In another web site a PC aided DS Oscilloscope is also used as a spectrum analyzer ,showing the main frequency and harmonics.
Can a standard DSO be used for this as well ?

 

Yes; otherwise called FFT. Note that cheap PC-based oscilloscopes do not have proper input circuitry (like the PoScope), and the ones that do aren't cheap, so you may as well get a standalone low end DSO that outperforms them all. Many DSOs also have USB communication abilities, and can dump data into custom programs. Of course, some suck and have none of this -- just need to spend a while peering at datasheets and reviews. On the other hand, you can make a soundcard 'scope for nearly nothing with some decent software.

 

Yes, horses for courses. But if you appreciate what the limits of a soundcard scope are, and can prepare a simple divider and limiter, or opamp amp, then blimey they make a great cheap little tool for audio work.

 

Not quite on topic, but good to know: Spectrum analyzers have mixers just after their input attenuators (This applies to digital sampling analyzers equally, even if they don't have a physical mixer.). Mixers generate harmonics. So how can you measure harmonics reliably with a spectrum analyzer? Usually, you can. But you have to use a certain procedure. Luckily, it's very simple. Since mixers generate more harmonics when driven harder, the trick is to increase the input attenuation until the harmonic levels are no longer changing.