Spectrum analysers. - Page 1
Forum section: electronics / beginners

author: nowlan
author_rank: Frequent Contributor
timestamp: October 12, 2015, 11:36:35 am
content: Hello, I had some questions about spectrum analysers etc.I was watching some youtube videos where the guy was comparing arials on his wireless dongles. I think he was using netstumbler to compare signal strength.This seemed like a cheap/simple way to see signals in the 2.4G region.I believe RF explorer are also capable of seeing how busy the bands are.Not sure if they arejustanother wifi chipset with a simple lcd+brain attached.Moving up the field we have hackRF and bladeRF. There was a lot of noise about the hackRF when it was released.I think everyone was waiting for version 2. I saw some Blue copy on the web a few months ago.Are these not a good substitute for Ham/hobbiest? Being able to transmit seemed to be of interest to more people.I think the more conventional specAn were the rigol DSA815 and SignalHound. The price increases so I guess you would have to a genuine need for one.---Perhaps I dont understand, but do these not just run a sweep via VCO and peak detect or is it DSP based?---I also had a question about what a Signal Analyser & VNA do. I asked a teacher about signal analyser, and my teacher said they had more smarts to decode the signal.I saw someone refer to the new tektronix RSA306 as a signal analyser.What exactly are VNA for? Do they plot on a smith chart instead of bode?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2015, 12:17:28 pm
content: Spec, of the old school variety: swept, usually of the GTO variety, spanning over a GHz total range. Though, going back through history, you'll probably see varactor, reluctance (i.e., magnetic saturation) and even vacuum tube tuning methods (specifically, reflex klystrons; there are other methods for frequency or phase control, like reactance modulation, but they aren't wide range).DSAs use sampling and compute the DFT, rather than sweeping. This potentially has a lower noise floor (all channels are received at once, rather than in turn), and has different dynamics as far as picking up transient signals (on a sweep, you'll see the transient come and go in a suspicious, and usually 'lucky', manner). Kind of the inverse of the difference between analog and digital scopes (perhaps the inversion shouldn't be unexpected, given the duality of time and frequency).Older DSAs were limited by the ADCs and computers of the day, and 100kHz was a typical bandwidth. This could be extended to other bands with frequency converters, but obviously not to higher bandwidths. Typical uses were for audio frequency, acoustic and vibrational testing, where the relatively low bandwidth is fine and the extreme frequency resolution (typically in the uHz) was desirable.Obviously, today we have no such limits, and DSAs can run "full speed", taking over from the swept designs. I don't know how much of each method is still around; I can imagine it would be beneficial to keep tracking or conversion stages, for better resolution and lower noise, perhaps. The ones with tracking generators also need to do more work (in the old days, you could convert the sweep oscillator to the in-band frequency, getting a TG practically for free).So, a DSA is a DSP spec.A VNA is usually a spec+TG as well, but has vector rather than magnitude readout. So it might plot on a Smith chart, or real and imaginary, or magnitude and angle. Typical uses are analysis of components, filters, signal chains, etc. Scattering parameters, coupling coefficients, resistance and reactance, gain and phase, etc.Tim