You can mean one of two things by frequency resolution:
- The frequency spacing between computed estimates (sometimes called the "bin width")
- The ability to resolve frequencies of the sampled signal independently.
When you zero-pad an FFT you essentially perform (periodic) interpolation between data points in the Fourier space. You can think of it as taking a long signal and convolving it (in the frequency domain) with the Fourier transform of the shorter window. If your window is rectangular, then you are essentially performing sin(x)/x interpolation in the frequency domain. You get a lot more estimates in the sense of #1, but they're very smoothed out.
You don't actually gain any frequency resolution in the sense of #2. You just get finer interpolation between points. The only way to gain resolution is to actually have more sample points of the original signal.
