Re: Weaver Image Rejection, are only images in band cancelled?

On Fri, 15 Feb 2008 13:49:31 -0600, Richard Owlett wrote:

Al Clark wrote:
[snip]

The Weaver method does work very well provided that the sin and cos is
calculated with high precision. I didn't use weaver at RF, I used it at
audio frequencies to create a set of continuous variable bandpass
filters with constant bandwidth.

Can you suggest good Google search terms? [weaver bandpass filter]
didn't prove useful. "continuous variable bandpass filters with constant
bandwidth" would be useful to me. Speed not an issue, will be doing
batch processing. I'll be able to use arbitrary precision math.
-- snip --

Why not FFT, window, and FFT back then?

I could see getting very confused trying to translate the Weaver method
into digital terms. In digital terms it's easy:

* Down-convert your data with a complex signal whose frequency is
the center frequency of your desired filter.
* Band-pass filter the baseband data from -fo/2 to +fo/2. After
doing all the math to design a complex bandpass filter centered
around DC, note that you've designed a filter that effectively
low-pass filters the real and complex parts separately and
identically. Slap your forehead.
* Up-convert the output of the bandpass filter with the same complex
signal that you used to down-convert.
* Throw away the imaginary part (or the real part, if that floats
your boat).

Note that this method only works if your center frequency is bigger than
the bandwidth of your filter -- if this is not the case, then things will
get mangled when you do the last step.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
.

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