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Re: How to check 125 logic states...?

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    We get asked to design some interesting stuff, don't we? :-)


Re: How to check 125 logic states...?

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yep. In this case, I had to replace a quasi-digital MUXing scheme, which
had a few minor problems:

1) a reasonable response time (say 50ms), divided by 120 keys, was hard
to achieve when the light source is modulated at 100kHz - it allows
about 40 pulses

2) the post-mux filter had a Q of 50, which meant it took a long time to
react, and an equally long time to shut up again (they did have a
blanking circuit for this reason), thus attenuating the hell out of the
filter output

3) alas, the high Q meant *any* of the digital edges (eg mux or blanking
FET) kicked the thing into oscillation

4) shine an IR remote control any where near it and it went bonkers -
very fast edges again set the filter oscillating like crazy

5) the very high Q, combined with a single opamp 2nd-order BPF, required
ludicrously high GBW. 12MHz wasnt even close (sensitivity goes as Q^2
IIRC for that topology)

6) to try and tame the quasi-digital MUXing, they dropped all the
relevant impedances. Unfortunately this meant using extremely bright
light sources and photo-darlingtons. Fine with the LED device, but alas
TOTALLY UNWORKABLE with a class-1 (or -2) laser.

Turns out they were digital guys, and casually ignored the laser source
until after it "worked" (albeit in a flaky manner). This alone killed
the existing project (after someone spent $100,000 having it developed).

I originally got called in to do the laser driver. Once it worked (aint
hard), the original "engineers" decided they needed much more laser
power, which they just cant have (apparently its bad form to burn holes
in peoples optical nerves)

there were a couple of mechanical design issues too - all the aesthetics
were done, and some bright spark had decided on green and blue overlays,
which we then attempt to shine RED light through. oops, howsabout more
gain then....I couldnt get them to change the colouring, but we did
reduce the colour "density" to allow more light thru.

So I added 120 bandpass-ish photodiode amps (using an astonishingly
cheap photodiode, which Fairchild made specially for us, QSB34CGR),
whipped up 16 summing-BPF's + HPF + HPF + DFD + comparators, and off we

even with all the analogue, it was cheaper. them photodarlingtons were
each about 40c more than the PDs. I also replaced 3 x $15 LED driver
chips with 2 SREGs and 12 dual transistors ($1 total), a $50 uP with a
$3 up, and a whole lot of $1 LEDs with $0.06 LEDs, and made the new one
a just little bit cheaper. Oh, and made it USB too.

PSRR turned out to be a real problem, and meant I couldnt use TL064's,
which are almost free, but never mind (turns out they have 10dB GAIN at
100kHz). In the process I found bugger all opamp datasheets with a
PSRR-vs-F curve, but got good at measuring it myself.

the 8th-order low-Q filters with 60dB passband gain werent too easy to
measure, so I bought a network analyser. by including opamp GBW in my
calcs and sims, I got extremely good results (reality = sim = calc), it
was just a pain to measure. 500s sweep time + go have lunch = slow way
to test 120 filters. not so good for the cholesterol either ;)

I did spend almost a week, near the end, trying to track down an
"oscillation" which was actually the response to optical square-waves
coming out of my TDS224 D-CRO LCD display. I happened to be standing
while moving my notebook, and saw the scope display change as the book
moved in front of it. much cursing ensued, and I dug out one of my
analogue CRO's - look ma, no wobbly bits. Grrr.....

then the time the MFG accidentally placed 4,000 of the wrong capacitor
(oops), that was tricky to find, the units may or may not work, and
erratically so. by accidentally making the reset capacitor 100x smaller,
  it violated the reset-vs-osc timing requirements, and the uP sometimes
locked up, depending on temperature, and supply slew rate. oops.

lots of fun. personally, I prefer power supplies.


Re: How to check 125 logic states...?

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That reminds me of a problem I had a few weeks back...

Debugging a particually elusive fault in a digital system that would
only show itself once every couple of days or so. On one of the digital
lines I captured an unusual packet about 100ns or so in duration,
sinusoidal in shape at 115MHz, and several volts in amplitude. Highly
unusual for a low impedance TTL digital line. Some sort of weird DC-DC
power supply oscillation I thought?

Of course, it would only get captured on the scope when I wasn't there.
This went on for quite some time when I finally managed to capture it
on the scope just as I was walking away. After much mucking around I
discovered that I could reproduce the problem every time by simply
standing up from the chair!

Then the penny dropped. The "packet" of sinusoidal data looked like an
LC resonant tank response, and standing up on the chair was generating
wideband static noise that could couple into my nearby circuit. Combine
the two and you would likely get the response shown. But where was the

As I was investigating the (rather horrible) ground system in the
design (not mine), I suspected the LC circuit was a parasitic in the
design somewhere. After ruling out many things I finally got down to
the scope itself - surely it's not my mega-buck 300MHz Agilent DSO?

Yep, sure enough, short the probe lead out and completely disconnect
from the product and the problem persisted! A very nice 115MHz
sinusoidal envelope packet of several volts amplitude when I stand up
from the chair. Obviously the LC resonant circuit was in the scope
probe itself, and the static was coupling into the probe! Different
probes produced almost the same result, and so did a piece of RG58

If you've got a high enough bandwidth DSO, give it a try!

BTW, an anti-stat coat and wrist strap didn't help much either, so much
for an ESD safe zone! :->

Dave :)

Re: How to check 125 logic states...?

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    It'd be even worse with the humidity often down below 30% in the
weather we're having in Sydney at the moment!
    I'm getting tired of throwing sparks when I get out of the car or
brush past something earthed.


Re: How to check 125 logic states...?

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mine is only a 100MHz/1Gs/s scope, so I wont see that. I could on my
500MHz analogue scopes, but I have to go digging to find them, and they
are too heavy to pick up with one hand :(

but 10pF rings at 115MHz with 190nH. I've seen way more than my fair
share of probes telling lies. Have you read linear tech's AN-47?

plus, of course, what can you expect from a windows-based scope.....

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Re: How to check 125 logic states...?

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There used to be the old trick of decoupling an emitter resistor in an
RF amp with a 1000pF cap with about 1/4 inch leads so that it
resonated at about 140 - 150MHz.  Gave a crude but useful frequency
controlled gain (quite broadband though).
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