From: dtmedin@catbyte.b30.ingr.com (Dave Medin)
Subject: Re: Oscilloscope triggering
Reply-To: dtmedin@catbyte.b30.ingr.com
Organization: Intergraph Corporation, Huntsville AL
Lines: 55

In article <C4vs0G.5ux@murdoch.acc.Virginia.EDU>, dgj2y@kelvin.seas.Virginia.EDU (David Glen Jacobowitz) writes:
|> >>Can someone out there explain exactly what the 'trigger'
|> >>feature found on oscilloscopes does?
|> >
|> 	{ lots og good explanation deleted}

<lots more deleted>
|> 	Is it just me, or does anybody else out there NOT like digital
|> scopes. My school has some beauutful 100Mhz HP that are digital with
|> all the bells and whistles, including soft-keys, which I think are a
|> loveley touch. ( that is, software keys. ) You don't forget that you
|> are dealing with a computer. Those scopes even with all their
|> neatness, still make the ickyest looking waves. Lotsa features, but
|> ugly output. And those are the best digitals I have ever seen. I've
|> seen a lot of cheaper digitals and they look terrible.

I think the hangup with digital scopes is that you have to know so much
more about them and how they work on a scope-by-scope basis, and
some of the functions are typically presented, in my opinion,
in a counter-intuitive fashion (HP has made some strides in their
54600 series, IMO). Automatic setups are fine for simple,
repetitive waveforms, but can give you some crazy results on more
complex events where you need to understand how the scope is
actually measuring/processing the event. For example, is the scope
in "equivalent time" or in "real time" sampling mode (equivalent time
being a mode where samples are built-up slowly by adding a delay to
the trigger event each sweep)? What was the scope's actual sampling
rate at the time? How is the data being massaged after capture but
before display, etc. One common misconception is the speed of the scope.

Is the HP scope you're using really a 100 MHz scope? Or is it a 20 MHz
sample rate scope (~5 MHz single shot significance) whose front
end including S/H can support 100 MHz waveforms (important for
equivalent time sampling)? The 100 MHz input in this case really
only helps you when your waveform is repetitive, or on a single
sample, when you get lucky and hit a transient event during a sample time.

So, there are a lot more variables in understanding how to get
useful information from a digital scope. I prefer an analog scope for
general use and the digital for events where I need storage for
later analysis or comparison, when the event is within the capability
of the scope. Now, for the price of true 100 MHz digital scopes to
fall...

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