Re: [SI-LIST] : What speed scope should I consider?

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From: Mark Randol (m.randol@motorola.com)
Date: Fri Nov 12 1999 - 09:34:42 PST


Larry McMillan wrote:
> They do. It's a 2GHz model called the TDS794. We have one and it gives
> very satisfactory performance. By the way, the Tek 580D is exactly the
> same as the 784 except that it has a black and white display instead of
> color and it costs a whole lot less. We have that one too, and it is
> also very good. Got rid of a couple of our 1GHz LeCroys when we got
> these, and now only use our two remaining LeCroys when both Tek scopes
> are already occupied. Tektronix doesn't make a black and white version
> of the 2GHz 794 or else we probably would have gotten it instead too.

OK, maybe I don't get it being a mostly analog type, but why is a
2GHz analog bandwidth with a 1-4GSa/s rate better than a 1GHz one
with a 2-8GSa/s rate? Sure, according to Nyquist you could see
up to 2GHz with a 4GSa/s rate, but if you're going to be able to
see much, you need a sample rate about 5-10X the analog
bandwidth.

Going with ROT (rule of thumb) BW~=0.35/trise, a 2GHz BW means
you can see a
trise>=175ps. But to hit it with a single sample point you need
a >5.7GSa/s rate. If you get lucky and hit it on the start and
end of the slope, you've got a pretty good measurement then, but
how many points on the slope do you need to really get it? Lets
just assume that 2 points on the slope will do it. To guarantee
you get at least 2 points on the line, you'll need twice the
sample rate, or >11.4GSa/s, or tsampling<=87.5ps. If just having
two points 50% of the time is ok, then 131.25ps or 7.6GSa/s is
sufficient.

If you have the luxury of being able to use repetitive waveform
sampling, you can get away with a lower sample rate at the risk
of missing some events. Personally, even though I know it can
work, I don't much trust it if it isn't "real time", or I have
other ways of verifying it.

Also, being mostly an analog type, I'd be concerned with aliasing
the frequencies over 0.5GHz when running the scope with the
1GSa/s rate on each channel. In reality, I don't figure that a 1
or 2GHz scope (analog or digital BW) in my mostly analog
applications is much use above a couple 100MHz anyway.

So what advantage is there to a 2GHz analog BW vs 1GHz when the
sample rates is halved? What am I missing? (I'm comparing a
TDS794 vs an LC584)

Thanks,

-- 
---------------------------------------------------------------
Mark Randol, RF Measurements Engineer   | Motorola SPS, Inc.
(480)413-8052 Voice                     | M/S EL381
(480)413-4150 FAX                       | 2100 E. Elliot Road
ryvw50@email.sps.mot.com                | Tempe, AZ 85284
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