Re: Interesting feature of an inexpensive function generator

On a sunny day (Mon, 1 Jul 2013 19:28:37 -0400) it happened "Charles" wrote in :

? Add a TTL chip to a less expensive one?

They use a special pulse generating diode to calibrate scopes. These items are peddled all the time on ebay at exhorbintant prices. Like $250 for what gets about $5 at the flea market.

Just search for Tektronix tunnel diode pulser on ebay to see one.

As far as I know, they are just diodes with RF connectors attached. You could probably roll one yourself if you knew the secret sauce.

Try:

Jon

Nice - thanks!

Tunnel diodes are rather scarce these days (like hen's teeth). Seems to me that one might be able to put together a similar pulser with a different sort of negative-resistance switching device, such as a "lambda diode" made from a pair of coupled transistors. The traditional lambda diode uses a pair of JFETs, but fast P-channel JFETs are also scarce. An "RF N-channel JFET, plus RF PNP bipolar" combination might work nicely. Keeping it small and compact (to minimize parasitic trace inductance) would probably be important in this application, so as to get the cleanest pulses possible.

Try Ebay for the Russian tunnel diodes. Cheap and most work. Get the Ge ones, not the GaAs types.

Search TekScopes Yahoo Group for "DIY TD Pulser". That is my version of the venerable Tek 067-0681-01 using russian TD still available from Ebay. Everything is there including all construction details, photos, and actual test results. YMMV but that one I built and put on TekScopes gives something like 35pS risetime and takes one evening to build.

Chris, how do you get 250 ps risetime? I tried to find the datasheet on Octopart and they have started using captchas for rate limiting. I could never get past that screen.

I did get into Mouser and found the part is made by TI and NXP. Neither datasheet said clearly what the output rise and fall times were. The best I could find was a table that had numbers like

The ~280ps risetime was found by measurement. If you want it guaranteed then you'd need a more expensive chip. It is only a typical value on even ADI's fast comparators ADCMP572 etc. In practice rather than paying a manufacturer extra to guarantee it, you might be better off buying a lot of 74LVC chips from the same batch, measuring a few and then knowing that this is what you have.

You can see the risetime figure briefly on the screen of the scope in the video I linked. I measured it previously with a similar scope and I got about 280ps, depending on supply voltage etc. That was for an old Philips chip, so now that would be called NXP. I have a few of those old chips left over, and I know they are fast. No idea whether they use the same chip layout and fab now, maybe JT can advise.

Chris

It was very difficult to follow the video. He kept rotating the time axis knob back and forth. Too fast to actually look at the waveform and try to understand what it is saying.

I am surprised the risetime can be that low. It takes very fast transistors to do that. If that is true, why is the propagation delay so long? You'd expect with transistors that fast, the prop delay would be down in the picosecond range instead of 4.5ns at 3.3V as shown here

"

I don't have any of these devices on hand and do most of my stuff in

Thanks,

JK

Note that the rise and fall time of the 74LVC04 is quoted in the datasheet with a 30pF or 50pF capacitive load. Instead, I measured the risetime with the output connected through a resistive back-terminating network to a matched coaxial cable, which would be expected to give much faster edges.

Each inverter in those 74LVC04 chips would contain several (an odd number of...) inverter stages in a chain. They would probably also have ESD resistors at the input which would somewhat slow down the propagation delay to make it likely to be several times greater than the best possible rise and fall times. I think they probably use a 0.5um CMOS process guessing from the absolute max. supply voltage rating. On a

Jim Thompson might know a lot more about it than that, and might even be allowed to tell us.

I no longer have access to a fast scope so I can't give you any plots, but I can confirm that you could get a pretty clean looking square wave with

Where do you see a spec for the rise and fall times? I tried several different manufacturer's datasheets and all I can find is the prop delay.

I can see the mfgr's wanting to eliminate every possible spec parameter that they can get away with. This reduces cost and removes the loss for devices that don't meet the spec.

But I'm old school. I want as many spec parameters I can get:)

JK

In any case, removing the capacitive loading will speed up the output waveform, both risetime and propagation delay.

Chris