[Premium-Rx] New radio selection

[Tim Shoppa]

"Tracey Gardner" <tracey.gardner at talktalk.net> wrote:
> With regard to your "brick wall" filter comments.
> I bought a surplus Racal RA1792 a few years ago, specifically because it had 
> a Racal 100Hz CW filter fitted.
> I soon became disillusioned with this filter, as in apparent bandwidth 
> terms, it performed a lot worse than the 250Hz filter in my Icom receiver.
> After doing some investigation I discovered that the Racal CW filter was 
> optimised for a  linear phase response and not for shape factor, which meant 
> that it's skirts were as wide as a barn door.
> I've been using an Inrad 125Hz filter, with a good shape factor,  in my 
> AR7030 for some years now and that doesn't ring at all.
> I'll be exchanging the Racal 100Hz filter for an Inrad one at some point in 
> the future, so if anyone needs a "barn door" ?

I do occasionally see linear phase crystal filters showing up,
usually in oddball bandwidths, in gear designed for digital HF
comms.

There are compromise filters out there, e.g. "Gaussian to 12dB" that
have linear phase over most of the passband but turn steep out in
the skirts where ringing will be down a good amount anyway. My
homebrew crystal filters are actually designed to be Gaussian to 12dB -
this is actually a very easy shape to realize using ladder filter
techniques.

I think that narrow "brick wall" filters do have very real value in very
densely packed CW bands. e.g. any ham contest weekend. But for general
operating I find them very tiring. And with typical hams matching
frequency only +/- 400 Hz, having a brick wall 100Hz wide in a contest
may be counterproductive unless you are working from a country that
only comes on the bands once a decade and always have a pileup calling you :-).

Tim N3QE