recovers in (as I recall) something like 10 ns.

There's nothing exotic, just a chain of opamps and diode clippers.

It would be more interesting at 3 GHz bandwidth. MMICS and tiny diodes. We were DC coupled, which helps recovery.

Too bad. That two-winding hack would have been slick.

Cheers

Phil Hobbs

Hmm well I wonder if you could make a transformer such that it would saturate with the big pulse... but let the little guys through... Maybe if Jeroen can knock the 600V down by a factor of 10 then some diodes can do the rest. (I have no idea if ferrites work at 3 GHz.)

George H.

Because a customer paid NRE for the design, and we have an NDA.

And it's 100 MHz, not 3 GHz. But the concept should scale: a chain of amps and diode clippers, arranged so that none of the amps ever saturate. That implies fairly low gain per amp.

Wire bonding would be great, if available. Or beam leads. Chip and beam-lead schottky diodes are available below 0.05 pF. It's the packages that dominate capacitance.

There are also photodiode limiting amps that would be interesting. Some work into the 10s of GHz.

Quit being a jerk. Nobody likes a jerk.

Commonly known a LOG amplifier. Good idea to handle a large dynamic range.

Ok, looks like the amp must be 3GHz in BW but "80MHz buckets" seems to indicate about 12nsec between them which is large.

Phil's nuvistors are a good idea since tubes can take a huge punch without penalties but they are only available as (mostly Russian) cold war era NOS. Also, I doubt they can do 3GHz but I may be wrong, maybe there are some. If you need only a one-off solution that doesn't have to live for half a century and you can find nuvistors that run in the gigeehoitzes it may be an option to buy some through an auction site. They often come in cartons of 10 or 20. The wire/pin inductance will already present a problem in a GHz application.

In ultrasound we have the same problem at a lower frequency range but it should scale. We have large pulses of more than 100V and right afterwards must acquire uV-level signals. Way I usually did that:

1. T/R switch. Essentially two diodes back to back in series with a small current through them. So small that Trr didn't matter but one could also use Schottky here. We couldn't because of the high pulse amplitudes. This makes sure that the amp doesn't fry. Possibly your main pulse energy is low enough that you don't need it.
2. An amplifier that is immune to any lingering saturation effect. My favorites are grounded gate FET stages. A professor at our university said this technique is stupid. He had no clue.

Another option may be opamps with GBW over 15GHz that have a very defined saturation behavior. Often you can only find out by trying because the datasheet is silent about it.

I'm speaking with more and more ignorance as I get further down in my reply -- size your grains of salt accordingly.

How many bits on the digitizer? 8? 6? Knowing that helps us know how much dynamic range you need on the protection circuit.

If it was 16 bits you could almost do it just by digitizing what you see. Surely the catalogs are just bursting with 16-bit, 3GHz ADCs these days!

I think people are misreading your "RF bucket" terminology -- I'm taking what you say to mean that the desire is to have a pulse that's 4ns long, centered in a 'bucket' that's 12.5ns long, with tails that die off quickly enough that the bleed into adjacent 'buckets' is less than 1e-5 times the pulse height?

Do you know in advance which bucket you expect the pulse to be due? Do you need to see the pulse at all in the "desired" bucket? Can you actively blank? If so, a T/R switch, timed for that bucket, may work.

Or -- could you bias a diode such that it normally has current flow, but your large pulse reverse-biases it? That, possibly followed by more mundane protection, at least has the potential to be pretty fast. DC bias may be a problem, but could be recovered if you're got a portion of the beam that's otherwise known to be at zero current.

Active T/R switches can have their issues because they can generate spurious little pulses on their own. Things that aren't really there.

That's ye olde passive T/R switch. Woiks, BTDT.

One trick with T/R switches is to use two diodes in series but opposite, ideally on one die or at least in one package and from the same wafer. Can be cathode to cather or anode to anode, whatever is on sale. Make sure the same DC current flows through both. Then the DC almost goes away.

On a sunny day (Wed, 16 Oct 2013 11:36:52 +0200) it happened Jeroen Belleman wrote in :

So that was the Higgs ;-)

Returning to my LOG amplifier idea... in my Garmin GPS chip designs I used basically a string of PECL stages with a DC loop around the whole deal to keep all the stages in the linear region, maintaining data and getting the RSSI function almost for free.

I would suppose your pulses are unipolar, which presents difficulty with just dropping into my kind of scheme... unless your pulses are bursts with lots of off-time between bursts, then you could use a very long time constant for the DC loop. ...Jim Thompson

Have you considered a delay-line amplifier? The general idea is to feed V(t) and A* V(t-T_0) into a difference amplifier, and hope the tail of V(t) can be cancelled and you get a good measure of the excess (at least, for the short time between T_0 and 2* T_0). This is one of the classic ways to prevent pulse pile-up.