Ping John Larkin: Tek 11801 setup

WRT the sampling heads watch out for words such as "un-tested" or "as is". It's like with spectrum analyzers, the input circuitry is often why something landed in the basement for many years, because someone fried it.

Thanks for the advice, Joerg. You don't have an 11801 (or similar) around your lab at home, do you? Just the 2465 and some assorted spectrum analyzers?

---Joel

Sorry, I don't. The fastest scope in the lab here is a 1GHz HP.

That's a great combination for TDR. Mainframes (11801 or 11802) aren't too hard to come by, often under $1000, but the SD-24 heads tend to be expensive. If you can get a working head for under $1000, that's a good deal. The last Tek catalog would have priced the combo for something like $45K.

A head with one channel blown will still do single-ended TDR. But if both channels work, you can do TDR and TDT simultaneously, which can be very cool. The TDR channel is an awesome pulse generator.

This is from an old monochrome 11802+SD24, a test trace on a 4-layer board.

ftp://66.117.156.8/Z250A.jpg (tdr'ing from J28 to J29)

ftp://66.117.156.8/Z250_TDR.jpg

The layer1 (first cursor dot) and layer3 (2nd dot) traces were supposed to be 50 ohms. I think the fab house played with my stackup. The last segment, layer4, is pretty good.

John

too

Now if someone would build those sampling heads as USB pods for the PC ...

will

too

That's on my list of things to do. I wouldn't mind working with somebody who'd do the USB stuff and the PC software. I have a really slick deconvolution algorithm that builds a FIR filter that turns a really ugly TDR step into a perfect one, so the front-end gets a lot easier. Making a truly clean picosecond current step is hard on FR4.

I figure you could sell a ton of them to PCB shops alone, if the price is reasonable, below $1K maybe. Along with my fixtureless bare-board tester.

So many ideas, so little time.

John

Hi John,

There's one on eBay right now from a reputable seller with a 14 day warranty for $700. I'm tempted to grab it... but of course that begs the question of how would I test it when it got here? When they fail are they usually obviously broken, as opposed to just having significantly degraded specs?

Forgive my ignorance here, but what's the difference between the 11801 and the

Nice, I'd seen your PCB picture before but not the screen shot.

Thanks,

---Joel

I think the 1GHz spectrum analyzer has better prospects right about now, Joerg. :-)

An SD-24? Ship it to me and I'll test it. I've seen heads with dead channels, channels that were goofy but sort of worked, and heads that the scope wouldn't recognize at all. I don't think we've ever had a head that failed here.

The 11801 has four sampling-head slots. The 11802 has two slots and two 6 GHz delay lines with trigger pickoffs. You can see the leading edge of the thing you're triggering on if you use the delay lines. Samplers usually need a pre-trigger, in the 40-70 ns range.

The later color versions, 11801A and such, never offered the delay lines.

John

will

too

something?

I might know someone, will send PM.

Not just them but also cable guys and so on.

Oh yeah!

Thanks, much appreciated -- if I get it I'll take you up on the offer.

The auction link is here:

- it says he has 2 available, and if you look at his feedback he's sold at least 3 others lately, so apparently he has a whole bunch sitting around.

So they must be relatively robust then...?

That makes the 11802 sound rather preferable to me...

---Joel

Oh, hey, did you get a chance to try out those DIP switch attenuators yet to determine the frequency response?

Yes, it would most certainly have a larger market. A market that is there and nobody takes it.

Too good to pass up, so I got one. I left one for you.

In general, yes. One rarely needs 8 channels.

John

No. I'm working my way through the other tiles on the board, the pulse drivers.

John

Would a 547 and 1S2 suffice? Or 567 with S-6 and S-52?

Those work, but the TD step waveform is sloppy, and the display is not quantitative, so it's hard to measure trace impedances and prop delays accurately. The 11801 is a beautifully precise machine, and a lot faster.

I got started in fast electronics with a 547/1S2. Very cool in its day. I still have a few around.

John

Well that increases the pressure just a bit. :-) OK, I bought the second one... I'll have to start keeping an eye out for an 11802 now (there's one for $1200 listed, although that seems a little high?)

---Joel

Say John,

Are you still out there? I've e-mailed you a couple of times about those SD-24's and haven't heard back.

Thanks,

---Joel