Spectrum analyzer- What are the different trade-offs?

Recently I read about spectrum analyzer and I understood it a bit except that I have few doubts.

Wikipedia says

"The bandwidth of the band-pass filter dictates the resolution bandwidth, which is related to the minimum bandwidth detectable by the instrument. The smaller the bandwidth, the more spectral resolution. However, there is a trade-off between how quickly the display can update the full frequency span under consideration and the frequency resolution, which is relevant for distinguishing frequency components that are close together"

I couldn't quite understand the reason behind this. Could you please help me in sorting this out?

Thanks in advance.

It's like the difference between a wide-angle lens and a telephoto lens. The wide angle shows you a large field but with less resolution. The telephoto gives you more detail, but you have to take more pictures to see the same area you saw with the wide angle.

The spectrum analyzer takes a picture of a portion of RF spectrum.

wa1kij said:

It's like the difference between a wide-angle lens and a telephoto lens. The wide angle shows you a large field but with less resolution. The telephoto gives you more detail, but you have to take more pictures to see the same area you saw with the wide angle.

The spectrum analyzer takes a picture of a portion of RF spectrum.

Click to expand...

To learn about technology basics it is advisable NOT to rely on Wikipedia ONLY. Find a textbook to learn about the basics!

A spectrum analyzer is a radio receiver with a narrow and adjustable bandwidth, tuned (swept) by a local oscillator across the desired band. As you tune the narrow receiver over a wide band, once there is a CW signal, the video response of the receiver needs certain time to allow the CW signal peak to grow up to the full amplitude before the receiver passes to the next frequency "window".

Therefore, the RBW, bandwidth of this receiver, and sweep rate or speed, and VBW, the video bandwidth, are related. If you sweep too fast with the narrow RBW, the video signal cannot grow to the full amplitude, and you will lose the calibration.

See here, page 22 "Sweep time"
**broken link removed**

What implications does this have for my Vector Network Analyzer. I have been performing measurements using the maximum 1200 points a relatively large frequency range with the smalles resolution bandwidth of 10Hz.

The S parameters I measure tend not to match those provided in the datasheets and simulation models.

darcyrandall2004 said:

What implications does this have for my Vector Network Analyzer. I have been performing measurements using the maximum 1200 points a relatively large frequency range with the smalles resolution bandwidth of 10Hz.

The S parameters I measure tend not to match those provided in the datasheets and simulation models.

Click to expand...

It depends; does the network analyzer make BW selections for you according to other setup criteria such sweep rate or speed, or, do you select such things as BW, sweep rate and so forth independently?

On the old HP 8410 series VNA nothing was set automatically, and it was necessary to know the setup trade-offs between BW and sweep rate. Higher sweep rates required the wider BW settings.

Setting for a resolution of 10 Hz is probably not suitable for sweeping anything as broadbanded as non-frequency selective components, but is useful perhaps in sweeping crystal filters (or Quartz crystals) or filters where the higher resolution would be useful.

Jim

Another consideration is whether the signal(s) you're looking at are continuous or transient in nature. If continuous, you can take as long as you like to sweep the frequency range in question. If transient, the longer the sweep takes, the more likely you'll miss a signal that was there.