Re: Ping John Larkin: Tek 11801 setup
- From: John Larkin <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 22 Apr 2008 14:05:36 -0700
On Tue, 22 Apr 2008 13:38:45 -0700, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:
John Larkin wrote:
On Tue, 22 Apr 2008 11:42:43 -0700, "Joel Koltner"
<zapwireDASHgroups@xxxxxxxxx> wrote:
Say John,
For characterizing PCB impedances (and perturbations caused by all the
circuitry you're hanging off of those nice, controlled-impedance lines), will
a Tek 11801 mainframe and an SD-24 sampling head suffice? It almost seems too
cheap, with each going for ~$1k on eBay these days -- am I missing something?
Thanks,
---Joel
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.
Now if someone would build those sampling heads as USB pods for the PC ...
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
.
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