[sdiy] I am in love....

[Magnus Danielson]

From: Jim Patchell <patchell at silcom.com>
Subject: [sdiy] I am in love....
Date: Thu, 25 Oct 2001 14:09:57 -0700

>     We just got a new toy at work...(big surprise considering we are
> supposed to be cutting expenses...I requested this thing a long time ago
> though), an HP, er, I mean Agilent, 54622D Scope/Logic Analyzer.  This
> thing is really cool.  It is a two channel DSO (200MHz sample, 100MHz
> BW), and a 16 channel logic analyzer, all built into one.  The scope
> display is syncronized with the logic timing diagram...really neat for
> debugging circuits that combine both analog and digital circuits.  I
> wish I could afford to have one at home :-(  (base price for this thing
> is $4000).  Of course, I suppose I could borrow it on week ends...

Indeed. Actually, I'd like to point out that some "digital" problems
is really analog. This is where these babbies can be *REALLY* a
mission saviour! How many times have I been able to save a troubled
debugging with a scope? - A million times! ... and counting! (OK, not
a million, but many times). Now, the problem with most of todays
scopes is that they almost never have more than 4 channels, and most
of them can only trigg from one channel anyway. A few scopes allow you
to do more elaborate triggers. Now, you also want to look at
complementary signals to see the after effects and some just to see
where you where when it happend. Achieving this with a 4 channel scope
is sometimes just not possible due to all constraints, and you can't
use the big logic analyser either, since you got to have an analog
view of one or two signals. This is where this thing comes in.

Tektronix once made a very odd instrument, the TLS216. It is neither a
logic analyser or a scope, it is both! This baby had 16 channels of
500 MHz BW 2 GS/s - on all channels simultaniously! It came with 16
FET probes, color display, 3.5" floppy... VERY cute tool. I once
demostrated to the designers of a research project why they probably
had biterrors. They had noted that when they pushed many '1's out
through the CMOS SRAM memories (this was in the 20 MHz part of a
system which had 622 Mb/s interfaces using Gigabit Logic GaAs chips)
So, I just happend to have unpacked an TLS216 in another researchlab
one floor down, so I asked them to set things up while I got the
scope. I started to hook up on the necessary control pins and outputs,
just as if I where using a logic analyser. I just "happend" to hook a
probe to the power line. Now, to their big supprise - but not mine,
the power dipped considerably (from 5.0 V to 4.5 V) exactly as the
outputs pushed out '1' after driving '0's. They learnt that digital is
indeed analog, and that you *SHOULD* take the decoupling capacitor
buissness serious. OK, in this story we are also talking about what
was most probably the last use of that card, the research it
represented had allready acheived its goals and new steps had to be
taken (and BTW, many many steps have since been taken can I say some
6-7 years later, still walking and talking). However, I think the
point here is that the lesson to be learned is that sometimes there is
simply no replacement for looking at things in the analog domain, and
combining that with digital signals.

In the end, when you do professional work, just having the right tools
around and knowing how to use them can save you a lot of money and cut
times tremenesly, you might also learn for the next project, and then
save time by doing it better from the beginning. That's why I have
ensured that we have some pretty cool tools just standing around in
the lab doing nothing for many months, and then suddenly when I learn
of a certain problem I pull out one of "those" golden instruments and
can help cutting down to the real culprit.

Those that have visited my home can confess that I have a tendency to
keep alot of instruments around.

Luckilly, when doing synths, many of the properties we deal with does
not require very expensive and elaborate instruments. For most of the
times will a cheap 2 channel analog scope with 10 MHz be more than
sufficient, toss in a DVM which does voltage, current, resistance and
can beep you and you are well equiped. Most survive without the scope.

Cheers,
Magnus - gear freak? Who? Me? Naaaahh!