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Measurements >> RF Measurements >> Spectrum analyzer
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Message started by raja.cedt on Aug 19^th, 2010, 5:23am

Title: Spectrum analyzer
Post by raja.cedt on Aug 19^th, 2010, 5:23am

hello,
can any one please tell me what is RBW and VBW in spectrum analyzer? Everybody is telling that lesser the RBW more accuracy in the measurement. can any one clarify this or direct me towards a ref..

Thanks,
Rajasekhar.

Title: Re: Spectrum analyzer
Post by aaron_do on Aug 19^th, 2010, 6:28am

Hi,

I suggest you google some documents on spectrum analyzer fundamentals. The documents by Agilent are very helpful.

A spectrum analyzer is similar to an AM receiver. Anyway RBW is the resolution bandwidth and if you are familiar with receiver design (which is basically what a spectrum analzyer is), it is your channel filter bandwidth. i.e. it is the filter after the frequency translation. A narrower RBW will lower the noise floor potentially making your signal more clear. After the RBW filter, the signal is basically amplitude detected using some kind of rectifier. And following this rectifier is another filter which is basically your VBW filter. Reducing the VBW is akin to averaging the signal power. It does not reduce the average level of the signal or noise, but it removes all the large spikes you see in the noise also potentially making your signal clearer. So to sum up, reducing RBW lowers the noise floor, while reducing the VBW smooths out the noise.

Modern high-end spectrum analyzers are implemented quite differently. Basically they can digitize the signal much sooner, but the basic principles of RBW and VBW are the same.

Anyway like I said you're much better off reading some of those Agilent documents on spectrum analyzer fundamentals.

cheers,
Aaron

Title: Re: Spectrum analyzer
Post by raja.cedt on Aug 19^th, 2010, 10:39pm

hi
tx for your reply...i have gone through some documents, so finally what i have understood is if RBW=1hz then we will get correct noise..is this correct?

Thanks,
Rajasekhar.

Title: Re: Spectrum analyzer
Post by pancho_hideboo on Aug 20^th, 2010, 8:28am

raja.cedt wrote on Aug 19^th, 2010, 10:39pm:

so finally what i have understood is if RBW=1hz then we will get correct noise..is this correct?

Not correct.

Noise marker mode in Spectrum Analyzer where dBm/Hz or dBc/Hz is displayed has no relation to RBW directly.

Title: Re: Spectrum analyzer
Post by RFICDUDE on Aug 22^nd, 2010, 6:05pm

A couple of good references on measurement of noise and noise-like signals includes

http://cp.literature.agilent.com/litweb/pdf/5966-4008E.pdf

http://cp.literature.agilent.com/litweb/pdf/5965-7009E.pdf

http://cp.literature.agilent.com/litweb/pdf/5952-0292.pdf