[sdiy] Slightly OT, Logic Analyzers...

Jay Schwichtenberg jays at aracnet.com
Mon Aug 5 18:52:23 CEST 2002


Prot,

It really depends on what level (speed/width/sample depth) that you want to
go to. The old Tek DAS units (I think that the model number was 9100) are a
UNIX box with a lot of electronics to support data acquisition. On the low
end it's doable (100 kHz-1mHz) with good design and PCB layout. With the
high end (1mHz and above) you get into the rocket science of ECL/high speed
logic, transmission lines and trigger engines. Typical input circuit will
have input protection and then some sort of comparator. Depending on the
speed you may be able to use comparator chips but for real extreme cases
discrete is used.

Over the years there have been several articles I've seen that have home
built stuff. Can't remember where it was. You might see if there is an
archive for Nuts & Bolts or Circuit Cellar. That would be my best guess on
where I saw it. Also Circuit Cellar has ads for some PC based logic
analyzers.

For me this would be something I'd pick up on e-bay. If I could afford it
probably wouldn't be good enough for 'real' work (10-50 mHz) but would be
good enough for home. I don't know what an small old HP or TEK (12XX, stay
away from the old DASs)would run. Also if you go that route make sure it
comes with probes and clips. These are not cheap and can add considerable
cost to a unit.

Good luck.
Jay

> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of The Proteus
> Sent: Monday, August 05, 2002 1:44 AM
> To: Synthesizer DIY List
> Subject: [sdiy] Slightly OT, Logic Analyzers...
>
>
> All,
>
> 	I've been contemplating designing a very small logic analyzer with
> some Linux software to run it. One of the features I'd like to have is
> support for multiple logic types (TTL, CMOS, LVTTL, etc...). Now, from
> what I'd assume, the logic capture hardware is basically a latch being
> driven at high frequencies, and then that latch data is being stored in a
> buffer memory somewhere. That's all simple to me... the question I'm
> kicking around currently is how to get the input voltages in line with the
> latch. If you've got a 5V latch and the input is a LVTTL signal, how do
> you translate the high, low, and threshold voltages appropriately? This
> isn't a big deal for basic TTL<->LVTTL or CMOS<->TTL type circuitry, but
> when you get down to 1.8V and 1.5V Vcc rails, the appropriate level
> translator could get quite tricky to design - especially in commercial
> logic analyzers where there are 128+ channels.
> 	Does anyone have any ideas on a good, low-cost level shifter I
> just described? Yes, my goals are lofty to top out at commercial analyzer
> channel counts, but hey... let a young kid have his dream. ;-)
>
> Regards,
>
> Prot
>
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