Protection for sensitive RF front-ends

Tough to do well, I think. (JL and Joerg would be the go-to guys on the group for that sort of thing.) At lower speed, back-to-back LND150s in series with the input are a big help, but unfortunately they look like

1k resistors up to their I_DSS (1.6 mA) and so they'd slow down your sampler pretty badly.

IIRC Tek used to do it by putting extra diodes in series with the sampler drive, and hanging a resistor between the sampling cap and the buffer amp that runs the sampling loop. That way the bridge and sampling cap all charge up together.

Their sampling circuits book is an excellent read--it covers up to about the S-4 generation (25 ps), but unfortunately not the 118xx ones (17 ps). .

At a quick squint I don't see a lot of input protection there, which is sort of an RF tradition.

Still should be easily fast enough for what you're doing.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

I've used (maybe sacrificial) jfet gate diodes a few times to protect more expensive opamps and rf transistors:

If it's pure RF at a high enough frequency, a pin diode limiter can be used.

--
Grizzly H.

From my experience with fast samplers, there is no protection at all. Anything in that direction would spoil the performance. It's all care and forethought that saves you from disaster.

Jeroen Belleman (Who has --so far-- broken two Tek S-6 samplers in 25 years. :-( )

How about just shorting the input with a relay, until you turn the pulses on? Or, short it with an NC second section of the TEST momentary contact switch?

Peanut gallery:

- ESD is mainly RF, and pretty big at that.

- Fight back in kind.

- Instead of using a, say, 1V TDR pulse and sampler, do it at 100V, or

10kV. Now your hardware is beefy enough to withstand such treatment, and doesn't care.

- Bonus: you can get a pretty good step generator just by sparking through air. (You'd probably use something more controlled, like a reed or vacuum relay.) ESD itself is standard around 1-5ns rise (IEC 61000-4-2).

- Plus, you can use the generator head for ESD testing and physics experiments!

There's also a lot of wideband energy, so you can cut down on the energy seen by your critical devices by reflecting or absorbing the rest. TDR could be done with bandpassed wavelets, for example. In which case you're just doing old fashioned RADAR, with everything but the rotating antenna.

Of course, your time resolution always follows the total bandwidth, whether it's baseband or not, so this isn't really desirable. Perhaps instead of DC-1GHz bandwidth, you'd go with bursts in the 4-6GHz range instead. Which avoids a lot of ESD energy thanks to filtering, but now you're dependent on quality cable too, which might hardly succeed at all -- like with a modest length of RG-58, or give or take whatever kind of nonsense the attached antenna might reflect at those frequencies.

Continuing this aside... how would you build a 10kV sampler, anyway? Tempting to use the input signal (i.e., assuming the return event is still ~kV), plus a triggered spark gap (e.g., UV radiation from another, carefully timed spark) to measure it. But sparks are pretty bad at timing and breakdown to begin with. Perhaps that's the kind of domain you'd harness electron physics for your advantage? Perhaps use the input signal as a kicker for bunches in a cyclotron, so you get the synchronous detection needed, and for the anode, a series of small plates, or perhaps an interleaved binary sequence or delay line, giving the electron equivalent of a smear camera for the readout.

If you've got a precision timed pulse generator, you can certainly still use the diode gate method. Tube diodes of ~1pF and 10kV are plentiful, unfortunately I wouldn't expect their bandwidth to extend much beyond

500MHz or so.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com 


"Clifford Heath"  wrote in message news:jK98y.223826$M%.3160@fx20.iad... 

I want folks' opinions about options for RF input protection. 

I'm thinking about building a TDR (for probing antenna cables, etc) 
using a couple of HSMS-286K double diodes (0.18pF) for the sampler. 

Low capacitance in the protection diodes is obviously a win, but not at 
a big cost to actual protection. Mainly worried about ESD here, not 
bozos plugging it in to 240V mains. 

I'm thinking I should be able to get down to under 500ps edges using a 
TinyLogic driver, maybe even 200ps (beyond that is more than I can hope 
for with my equipment and state of knowledge). Faster would be better, 
and of course jitter matters more than raw edge speed (time delay made 
using comparators and ramp generators as has been discussed here; 
absolute accuracy isn't that important to me), but as far as protecting 
the inputs, how would you go about it? 

Clifford Heath.

A couple decades ago I worked for a company that sold TDRs for RJ-45-term inated CAT-5 ethernet. *Our* bozos would plug them into RJ-45 phone lines a nd kill the front ends with ring pulses. Our brute-force solution was zener s from the I/O pins to ground. The TDR measured pin impedance to ground dur ing POST and baselined readings from that. The zeners were too slow to shor t out the TDR pulses but swallowed ring pulses whole.

I don't think zeners will work for you, but dumping ESD to ground is simp ler than trying to block it.

Mark L. Fergerson

What's this guy been smoking?