# Spectrum analyser and resolution bandwidth

For the preliminary part of a lab I had to do, I was asked the following question:

I get how to do 1.1, but with 1.2, I really don't get what's going on; I really would've thought that the resolution bandwidth would be equal to the bandwidth of the low-pass filter. Furthermore, I'm pretty sure a spectrum analyser has a variable band-pass filter in it, so I really don't get what's going on here.

Any help would be much appreciated. For the record, this isn't part of an assignment or anything like that, it's not worth anything, it's just something that's bugging me.

Thanks.

The bandwidth of the lowpass filter is the one-sided bandwidth i.e. $B_{LPF} = f_{max}-0$. If you shift this LPF up to some center frequency, $f_c$, then the bandwidth becomes $B=(f_c + B_{LPF}) - (f_c - B_{LPF}) = 2B_{LPF}$.