Question: 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?

Answer: 
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.
![enter image description here](https://i.stack.imgur.com/f6Pyj.jpg)
Waveform Courtesy of AC-DC (Hell's Bells)
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](http://www.keysight.com/en/pdx-x202152-pn-N9040B/uxa-signal-analyzer-multi-touch-3-hz-to-50-ghz?cc=US&lc=eng). 

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

[![](https://i.stack.imgur.com/dBHHP.png)](https://i.stack.imgur.com/dBHHP.png)
Above: Scope measuring multi-channel pulses