[Elecraft] K3 filter performance

[Larry Molitor]

--- "Joe Subich, W4TV" <lists at microham-usa.com> wrote:
 For a died in the
> wool 
> RTTY DXer, a steep sided 270 - 300 Hz filter would
> be nice
> instead of 370 Hz from the "250 Hz" filter.

Over the years I've spent a lot of time looking at
this issue. My interest is primarily digging weak RTTY
DX out of the noise but I do occasionally get into a
contest. 

I've played with a lot of radios and filters but most
of my real testing was done with a FT-980, a TS-2000,
and a FT-2000. 

While I have seen good weak signal performance with a
250 Hz filter, it was the exception not the rule. As
an example, the improvement on the FT-980 going from
the stock SSB filter to the dual CW filter was
dramatic. But going to the 250 Hz CW filter lost about
6 dB in ability to properly decode weak RTTY signals.
It was even worse on signals that had polar flutter. I
no longer have the plots of these FT-980 filters but
as I recall, the 250 Hz filter was a bit peaky in the
middle and had poor group delay characteristics
extending well in from the corners.

As you say Joe, the determining factor is the passband
ripple/group delay. In a typical bandpass filter the
group delay goes to heck at the corners. But the shape
of the "corners" varies from filter to filter. I've
run a number of "ham filters" on a network analyzer
and plotted group delay. Seems like no two filters are
the same even if the same part number. This I believe
is due to ham filters being so cheap and manufacturing
process control being minimal to keep the sell price
down to what we can afford.

So I would say, if you have a 250 Hz filter, no matter
where it's placed in any radio, give it a try on very
weak signals. Switch between a wider filter and the
narrow filter and see if there is any degradation. If
not, use the narrow filter.

Other wise, plan on not using any filter less than 300
Hz in passband width (note - this is NOT the 6 dB down
width!!! very important!!) if you want optimum weak
signal performance.

The DSP filters and DSP modems following the IF
filters cannot re-constitute the "distorted" signal so
it is important to keep the RTTY (and CW) signals well
away from any spot in the passband that can cause
phase distortion and/or pulse stretching.

Detailed high resolution plots of the filter passbands
in question would make these decisions a lot easier.
(hint, hint)

73,

Larry - W7IUV




      ____________________________________________________________________________________
Never miss a thing.  Make Yahoo your home page. 
http://www.yahoo.com/r/hs