Re: [Discuss-gnuradio] filter ssb

[mike revnell]

I've spent the evening having a more detailed look at the filter.

Attached is a new set of tap coefficients and a cleaned up demod which 
uses them. These have less leakage of the passband into the oposite 
sideband.

You can switch sidebands by conjugating the tap set.

It's very interesting to put a fft window on the output of the demod filter.

The easiest way to control the bandwidth of the demod is to change the 
bandwidth of a
post lowpass filter.

The sharp edge of the response is set by the prototype lowpass filter. 
The less sharp edge is the dc transition, as in the hilbert transformer. 
The only way to make it sharper is to increase the length of the filter.

The basic idea is based on the analog complex down converters used in 
the VLA. They are also known as sideband suppression mixers.

When used in this way the tuning doesn't change when switching sidebands.

It might work to offset the center frequency to move the sharp edge to 
the carrier. A spectral inversion about the center of the passband will 
also work, I think this is the basic idea behind the weaver demod.

The wide transition band at dc is why I think this is more an example of 
DSP than a practical solution. If you are willing to live with a long 
filter it should work ok.

cswiger wrote:

> In fact, here's a plot of the upper/lower sideband passbands
> for the two sets of filter taps, carrier point at 50

> http://webpages.charter.net/cswiger/lsb-taps_usb-taps_rev.jpg
>
> I don't understand enough of the math in hfir.sci, but all we
> need to make radios is: put the sharp side of the passband
> next to the carrier point, if possible, and how to vary
> the width to values from 600 to 3600Hz.
>
> --Chuck
>
>
>
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ssb.tar
Description: Unix tar archive