Narrow pulse generation

Hello,

Anyone have good ideas on generating 5nS pulses into a 100ohm load, but the load can be anything from an open circuit to a dead short. 5 to 10 Volts would be great. It's not a beauty contest on the shape so I want to do this cheaply using generic parts I have laying around if at all possible. I would want to be able to trigger these repetively at up to say a few hundred Hz. It's for some TDR tinkering.

thanks

Reply to
Anthony Fremont
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Seems to me any logic gate can drive faster than that, but you might not get the voltage swing you want. Raises the question of why you want such a high voltage? With such a slow rise, you might want to test very long cables? TDRing nearby typically uses 25pS rise-times with about 100mV swing. But I think with a typical logic gate you can see all the classic TDR cases with a few feet of coax for sure.

Reply to
a7yvm109gf5d1

You can use the output of some waveform generator (or a astable multivibrator) to feed a high pass filter (a simple C-R with a diode in anti parallel to R to avoid negative pulses). Then the filter output feed to a schmitt trigger like 74hc14. The pulse wide is controled by the values of R and C. Easy and cheap!!

Good luck

Reply to
mario.sardon

Thanks for your reply.

The voltage I chose was arbitrary, I was just thinking of something battery powered.

Those rise-times would be nice, but not really necessary for me AIUI. I'll just be using an average scope for the monitoring so anything should work for tinkering purposes.

Since I'm not very clever with analog stuff, how can I make the logic gate switch on, wait a few (adjustable would be cool) nanoseconds and then switch back off on command?

Reply to
Anthony Fremont

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Reply to
Andrew Holme

Figure 4 shows the basic method of how it's done:

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Of course you have to use much faster logic and then amplify it. Or use tons of parallel buffers and step up via a stripline transformer. Sourcing 5-10V into a dead short might turn out to be a challenge though. At least I'd ask the local uitility to spool up their peaker plant and warn the fire district before moving that big switch. A notice to the FAA would also be good so they can alert pilots that this big bright flash is not a UFO or meteorite that has crashed ;-)

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

You don't do TDR with a pulse - you do it with a step; you reset the step when the last reflection gets displayed (which will depend on the length of the line). That gives you a nice graph of the cable's impedance along its length. A pulse will just differentiate that and smear out all of the pips.

Generating the step is left as an exercise for the reader? Heck, it's just a step generator - you just go from 0 to 1, and the risetime will depend on the chip, when a trace is done, reset to 0 until all of the reflections are back, and give another step. At 1' per nanosecond, you probably won't have to wait long. :-)

Good Luck! Rich

Reply to
Rich Grise

Electrical TDR usually uses a step, not a pulse. My TDR scopes all seem to use a square wave, 100 KHz ballpark, to generate a repetitive step, although you might go slower to shoot out really long cables.

An AC-family logic gate should work.

John

Reply to
John Larkin

LOL You don't think I'll have trouble keeping the voltage up do you? ;-) I guess I better allow for that then. I would like the output impedance of the generator to be somewhere in the vicinity of 75 Ohms, adjustable from 50 to 100 would be ideal. Thanks for the link. :-)

Reply to
Anthony Fremont

That would certainly mean some paralleling or transformer-based combining so you can have fast logic and boost up the voltage. But as Rich already mentioned TDR is usually done with a transition, not a pulse.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

I'd go all out and use PECL or something like that.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

I don't know the details, but when I was doing video speed analog, we used HP ATE. Anyway, it tuned itself up using a pulse that they generated with a diode specifically designed for flat pulse generation. If you do a patent search, Tek and HP have many pulse generation circuits.

This particular HP circuit was designed into every DUT board.

Reply to
miso

That certainly sounds easy enough and I happen to have some 74AHC04s. Will those work ok?

And now, how do cheap micro based TDRs measure those nanosecond response times? They obviously aren't using a CCP module to time them. There must be a cheezy trick of some sort. ;-)

Reply to
Anthony Fremont

Should. The source impedance of the tdr step should be 50 ohms. Put several gate sections in parallel to get a stiff source, and then put a series resistor, 47 ohms maybe, to make a net 50 ohm source. Run that to a tee connector at the scope input (assuming a hi-z scope) and the other tee port is the tdr thing.

Dunno. The old Tek 1503 (?) handheld used a classic diode-bridge sampler. Nowadays you could just trigger a fast adc, once a tdr step, with the adc trigger slowly walked in time to make the range sweep.

John

Reply to
John Larkin

By 'fast adc,' I think you are addressing yourself to the sampling window (which needs to settle quickly) and not so much to the length of the actual conversion time. That may need to be made clear to Anthony.

Jon

Reply to
Jonathan Kirwan

Right. You only need to take one sample each TDR edge, and slowly walk the sample delay to build the waveform. You can even take multiple samples at the same delay, over several shots, to average out noise.

No rush.

John

Reply to
John Larkin

I can understand that, you just build up the information over many pulses one piece at a time. My question then becomes, how are you "walking" your samples so precisely? What is generating the time delays?

Reply to
Anthony Fremont

5nsec pulses don't sound too hard - we got 0.5nsec FWHM at 5V into 50R and 7.5V for anything longer.

We did use a fairly expensive HP RF transistor as the output stage.

On the way there I got 800psec FWHM out of a bunch of dead cheap BFR96 wide-band transistors.

Farnell doesn't stock them any more, but you might like to look at the Philips BFR106 or the BFG590, which are listed as limited stock in my Farnell catalogue. The BFQ68 looks like a keeper and is a lot sexier, but contains berylium oxide and costs a lot more.

We drove our transistors in long-tailed pairs from 100k ECL using BFT92 or BFT93 PNP parts as level shifters - modern CMOS would be fast enough for your purposes, but note that the thin base-emitter junctions in wide-band transisors often break down when reversed biased by more than 2V (check the data sheet!).

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

For example, you can build an LC or RC section that scoots your bridge trigger time sliver by time sliver. You can use a varicap in the C or make the R variable. The latter is what I usually do. Early on with the SD5400 but when that got expensive I used dual gate FETs for TV tuners. Those cost only pennies but have very low inherent capacitances. Low stray capacitance is very important here. You can easily scoot a few hundred psec. Personally I avoid digitally adjustable delay lines for noise reasons.

That SD5400 was very handy. Since contains four matched devices I could servo it an literally steer resistance. Very smooth. But all good things will some day end .

BTW the scope connection in Hi-Z that John mentioned needs one word of caution added: Some of the glitzy "modern" digital scopes have the nasty habit of reducing their bandwidth when cranking up the channel gain.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

_Sounds_ like a piece of cake, thanks. I should have most of what I need laying around here somewhere. ;-) I'm not sure my old Hitachi scope is going to be up for it, but I should see something with 100' piece of Cat-5.

Reply to
Anthony Fremont

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