Joerg said:
Hello Tom,
Phase shift versus time can be tough (meaning expensive) in frequency
domain if you are looking at just a couple degrees or so. But for droop
you are right, we measure that with spectrum analyzers. That stuff is a
serious concern when designing pulsed Dopplers for ultrasound. Spectrum
analyzers just don't like to trigger that well, mostly they provide a
BNC jack in the back for this and you have to concoct some circuitry to
drive it (but only once).
....
Well, like I said, it doesn't seem all that difficult with modern
frequency domain instruments like we build here. They can trigger on
all sorts of things, including things that scopes can have lots of
difficulty with--like triggering on energy above a particular level in
a particular frequency band. If you calibrate out cable lengths
properly, you can get phase accuracy to a fraction of a degree. How,
with a scope, do you detect signals at -90dBc and determine that they
are or are not harmonically related to the fundamental? How do you
analyze phase noise versus frequency with a scope? How do you plot a
color map of amplitude versus frequency versus time with a scope, and
then display a trace of amplitude versus time for any particular
frequency? I'm not aware of any scopes that digitize with the required
spurious free dynamic range to do that as well as modern spectral
analyzers, and I don't believe they come with software that does as
good a job as what you get with spectral analyzers for frequency-domain
applications. On the other hand, if you need really wide instantaneous
bandwidth (perhaps greater than 100MHz), you probably ARE better off
with a scope--for now.
In general, I'd say that if you are analyzing a signal and want to know
its time-domain behaviour, like rise time, overshoot and the like,
you're better off with a scope; but if you want frequency domain
information like phase, harmonic content, accurate frequency
information, and spurious versus frequency, and even the time
progression of any of those things, you are better off with the proper
frequency domain analyzer.
Cheers,
Tom