[TenTec] WIDER FILTER FOR WEAK CW DXING?

[W.D. (Doc) Lindsey]

Good Evening:		

I always thought I was greatly blessed to start out as a rock bound Novice
in late 1960 [g].  Especially since I had a super-wide BW receiver called a
Hallicrafters S-38D.   Seemed like I could hear the entire Novice CW band
without even having to fine-tune.  I was forced to learn to pick out
whoever might be calling me.  Actually, turned out that stations *had* been
replying for a couple of days before I could finally recognise *my own
call* [c].  Eventually my frustrated elmer actually drove over to my house
to point this out!  From then on things began to improve....but verrrrrryyy
slowly.

But the net result of those early trials of 47 years ago is that now I
really CAN copy CW in my head easily, and usually much better without
filtres bands are crowded or there is lots of QSB/QRN.  I actually don't
even particularly want a really complicated rig, preferring the simplest
(the "ZEN?") CW rigs I can find.  Others of course will differ completely,
and that's their schtick.  

Oh, forgot to say that it actually took me over four years to memorise the
Morse code, so that I could take the Novice test in 1960!

73,
--Doc/K0EVZ 


> [Original Message]
> From: Dr. Gerald N. Johnson <geraldj at storm.weather.net>
> To: <tentec at contesting.com>
> Date: 6/16/2007 1:23:02 PM 
> Subject: Re: [TenTec] WIDER FILTER FOR WEAK CW DXING?
>
> On Sat, 2007-06-16 at 11:54 -0700, Kevin Purcell wrote:
> > This is very common amongst VHF/UHF weak signal DXers (including  
> > EMEers) too.
> > 
> > It's also little know but wider bandwidths for SSB improve  
> > intelligibility too especially for the very weak signal case (see the  
> > graphs on articulation index versus bandwidth in Sabin's book on SSB).
> > 
> > The reason to use narrower filters is to reduce QRM that your brain  
> > (or the RX) can't deal with.
> > 
> > Why do wider filters work:
> > 
> > 1. It avoids the problem of impulse noise being smeared in time by  
> > the limited bandwidth of the filter (and by group delay in the  
> > filter) so in noisy environments a wider filter can help a lot.
>
> The time response of various filter designs is very important. The
> classical Tchebychev filter approximations typical of crystal lattice
> filters and vintage Collins mechanical filters have a rotten time
> response. The amplitude response of those filters has sharp shoulders
> and a flat passband. The filters with the best time response
> (Butterworth or even better Bessel) have the least flat frequency
> response, rounded shoulders, and gentle skirts. Its hard to make ladder
> filters give a Tchebychev response while Butterworth and Bessel are
> easy. Tentec filters are ladder filters. I don't know about Inrad
> filters. I do know that an aftermarket Collins filter I bought for CW in
> my FT-857 doesn't ring significantly. And on VHF it sometimes enhances
> S/N slightly on very weak signals and always does that better than the
> audio DSP.
>
> It is possible to have sharp corners, flat frequency response, steep
> skirts, and a good time response but that is ungodly expensive in
> hardware because the design has to use many extra resonators to correct
> the phase response. In a digital filter it is handier to have the extra
> poles to achieve both goals, at the cost of total time delay.
>
> The worst CW filter I've ever used was a 270 Hz from Kenwood in a
> TS-430. Pure noise going in, had a distinct pitch coming out. It wore my
> ears out in a couple hours of FD fighting that tone to copy desired
> signals.
> > 
> > 2. Your brain is an interesting signal processor. It can act as a  
> > very narrow (50Hz or so) adaptive filter but without the usual  
> > problems of group delay and the like of a real filter.
>
> Probably because it does a time correlation and the wider the noise
> bandwidth, the poorer the correlation of the noise so cancellation is
> easier. The narrower the noise bandwidth, the more coherent the unwanted
> noise is the harder it is for brain or DSP to eliminate.
> > 
> > It was mentioned in EMRFD and you can find it on VHF DXer websites  
> > too. SM5BSZ comes to mind.
> > 
> > On Jun 16, 2007, at 6:19 AM, Jeff Frank wrote:
> > 
> > > I almost always have easier and clearer copy on weak CW signals  
> > > when I use the 1.8 kHz filter vs. the .5 or .25 kHz filters in my  
> > > 6.3 MHz IF. I've got nothing in the N1 position. Is this the way  
> > > it's supposed to be?
> > 
> > --
> > 73 DE N7WIM / G8UDP
> > Kevin Purcell
> > kevinpurcell at pobox.com
> > 
>
> -- 
> 73, Jerry, K0CQ,
> All content copyright Dr. Gerald N. Johnson, electrical engineer
>
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