Is is possible to measure rise time or fall time with a apectrum analyzer?

[blueskull]

Hi,

I'm making my decision on buying a spectrum analyzer for measuring signal's rise and fall time. Will that work? In theory the fall and rise edge can be estimated using any sweep spectrum analyzer, or they can be accurately reconstructed using  a zero span real time spectrum analyzer. The signal intended to be observed is pule with tr/tf in range of 0.5~5ns.

What I want to know is practically is that possible? I don't have too much budget, so a 2+GHz o'scope is really out of my reach. I can afford a 6+GHz real time spectrum analyzer though.



Thanks,
Bo Gao

[KJDS]

5nS implies a baseband bandwidth of about 100MHz, so much to high to measure on a spec ana

[katzohki]

Oscilloscope is usually the preferred test equipment for rise time, at least for me.

This is an interesting read:
http://cp.literature.agilent.com/litweb/pdf/5988-8008EN.pdf

It indicates that you may not need a 2GHz scope, but closer to 800MHz.

Sometimes the best option is a rental.

[tggzzz]

I'm making my decision on buying a spectrum analyzer for measuring signal's rise and fall time. Will that work? In theory the fall and rise edge can be estimated using any sweep spectrum analyzer, or they can be accurately reconstructed using  a zero span real time spectrum analyzer. The signal intended to be observed is pule with tr/tf in range of 0.5~5ns.

It works. See https://entertaininghacks.wordpress.com/2015/08/11/measuring-digital-signal-edge-rates-without-an-oscilloscope/ for pointers to the theory and for how I measured ~600ps rise/falltimes.

Note there are limitations, particularly for the longer risetimes - but for those a 250MHz scope would be more than adequate.

What I want to know is practically is that possible? I don't have too much budget, so a 2+GHz o'scope is really out of my reach. I can afford a 6+GHz real time spectrum analyzer though.

Lucky you! The SA I used cost ~$20, new.

[tggzzz]

That's cool man, very appreciated. You just saved me $5000!

You're welcome, but make sure you understand the limitations before you rely on the results.

[T3sl4co1l]

An ideal square wave has harmonics that go as 1/N (i.e., -20dB/dec).

An ideal triangle wave has harmonics that go as 1/N^2 (-40dB/dec).

An ideal trapezoid wave (a square wave with finite rise/fall edges, such as you get from a PULSE source in SPICE) has harmonics that go inbetween these: namely, 1/N for harmonics 1 to X, and 1/N^2 for N > X.

The rolloff point X corresponds, more or less, to the risetime of the waveform (i.e., give or take a constant of perhaps 2*pi or so?).

It's tricky because you might not see harmonics at the correct levels (the signal path needs to be clean, no ringing -- ringing is the manifestation of harmonic amplitudes being raised or lowered by reactive elements), and the harmonics might be so far down that they're in the noise floor by the time you can begin to use this as a basis.

The tradeoff between gain-bandwidth (or RBW, in this case) and dynamic range (usually < 80 or 100dB for a spec), and their impact on the measurement (its accuracy, or the ability to measure it at all), is very much equivalent to the tradeoff between time and rate on a digital FFT type analyzer (or other signal processors).  Which gives a very solid limit on how much risetime you can analyze (namely, because a sampled data set can't tell what the rise time is, in any better terms than simply "sample rate or greater").

Tim

[tggzzz]

The rolloff point X corresponds, more or less, to the risetime of the waveform (i.e., give or take a constant of perhaps 2*pi or so?).

I'll rely on Howard Johnson's graph for that.

It's tricky because you might not see harmonics at the correct levels (the signal path needs to be clean, no ringing -- ringing is the manifestation of harmonic amplitudes being raised or lowered by reactive elements), and the harmonics might be so far down that they're in the noise floor by the time you can begin to use this as a basis.

Yes indeed.

My experiments used a gate driven by both

• a square wave: the spectrum was pretty horrible for the reasons you mention, but I eyeballed the graph and made a guess.
• a noise source, later on: the spectrum was far easier to interpret, and I was surprised at the close correspondance with the value guesstimated from the square wave

If I had the choice, though, I'd prefer to use a scope!