what's the difference between VSA and Sweep-time spectrum analyser? - Page 1
Forum section: electronics / testgear

author: a7458969
author_rank: Newbie
timestamp: July 03, 2017, 01:04:26 am
content: I am looking for a spectrum analyser to study for a long time .And I notice cmu200 have very good Parameter. It have a SA from 10mhz to 2.7ghz.and as a VSA,the span could be full-span.I wondering how could it do that.as a Vector sinal analyser, It can't sweep right?Can I use it as a sweep-time spectrum analyser?

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: July 03, 2017, 12:20:46 pm
content: Quote from: a7458969 on July 03, 2017, 01:04:26 amThe difference between a VSA and a conventional swept SA is that the VSA derives the frequency and phase information from the time domain through FFT, while the traditional swept SA pretty much uses a detector to get signal amplitude at the frequency it currently sweeps across.Most newer standalone SAs are a mixture of both (and VSA doesn't necessarily mean there is no sweeping involved), however most cell phone testers with SA functionality are actually VSAs.The CMU200, if I remember right (it's been a while since I had one, and there are others in this forum who spent much more time with its innards than I did) is a sweeping VSA, i.e. it sweeps through the the frequency span but instead of a (diode) detector it uses an ADC and FFT to derive frequency and phase information.

author: 1design
author_rank: Regular Contributor
timestamp: July 03, 2017, 01:40:53 pm
content: These a 2 different instruments for 2 different types of measurements.One is aSprectumanalyzer and the other on is aVector Signalanalyzer. The fact that both can show the spectrum of a signal is irrelevant in this context.The goal of a VSA is to demodulate a signal and analyze the spectral content in time domain, this is why it needs to digitize the signal BW in real time and use the right demodulator to analyze it. So it is a receiver for a variety of different signals/modulations(Pulsed(Radar) , TDD(GSM, LTE, WiFi) , FDD(PtP radios) , etc.)The goal of a SA is to depict the spectrum...that is it. How it achieves that is a matter of architecture. Usually it also supports zero span, but that is more or less it. You can use it as a front end for a VSA by sending the IF to a real time digitizer(oscilloscope) with the right type of SW to demodulate etc. and get a VSA that way. This is how we used to do when developing wide-band E-band radios when the VSAs did not yet support 2GHz of real time BW.And the new RTSA are not much else than VSAs with a new fancy way of showing the spectrum with resistance UI.So please, buy the right instrument for your application, for PtP and CW signals, components design etc. a VNA and SA should be enough. For transmitter design a VSA is a better option as impairments like IQ phase and amplitude imbalance can be measured. With wide-band signals they vary over the spectrum so this is even more important.

author: G0HZU
author_rank: Super Contributor
timestamp: July 03, 2017, 07:03:36 pm
content: In terms of a traditional VSA vs a classic RTSA I think the VSA can suffer from dead time as it typically acquires data then processes it (leaving a time gap).The RTSA will be able to process its data without leaving any subtle time gaps.

author: 1design
author_rank: Regular Contributor
timestamp: July 03, 2017, 10:23:51 pm
content: I agree with that statement, even some modern RTSA with VSA capabilities still show this behavior. For example the USB units from Tek.High end units that do all the processing in the on board FPGA, like the top end TEKs and Keysights have 100% coverage with no gaps.

author: w2aew
author_rank: Super Contributor
timestamp: July 04, 2017, 03:30:03 am
content: For the most part, modern Vector Signal Analyzers (including RTSAs) can be considered a superset of a spectrum analyzer functionality and vector signal analyzer functionality. Most modern VSAs can do all (or nearly all) of the measurements that a conventional swept spectrum analyzer can do. In fact, many manufacturers VSA/RTSA units are built inside of their swept analyzers - meaning that conventional spectrum analysis is handled by the conventional swept analyzer hardware, and the signal analysis is handled by a separate hardware path that is switched inline. There are common blocks, like the preselector for example. However, this also means that many of these analyzers are "mode driven", meaning that you are either in Spectrum Analyzer mode, *or* in Vector Signal/IQ Analyzer mode, *or* Realtime SA mode, etc. Also, for wideband signal analysis, the YIG preselector must often be bypassed...Other manufacturers have built RSAs as a VSA from the ground up, without the conventional swept hardware. Even though this is the case, swept spectrum measurements can also be made, they're just done a little differently - basically by acquiring data within the realtime BW and computing the spectrum (in segments for wide spans) using FFT or Chirp-Z transforms. These analyzers don't give you a "sweep time" control because they're not sweeping like a conventional swept analyzer. However, the sweep times for wide spans with narrow RBW settings can be dramatically faster than a conventional swept analyzer. Also, these analyzers are generally not mode driven - meaning that you can view spectrum results simultaneously with time-domain vector measurements, modulation domain measurements, etc. - all time correlated.