6
\$\begingroup\$

I'm interested in measuring the spectral amplitudes / frequency content of RF frequencies up to 30 GHz. This can be done using a digital sampling oscilloscope (I'll call it a SO) with FFT or a digital spectrum analyzer (SA) [or a fantastic digital storage 'scope, noted below]. As I understand it, a SO samples the signals directly with known sample jitter, then recreates the signal using regression. A SA, on the other hand, first downconverts the high-frequency signal with a mixer, then samples. It would seem that a SA should deliver greater frequency resolution given comparable ADC sampling rates to the sampling oscilloscope.

What are the limits of functionality of each type? How is one better than the other at spectral analysis? (They both rely on the FFT, right?) What makes either expensive?

Unrelated POIs: 32 GHz Agilent, 120 GS/s Lecroy, 100 GS/s Tek, Gameboy SA.
edit: There seems to be some confusion between digital storage 'scopes (DSOs) and digital sampling oscilloscopes (what I called SOs) -- they are not the same, although they both sample digitally. I've also updated the question.

\$\endgroup\$
  • 1
    \$\begingroup\$ one thing your missing here is that for a standard sampling scope to detect a signal at 30Ghz it needs to sample at twice that frequency (Nyquist) and accuracy can suffer if it can't sample even faster. Also multiply by 10 to 20 the cost of the scope or SA for 30Ghz to include the cost of probes and additional equipment needed at those frequencies. As to the cost, parasitics at these frequencies are dominant, 'resistors' and 'capacitors' are built with metal structures on RF substrate. No offence but if your even asking this question your not ready to be working at those frequencies. \$\endgroup\$ – Mark Dec 29 '10 at 2:08
  • \$\begingroup\$ @Mark, I am not missing that - it is obvious enough not to include basic signal theory. I understand RF and sampling electronics. I am looking for working differences in the tools themselves when working at these frequencies. \$\endgroup\$ – tyblu Dec 29 '10 at 2:19
5
\$\begingroup\$

Well, you're not going to be making RF measurements up to 30 GHz without spending a bunch of money, so either path is big bucks.

Typically, Spectrum analyzers are used to do frequency domain measurements. You'll get a display of power vs frequency on the display. The controls in the SA are setup for relevant things, Center frequency, bandwidth, resolution bandwith, signal powers in dBm/dBc etc.

Digital oscilloscopes don't directly have sampling rates to directly sample a 30 Ghz signal, so they'll undersample and assume that the signal repeats. probably a safe assumption, although with no front end filters built into them, you've got dynamic range issues, as well as aliasing concerns that aren't present in a Spectrum Analyzer. You won't directly get spectral plots out of a Digital oscilloscope, you'll need to do an FFT on that. Now, that opens up a can of worms. FFT bin width/windowing function selection, etc. All stuff that can be worked through, but another question to deal with.

You won't get eye diagrams out of a spectrum analyzer, it's a useless measurement @ RF. That's a demodulated signal measurement.

Ultimately, if you want time domain data, then use an oscilloscope. If you want Spectral information, use a spectrum analyzer.

\$\endgroup\$
  • \$\begingroup\$ Dave, signatures are discouraged here, as they are repetitive with your nameplate which is posted below your question/answer. \$\endgroup\$ – Nick T Dec 27 '10 at 19:50
  • \$\begingroup\$ Hey @Dave, I edited the question a bit for clarity. \$\endgroup\$ – tyblu Dec 28 '10 at 12:19
  • \$\begingroup\$ It's not clear to me what you're trying to measure. Could you clarify that a bit? \$\endgroup\$ – rfdave Dec 29 '10 at 4:40
2
\$\begingroup\$

A sampling scope can indeed make a good spectrum analyzer. The three main limitations are in the areas of span width, dynamic range, and spur rejection.

The span width of a modern sampling instrument is usually constrained by the sample clock rate and/or various throughput limitations of its baseband FFT pipeline. Meanwhile, its dynamic range is lower than that of a classical spectrum analyzer because the front-end sampler 'sees' the whole DC-to-daylight spectrum worth of noise. The instrument folds the noise at every harmonic of the sampling clock, so noise and signals near all of the resulting aliases are visible at once.

Using a sampling front end with an FFT baseband analyzer section is not a new idea -- check out the HP 71500A, and its main module, the 70820A introduced circa 1992. (Large .PDFs but worth the wait.) This 'microwave transition analyzer' was a bit too far ahead of its time due to the wimpy processing power available, but the concept is very sound, and it can be implemented very economically compared to a traditional microwave spectrum analyzer.

\$\endgroup\$
0
\$\begingroup\$

You use a sampling scope for repetitive signals, e.g. clocks and other signal-integrity issues. You use a regular DSO (that is the correct term, not a SO) for most everything else. You use a scope at baseband and a spectrum analyzer for RF. The FFTs in scopes are for lower-frequency signals, not at RF.

\$\endgroup\$
  • \$\begingroup\$ DSO is Digital Storage Oscilloscope. I'm talking about digital sampling oscilloscopes (not DSO). \$\endgroup\$ – tyblu Dec 27 '10 at 20:46
0
\$\begingroup\$

This is a guess (I would have put this into a comment but I don't have the points for it).

I would guess that Digital Sampling O-scopes probably use heterodyning to mix the incoming high frequency with a fixed carrier frequency, to bring it down to something more manageable, i.e. an Intermediate Frequency, just like a HF radio receivers do. For instance if you mix a ~5.1Ghz incoming signal with a fixed 5Ghz carrier signal, you end up with a ~.1Ghz beat frequency signal. (I've long since forgotten all the RF stuff my dad taught me years ago so I may have that wrong)

I suspect SAs do the same thing, at least those capable of working with RF frequencies, anyway.

\$\endgroup\$
  • \$\begingroup\$ Best not to guess! ;) Welcome to the site. You may want to edit your answer as it is incorrect: SAs don't mix down their inputs prior to sampling (source). \$\endgroup\$ – tyblu Dec 28 '10 at 23:59
  • \$\begingroup\$ Spectrum analyzers I've used do mix the input down before sampling. There's no ADC technology that can provide 90 to 100 dB of dynamic range across a 20MHz to 26GHz span in a single sweep, like the Agilent mxa on my bench can provide. \$\endgroup\$ – rfdave Dec 29 '10 at 4:43
  • \$\begingroup\$ Whoops -- I meant sampling 'scopes! \$\endgroup\$ – tyblu Dec 29 '10 at 6:05
  • \$\begingroup\$ The complaint that this answer is incorrect is itself ultimately incorrect. Sampling is itself a type of mixing and undersampling is thus a form of downmixing, one is simply multiplying by an impulse train rather than a sinusoidal local oscillator. \$\endgroup\$ – Chris Stratton Feb 7 '11 at 8:22
  • \$\begingroup\$ Thats is correct answer. Consequently, SA have a frequency resolution few thousand times better than oscilloscopes. Other limitation of FFT method, is speed. Try to run FFT against 0.5 GSamples. It may take half an hour on small CPU, used in scopes, if they allow it at all. \$\endgroup\$ – user924 Aug 4 '11 at 12:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.