# Oscilloscope with FFT or a Spectrum Analyzer?

Could someone explain to me please which applications demand one or the other and why? As far as I have read it's all about the 'dB'; is that true? And why?

At first I can see Digital Storage Oscilloscopes (DSO) with FFT function and Spectrum Analyzers (SA) as being the same thing...they will get a signal from the Time Domain, and convert it to the Frequency Domain and we can check all the harmonics and frequency components of a signal and analyze it in a whole new way.......But since DSOs usually are much cheaper than SA, I keep wondering what functionalities the SA will offer that a DSO can't. Is it about precision, speed of calculation (my DSO FFT is really slow), bandwidth (cheap DSOs usually go only up to 100MHz), or does it just depend on the models and not on being a DSO or a SA? Is there more that I don't know about and you can tell me?

• It depends on: your frequency range of interest, types of devices you're working with, amounts of funds available. Please advise. – Nick Alexeev Dec 5 '12 at 4:11
• I just a general answer...for example, frequency range is not related with being an oscilloscope or a SA, it's just related with what you buy...it seems to me the worlds are fusing together – mFeinstein Dec 5 '12 at 4:25

To answer simply - an oscilloscope is an essential tool for any electronics lab, whilst an SA is generally not (unless you are an RF engineer, and even then you need a good scope) and for a good quality one much more expensive in comparison (though Rigol have just brought out some pretty powerful SAs at decent scope type prices)
The FFT function on your average DSO will do for most work, so unless your frequency range of interest is e.g. > 500MHz or so (if it is let us know), then the DSO is the tool of choice.

Basically one does amplitude versus time (scope), and the other does amplitude versus frequency (SA)

Scope example:
Say you have a digital signal that is intermittently working, you could check on the scope and look for over/undershoot, ringing, noise, gltiches, etc.

(simple) SA example: Say you have a signal and you want to check the harmonic components of it, you can look on the SA screen and check for harmonics (e.g. a pure sine wave should just be one single spike on the screen, at it's frequency, a square wave would be a decreasing series of odd harmonics)

Square wave on a Spectrum Analyser:

The same signal on a scope would look like this:

• The comment about the FFT function on a scope "will do for most work" within the frequency range is highly misleading - the critical issue is that scopes have terrible dynamic range compared to a spectrum analyzer, most being only 8 bit and a few 12 bit, either of which rapidly gets meaningless when looking at RF signals for which a log scale is appropriate. Something with 16 bits of dynamic range would be the bare minimum. – Chris Stratton Feb 20 '19 at 18:13
• Hi Chris, long time no speak :-) – Oli Glaser Feb 21 '19 at 22:50
• Point taken, maybe I should have worded things more carefully and gone into more detail. However I was just trying to paint a basic picture of the oscilloscope being almost always on the list of first few pieces purchased for you average lab. Of course things are changing all the time and since this was written there are quite a few budget scopes (Rigol, Owon, etc) with 10, 12, 14, and I think even 16 bit capabilities. Of course a dedicated SA will be better (or DSO/SA) but for low frequency work, if one wants an idea of the frequency/time then they can be OK. – Oli Glaser Feb 21 '19 at 23:08

An oscilloscope with FFT function uses built in mathematical analysis of the stored waveform to calculate the frequency content and amplitude of the signal. It is displayed on the screen as a frequency vs amplitude graph - just like a spectrum analyser.

A 'true' analogue type spectrum analyser, actually measures the amplitude at each frequency ( steps ) from the signal and does not need to do any maths on the measured amplitude other than that required to show the measurement values accurately on the screen.

It's true that many oscilloscopes offer a FFT function - but unless you are using a new expensive scope - the resulting display is rather more of a guide than being equivalent to a real spectrum analyser.

That said - the newer generation of combined digital instruments do truly offer the same spectrum analysis results and oscilloscope measurements that single task instruments would. They are not cheap however but are useful in that the frequency/analogue content can be synchronised with the digital oscilloscope waveform to identify those signals which are causing RF related problems or EMC.

• Just to add, I think they are called Mixed Domain Oscilloscopes – mFeinstein Jan 14 '15 at 12:23
• Digital FFTs are also more prone to harmonic noise when there is data outside the expected measurement range. This can be overcome with filters and/or appropriate experiment setup. – VoteCoffee Feb 18 '15 at 15:04
• Modern spectrum analyzers are going to use FFT-related processing too; the critical difference is that they have good dynamic range through the signal chain, while a DSO does not have enough bits in the ADC to do that - processing gain helps some, but generally not enough to get a high dynamic range spectrum from a low dynamic range ADC. – Chris Stratton Feb 6 '17 at 16:26

Scopes typically are digital now or DSO and can be bought from $50 to$5K depending on specs, performance, bandwidth. They can be interfaced on USB, IEEE488, PCI and many other ports. These offer storage for repetitive and 1 shot waveforms and math functions.

Spectrum Analyzers measure Spectral Density and Digital SA's use FFT to calculate the spectrum whereas RF SA's use dual or triple conversion swept scanning like a TV tuner but with very precise preamps, filters and Log converters since measurements are more convenient to display a wide dynamic range such as 100 dB. They are used for seismic, audio, mechanical bearing analyzers in large turbines, radio, microwave, optical spectrum and more. They can be useful for doing Bode plots, filter plots , RF emanation test, Radio tests, antenna design, Radar, Cellular design and test verification.

There are literally thousands of different applications for Spectrum Analyzers besides for Radio Engineers in all fields of Industry where Engineers need to analyze the spectrum in a particular device, whether it is mechanical, optical, or electrical. I know one family relative that use one to analyze Gigawatt GE turbines in Japan for bearing harmonics, which is a strong indicator of product quality and aging factors.

Network Analyzers are even more precise than SA's and have built-in tracking generators with dual inputs so that a transfer Function can be measured. They come in wide ranges of frequency and can be used for measure phase margin in SMPS for stability tests or PLL test or Insertion loss, Return loss , SMith Charts etc. and can be as accurate as 0.1dB from .1 to 50 GHz or a sub-range of interest like 0 ~ 1MHz These can cost \$100K each. HP and Anritsu are the two top suppliers in America.

But for plain audio, there are free software tools to display Audio signals and Spectrum Analysis using the MIC, Line IN or internal audio.

e.g. Audacity is one program. I still have the old Cool Edit Pro 2. Version. Waveform Courtesy of AC-DC (Hell's Bells)

The difference is that the spectrum analyzer has a mixer frontend allowing it to shift the frequency range it is listening to, while an oscilloscope remains fixed at the lower end.

This means that it is possible to see signals at higher frequencies, and at the same time, signals outside of the area being looked at are filtered out, so you can adjust the ADC prescaler for a better resolution.

On the other hand, mixers don't like DC at all, so in normal EE work, you won't be able to use a spectrum analyzer in place of an oscilloscope either.

Current day spectrum analyzers (SA) are seldom fully sweep tune. Most do FFT and stitch channels together to form a frequency span.

Besides a class of modern SA measurement such as Vector Signal Analysis, does not stitch channels, but rather, measure the entire channels base on the IF sampling rate. The analysis bandwidth, which is is usually around [IF sampling rate/1.25] is up to 1 GHz, for the highest end SA -- Keysight UXA.

Non exhaustive of scope vs spectrum

1. Scope digitize from baseband to the desire frequency range. SA downcovert the RF signals and digitize at IF
2. Being able to digitize at IF allow SA to have better vertical resolution. A scope vertical resolution is mostly 8 bit, while SA is up to 14 bit. (Digitizer designers trade sampling rate with vertical resolution)
3. A scope is useful for time domain analysis. A spectrum is better for frequency domain analysis. SA having a better vertical resolution will have a better performance in S/N ratio, allowing one to see signal at very low power level. While scope having higher sampling rate will allow better time resolution of certain kind of measurement such as rise time.
4. A scope can be more than one port while SA is one port. Hence a scope is able to perform multi channel time domain comparison such as phase, pulse rise time...etc

Above: Scope measuring multi-channel pulses

There were a few correct differences mentioned above, I will try to systemize:

1) Bandwidth (oscilloscope's bandwidth usually wider, but the working band cannot be shifted). I.e. for example Oscilloscope modes are: 0-1kHz, 0-10kHz, 0-50kHz, 0-250kHz, 0-500kHz, 0-2MHz, 0-20MHz,0-100MHz signals, having max sample rate at 500 MSamp/sec. When one looks at FFT, he can see only these 0-100 MHz band. Spectrum Analyzer may have narrower bandwidth, but it can roll across frequency scale: i.e. for example, bandwidth 40 MHz, sampling frequency 200 MSamp/sec, and working frequencies: 0-6.3 GHz. I.e. Spectrum analyzers modes will be: 0-40MHz,10-50MHz, 20-60MHz, 30-70MHz....6260..6300MHz. So one can see, that SA has a tunable band filter instead of anti-aliasing LPF in the oscilloscope.

2) Dynamic range. ADC of a spectrum analyzer has a much better resolution.

3) The spectrum analyzer has a low-noise amplifier, oscilloscope doesn't have it. Low noise amplifier, is a special, radio-frequency amplifier, which works in a big range of frequencies, add very low noise to signal.

4) Oscilloscope and spectrum analyzer have different ways to set up triggers. The oscilloscope is oriented on a shape of signal in the time domain, SA is oriented on capturing certain shapes in the frequency domain.

5) The oscilloscope cannot demodulate signals, a spectrum analyzer usually can (because it is virtually an SDR-receiver).

Summarizing: an oscilloscope is an extra-wide band millivoltmeter. spectrum analyzer is a pretty narrow-band receiver, whos main goal to convert radio-waves into the baseband signal (I and Q components) with as low as possible lose and noise.

Another application for a spectrum analyzer is where you'd want to hunt down a source of interference. Latest-gen handhelds make this a lot easier too. For example, in addition to the spectrogram and standard spectrum analyzer measurements, these instruments can make interference-specific measurements such as carrier/noise (C/N) and carrier/interference (C/I). A trace mathematics (diff mode) can help you find, monitor, and characterize interfering signals. Another feature is the ability to record spectrum over a specified time. This allows you to find intermittant faults and frequency variations, over time. Great feature. Personally, I'd go for both: Scope + SA. It just makes your bench more useful, long term.