[hpsdr] spectrum increased frequency resolution?

[Frank Brickle]

On Tue, Jun 9, 2009 at 5:12 AM, John Melton <John.Melton at sun.com> wrote:


> I have modified ghpsdr to have the FFT use 32768 samples (4096 actual
> samples and the rest padded with zeros).  This allows me to do a 16X zoom on
> the bandscope (i.e. I can see 3840 KHz) where I get a 1:1 sample/pixel
> display.  You really cannot see much additional information. It is simply
> the original display stretched out.  After all, it only had 4096 samples to
> start with.


The way to think of what you're seeing is something like this. Increasing
the number of points in the FFT increases the number of bins *between* the
ones you see already. But unless you do some other, additional work, there
will be leakage among the new bins. That's why I said you need to be careful
about windowing. Furthermore, to keep up the accuracy of the power
estimates, you need to use other techniques to reduce the variance in each
bin. That involves making multiple looks at data in each frame. Simple
averaging amond frames is merely one of the simplest ways to do that.

Polyphase and multitaper estimation is probably the most common method used
currently since it's not too difficult to implement and the computational
complexity is not bad. However, if you're really concerned with zooming in
on arbitrary segments and getting truly high-resolution power and frequency
estimates, then ARMA techniques are where you want to go. But they can be
expensive.

73
Frank
AB2KT

-- 
You better buy yourself a 'lectric guitar / You better play / There's no
other way / To beat the blues... -- Pete Townshend, "My Baby"
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