For its spectrum display, QIRX offers the selection of different "FFT Windows". One of their uses is to allow a single peak
in the display to be confined in a narrow region, and not do decay slowly in case that peak does not hit the exact center of a FFT bin.
I will not go into further details here, in case you have more interest in the topic you might consult National Instrument's
Application Note 041 "The Fundamentals of FFT-Based Signal Analysis and Measurement"
, describing the topic to some detail.
Here, we are interested in the practical use of these "FFT Windows".
They come into play if you are keen on separating two neighboring peaks in
a high-resolution spectrum, like the one in the picture with the un-collapsed drop-down box.
All of these windwos, except the "None" have the (adverse) effect that they widen the peak, meaning that a narrow single peak will occupy
more than one frequency bin in the spectrum.
Then, why not stick with the "None" altogether? Because another property comes into play, the "sidelobes". These (also unwanted)
properties mean that a peak widens on decaying into its neighboring bins. And the "None" widens very much.
But how many bins are occupied by using a FFT Window? The answer is given in the inset of the spectrum. It is the NENBW
property, indicating the
number of FFT bins a single peak will occupy in the spectrum. "NENBW" means "Normalized Equivalent Noise Bandwidth".
Here is a short list of the recommended use for the offered FFT Windows:
None: Used for: Nothing, just for comparison how much sidelobes it produces.
Hann: Excellent allround FFT Window. NENBW is 1.5, rounded to two in the inset. Good choice for e.g. Doppler effect
measurements, mentioned above. Not recommended for accurate level measurements.
Hamming: Like Hann, a very good "Allrounder", often used in other applications as well. NENBW is 1.3, rounded to 1.
Best separation between two neighboring peaks, due to its low NENBW. Not to be used for accurate level measurements.
BlackmanHarris7: Lowest sidelobes of all offered FFT Windows, meaning low-level peaks not too near by can be resolved.
HFT70: "Flat Top" Window with low sidelobes. Due to its "flat top" it is used for accurate level measurements, e.g. using
the calibrated scales for the RSPs or the RTL-SDRs with a R820T tuner.