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'[EE]: DIY HP logic analyser probes (was Re: budget'
2004\07\20@120912 by Philip Pemberton

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In message <spam_OUT3e0a4bc4040720050919902eebTakeThisOuTspammail.gmail.com>>          Josh Koffman <.....joshybearKILLspamspam@spam@GMAIL.COM> wrote:

> I can't commit to one now, but it does seem like a worthy cause. Keep us posted!
The design work is mostly done. The big problem is going to be soldering a
bunch of 0603 parts together. I'm going to LART the idiot that said "0603 is
about 2mm by 4mm". Closer 1mm by 2mm. Ick.
But I've got the parts! Yes, including the wire. I'm off to crimp an IDC
connector onto a cable - after I've sorted that out, I'll take a look at the
compensation circuitry (the 0603s). Two resistors and a capacitor per input
on the analyser.

Schematic for the compensation circuit, from an Agilent appnote:
                Ra          Rb
To target  ----|RRRRR|--X--|RRRRR|--X---- To analyser
                       |           |
                       |----|C|----|
                            Ca
Ra = 250R
Rb = 90k9
Ca = 8.2pF

Later.
--
Phil.                              | Acorn Risc PC600 Mk3, SA202, 64MB, 6GB,
philpemspamKILLspamdsl.pipex.com              | ViewFinder, 10BaseT Ethernet, 2-slice,
http://www.philpem.dsl.pipex.com/  | 48xCD, ARCINv6c IDE, SCSI
... Life, don't talk to me about life.

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2004\07\20@122534 by Josh Koffman

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Well, that I could help with if you're not happy with the soldering. I
do a bit of SMT, and since I have steady hands, I seem to do ok!

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adams


On Tue, 20 Jul 2004 17:09:16 +0100, Philip Pemberton
<.....philpemKILLspamspam.....dsl.pipex.com> wrote:
> The design work is mostly done. The big problem is going to be soldering a
> bunch of 0603 parts together. I'm going to LART the idiot that said "0603 is
> about 2mm by 4mm". Closer 1mm by 2mm. Ick.
> But I've got the parts! Yes, including the wire. I'm off to crimp an IDC
> connector onto a cable - after I've sorted that out, I'll take a look at the
> compensation circuitry (the 0603s). Two resistors and a capacitor per input
> on the analyser.

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2004\07\20@140509 by Philip Pemberton

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In message <EraseME3e0a4bc404072009172eef1cfdspam_OUTspamTakeThisOuTmail.gmail.com>>          Josh Koffman <joshybearspamspam_OUTGMAIL.COM> wrote:

> Well, that I could help with if you're not happy with the soldering. I
> do a bit of SMT, and since I have steady hands, I seem to do ok!
Well, I've just built one interface, with the aid of a blob of Blu-Tack and a
bit of solder flux - not to mention the 1mm bit on the solder station.
The cable was easy enough to make, though there's a bit of a "gotcha" you
need to watch out for. The metal back panel (on my 1651B at least) is rigged
to make it damn hard to insert an IDC connector instead of the "official" HP
connector.
To get around this, crimp the connector on to the cable in the normal way.
The polarising tag should be facing up and the cable should go in vertically
with the red (or blue) wire on your right, looking at the back of the
connector. Before you fit the strain-relief clamp, use an X-Acto knife to cut
off the short (<5mm) "tail" of cable on the bottom of the connector. Try and
get it flush with the connector body. Bend the long section of wire over,
hold it tight and push the strain relief clamp on. Do the same for the other
end of the wire.

I soldered up the compensation components without a PCB. Using a PCB is
probably a good idea - I found it rather difficult to stop the parts moving
around. You want to solder Ca and Rb (the 90k9 resistor and the 8.2pf cap)
back-to-back, then solder the 250R onto one end. Solder this to a small piece
of PCB board, then solder wires on and cover in Araldite or a similar potting
compound. Solder a pin onto each end - round 0.6mm diameter or thereabouts is
probably best. If you want, you could solder a grabber probe on, though I
couldn't find any reasonably priced good-quality grabbers in the Farnell
catalogue. If anyone happens to know of a source for grabber probes, please
let me know.

The IDC cable should be good to around 25MHz (best estimate). Given the fact
that the even pins on the IDC connector are all grounds, and the two odd
numbered pins on the ends (1 and 39 iirc) are both +5V, you may be able to
get a bit higher. If someone with a HP 165xB and a 100MHz (10nS) pulse
generator would like to try this out, I'd be interested in the results :)

To do: More experimentation, maybe etch some PCBs when the cut on my finger
heals. Lessons learned: Don't pick a PC case up unless you're holding it on
the base. That and "PC cases are like gigantic razor blades"...

Later.
--
Phil.                              | Acorn Risc PC600 Mk3, SA202, 64MB, 6GB,
@spam@philpemKILLspamspamdsl.pipex.com              | ViewFinder, 10BaseT Ethernet, 2-slice,
http://www.philpem.dsl.pipex.com/  | 48xCD, ARCINv6c IDE, SCSI
... It is fatal to live too long.

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