spectrum analyzer settings

When changing the Res BW or VBW on a spectrum analyzer, my desired signal level does not change significantly but the amplitude of the spurious (and noise floor) changes. Of course the noise floor changes but why does the spur level change?

tnx

What kind of signal are you referring to ? Narrowband signals level is not affected by RBW setting as, instead, wideband signal level is.

Hi albbg,

The desired is narrowband (CW). Down low (~60dBc), I believe I have both narrowband spurs and broad noise spurs.

tnx

See attached. I get a constant amplitude measurement from the desired (9.275GHz) and narrowband spur (9.225GHz) but I get different amplitude readings from the wideband spur at 9.172GHz based on the Res BW. Typically I measure using 801 data points but I see this one was captured at 401 data points.

One of 3 things is going on...
1) The spur is very narrowband, and the sweep time you are using is not allowing the spur's true energy to be measured. The spectrum analyzer sweeps by before the resolution bandwidth filter responds fully.
2) The spur (especially the one at marker 3) is not really a cw tone spur, but a broad noise source. If it is a broad noise source, then obviously changing the resolution bandwidth will give you a different dBm/Hz reading.
3) the spur is not a tone, but is some sort of pulse modulated signal. So it may not be "on" when the spectrum analyzer sweeps by its frequency. By making the RBW bigger, you can statistically catch more of the signal as it sweeps by, since the receiver filter is wider.

Rich

From your picture, the spur at 9.172GHz is a wideband signal, then point 2) or 3) of biff44 analysis apply. Are you sure it is generated by the signal ? What happen to the spurs frequency if you move the carrier frequency ? I think the marker 3 spur could be noise spiffering from the spectral regrowth of a filter.

thanks Biff44 & albbg,

After further investigation 2) & 3) of Biff44 is definitely correct (braodband noise). The spur at marker 2 is a cw tone spur but marker 3 is not.

>> I think the marker 3 spur could be noise spiffering from the spectral regrowth of a filter.
I'm wondering the same. I'm use to the narrowband spurs. The noise spur had me wondering for a bit. By "noise spiffering", do you mean a reentry of a filter?