Differences between Signal Analyser & Spectrum Analyser?  - Page 1
Forum section: electronics / beginners

author: Well, That Was Unexpected
author_rank: Newbie
timestamp: November 10, 2015, 04:40:22 pm
content: Googling around I haven't found a clear or concise answer to the differences between a signal & a spectrum analyser? I tried nutting it out by just reading up on the two separately and then drawing my own conclusions. From what I can tell, they are different instruments with different capabilities, but he advent of modern techniques has somewhat merged or blurred the line.For example, references to products like this one from Keysight are what tend to make me a bit confused?http://www.keysight.com/en/pc-1000000520%3Aepsg%3Apgr/spectrum-analyzer-signal-analyzer?cc=AU&lc=eng] http://www.keysight.com/en/pc-1000000520%3Aepsg%3Apgr/spectrum-analyzer-signal-analyzer?cc=AU&lc=engIt references "Choose from an extensive selection of signal analyzer products from DC to 50 GHz, extendable to 1.1 THz with external mixers, for your spectrum analysis measurements." and then later "Combine your spectrum analyzer or signal analyzer with 89600 VSA software for deep vector signal analysis" The two statements are somewhat contradictory.Is that to say then, that spectrum analysers can be considered a type of signal analyser yet distinct in their own right?

author: awallin
author_rank: Frequent Contributor
timestamp: November 10, 2015, 08:45:58 pm
content: my 2 cents.. might not be worth even that..spectrum analyzer:- usually just one input that measures power in some bandwidth, never shows phase information- sometimes a tracking generator that outputs a sweep or fixed frequency in sync with the input- usually all analog design, not starting from DC but maybe 9kHz and up (1 GHz at least)signal analyzer:- might be analog or digital- digital 'FFT analyzer' might be DC to 10 MHz say- gives both magnitude and phase for DUT aka. "vector" (sounds so much cooler than magnitude+phase)- 2 or sometimes 4 inputs/outputs (think S-parameters)

author: tggzzz
author_rank: Super Contributor
timestamp: November 10, 2015, 10:06:45 pm
content: Quote from: awallin on November 10, 2015, 08:45:58 pmThat's a network analyser.A signal analyser add the capability of "interpreting" the power/phase information as specific type of signal modulation, e.g. FSK, or PSK, or OFDM, or GMSK or, to include the "higher levels", GSM for example.

author: rfeecs
author_rank: Frequent Contributor
timestamp: November 10, 2015, 11:22:29 pm
content: A traditional spectrum analyzer measures amplitude only and produces a power vs frequency spectrum measurement.A vector signal analyzer converts the input signal to an IF frequency and measures both phase and amplitude. Consequently it can demodulate a complex signal.Keysight is calling both of these "Signal Analyzers". So a spectrum analyzer is a specific type of signal analyzer.A network analyzer has a source that provides stimulus to a network. The source and measurement are at the same frequency. The measurements can be scalar or vector.All of these are tuned receivers and they are often combined into one instrument. For example Keysight's PNA-X series of VNAs combines all of these to allow measurement of S-parameters, harmonics (magnitude and phase relationship), X-parameters, IM3 and vector corrected noise figure. Add pulsed measurements and you even have oscilloscope and real time spectrum analyzer functions.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 11, 2015, 04:38:35 am
content: Quote from: rfeecs on November 10, 2015, 11:22:29 pmThere are as many answers to this as there have been companies and marketing departments in this general field over the decades. This answer is as good a summary as any, but the marketing material (and operating manuals) of many instruments won't fit perfectly into these categories.

author: Psi
author_rank: Super Contributor
timestamp: November 11, 2015, 05:43:58 am
content: spectrum analyzer:- Very expensivesignal analyzer:- Absurdly expensive

author: Well, That Was Unexpected
author_rank: Newbie
timestamp: November 13, 2015, 04:20:33 am
content: Thanks for the answers, glad to know that I wasn't failing to understand or missing something obvious.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 14, 2015, 04:06:37 am
content: Historically anyway, specs were purely swept instruments: little more than a radio receiver, but with tight tolerances (emphasis on amplitude and logarithm correctness, usually), and automatic wide range tuning (using a variable oscillator, most popularly a YTO (YIG tuned oscillator: like a crystal oscillator, but tuned to an electronic resonance of a yttrium iron garnet (YIG) crystal, which varies proportionally with frequency). When swept at an appropriate rate (slow enough for the bandwidth settings) and plotted on a graph, you get a spectrogram.The average SA is little more than a computer wrapped around a very nice radio. ADC sampling measures the detector output, and DACs or switches control the signal path and frequency sweep. Only the rated bandwidth is acquired at any given time, sequentially across the entire band. Intermittent signals therefore have to be dealt with in a suitable manner (peak, average, quasi-peak, ...?).DSAs however were oriented more towards mechanical and acoustic analysis, made possible (obviously) by the advance of digital computation. Which was slow at first (< megasamples/sec), but when you're only looking at frequencies of kHz or less, this is more than good enough.Sampling is performed continuously, then the data converted en masse via Fourier Transform. This has several important implications:- The bandwidth measured is, in a sense, maximal. Any given frequency bin is derived from all the information acquired during the sample period. (Whereas a spec samples only the frequencies it is tuned to at a given moment.)- The amplitude is the peak sine component, for that frequency, for the entire sample period. It's not intermittent or modulated, it's not peak (over some time frame), or average (in the SA sense), or anything else. Itisa kind of average, but because the phase and amplitude over the entire acquisition period matters, it's a true vector average -- whereas the SA will yield more of an RMS average, now because it is phase-insensitive.- The bandwidth per bin (and usually per pixel, on a digital display) is determined by the sample rate and number of samples. For the SA, you still benefit from setting an unusually low bandwidth (= high frequency precision) for a relatively wide span, but doing the equivalent (more samples than there are pixels to display) with a DSA will result in aliasing -- the loss of information. (There are tricks to manage the losses around marginal conditions, like this. For example, sampling perhaps 3x too much data, multiplying it by a suitable function such as a Hanning window, then rescaling the FT result, perhaps with a weighing function such as antialiasing, or taking the RMS or peak value, when merging neighboring bins together -- because the window function acts to smooth the frequency display, this is now a reasonable step to take.)Nowadays, DSAs offer as much power as DSOs, which in turn offer more power than any analog instrument of either kind did, back when digital ones were still new. And algorithms and programs have been developed to deliver the same old functionality with the new machines: such as (phase insensitive) peak or quasi-peak readouts, or narrow frequency sweeping (think: using the DSA's ADC as an SDR).Disclaimer: I have used both analog and digital scopes, but not of SA/DSAs. So I don't know how they actually compare in terms of bandwidth and noise and tradeoffs and such. What I've seen, seems to be well in line with my intuition. So, don't worry about them, just get whatever's best these days. Or, get a vintage one if you like (but not, like, really old and crusty, they'll probably be more hassle).The main feature, as far as total acquisition bandwidth and update rate and all that, will be: because a DSA can acquire and process a huge amount of data at once, while an SA must do it one frequency range at a time: the DSA can acquire and display the correct spectrogram (peak, average, whatever) very quickly, even when very intermittent signals are present.A very typical use-case is a switching supply putting out noise modulated by 100/120Hz: the supply rectifier acts as a PIN diode, gating RF into the AC line; the diodes also generate noise due to reverse recovery. The resulting spectrum (on an SA) is a forest of dancing harmonics; a long averaging or "max hold" period is necessary to find the true amplitudes at all frequencies. A DSA with enough memory (and acquisition power, and processing power) to acquire >8.33ms of contiguous data, will be able to perform the same measurement in exactly one pass, period.Tim

author: Dago
author_rank: Frequent Contributor
timestamp: November 14, 2015, 05:53:05 am
content: One (possible) word of "warning": If you are buying a "spectrum analyzer" and run in to some interesting looking "signal analyzer" (for basic use they are usually pretty interchangeable, signal analyzer having more analysis functions) then keep an eye on the sweep range.I am not 100% sure but I think I've seen some "signal analyzers" that did not have any sweeping per se, they just captured like <100 MHz baseband directly. This would make them not too good for general spectum analysis work where you often want to look at wide sweeps (for EMI etc.). If you just need a narrow band for analysis then this is probably better than sweeping because you capture "all" the signal. Modern signal analyzers often have a hybrid approach where they have sweeping on top of which they capture a direct spectrum with an ADC so you can do pretty complex modulation analysis etc.I personally have a R&S FSIQ7 which is a signal analyzer:https://www.testequipmentconnection.com/10978/Rohde_Schwarz_FSIQ26.phpTo me it has looked/felt like a regular SA with a separate vector signal analysis side.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: November 14, 2015, 08:47:46 am
content: i thought signal analyzers while having interesting functionality have limited span and other adjustments that make them a kludgy tool to use

author: G0HZU
author_rank: Super Contributor
timestamp: November 14, 2015, 11:38:49 am
content: High end signal analysers are now being marketed as a replacement for a traditional spectrum analyser. At work we have lots of the older type Agilent PSA spectrum analysers and a couple of years ago I was tasked with evaluating the newer PXA signal analyser as a replacement for it.Agilent also produced this document at the time showing the differences.http://cp.literature.agilent.com/litweb/pdf/5990-3990EN.pdfI really like the old PSA analyser. I think it is let down a bit by its VGA screen but it does still feel and perform like a traditional swept spectrum analyser even though it has a digital IF.The newer PXA signal analyser looks really nice when they are both side by side as it has a much nicer display. But it definitely does not feel like a traditional spectrum analyser to me.The PXA I played with was an early version and I wasn't that impressed with its performance in the 'near IF overload' region. It also had some issues with how it (auto) managed its PLL loop BW on certain spans. it does this loop BW trickery to keep in line with the marketing graphs for phase noise. But it can catch out the unwary unless you turn off the auto feature or you can live with the issues it introduces.

author: Well, That Was Unexpected
author_rank: Newbie
timestamp: November 20, 2015, 01:39:39 am
content: Tim \ T3sl4co1l thanks for taking the time to write such a detailed response. I could find info on the history of the two instruments but was struggling to join the dots as to the differences. Your explanation does exactly that! Many thanks.I wish I was on the hunt for a DSA or SA but unfortunately I cannot even attest to owning my own CRO let alone DSO. I asked because Dave has both a DSA and a SA. The modern Rigol DSA815 with a tracking gen seemed to serve a very similar purpose to the HP 35660A (minus the tracking gen of course.) That is in terms of working in the frequency rather than time domain and taking, again seemingly, similar measurements. Hence, I was wondering what the HP 35660A brought to the table? What could it offer over a modern SA like the Rigol? Was it just for the merits of the blog and getting to look at different gear or was there a key difference that would make it handy to have?