Peter,
I have the Inrad 763 filter (600Hz CW, 9000.7kHz IF) installed on
the roofing filter board, Inrad 759 filter (400Hz CW, 9000.7kHz IF,
N2) and Inrad 750 filter (400Hz CW, 6299.25kHz IF) installed in my
OMNI-VI+. Operating on 80/40M during the early evenings is a
particular challenge with the atmospheric noise, static crashes and
QRM from SSB stations closing in on the CW sub-band.
Operating without the roofing filter, 2.4kHz first IF and 500Hz TT
filter, the QRN and QRM is dramatic. Activating the -20dB
attenuator and/or reducing the RF gain reduces the overall noise
power mentioned by Gerald below. Switching in both 400Hz filters
(1st and 2nd IF) plus the 600Hz roofing filter SUBSTANTIALLY
reduces the noise on weak signals. I attribute the improvement in
S/N ratio to the total shape factor and quality of the filters.
And, this improvement is without using the LP/NR DSP settings, with
their unique contribution of artifacts (ringing, popping, etc).
For many years I used the TT dual-500Hz and TT Dual-250Hz filters,
in spite of their passband ripple and perceived blow-by. Upon
installation of the Inrad filters, you can immediately hear the
improved shape factor and low PB ripple. It is startling and
clearly obvious with the first listen.
I have the same issues with diminished hearing, and the Inrad
filter combination was a sound financial decision for the return in
weak signal performance improvement. The reduction in fatigue from
noise and QRM has made operating the OMNI a continuing pleasure.
73 es tnx bill n4lg
PS: I have no pecuniary or other interest in INRAD other than being
a satisfied customer.
At 11:52 AM 3/7/2007, Dr. Gerald N. Johnson wrote:
>On Tue, 2007-03-06 at 15:53 -0700, Peter Hoon wrote:
> > Fellows,
> >
> > Am hearing disabled, own an Omni 6 Plus mostly for CW, and
> cascade the 250
> > Hz 6.3 IF CW filter along with the "low note" 221 250 Hz CW
> filter in the 9
> > MHz IF (position N-2).
> >
> > In reality, my stock 2.4 kHz filter is always in line along
> with both the
> > 250 Hz CW filter.
> >
> > If I were to add the Inrad 765 500 Hz roofing filter offset for
> the special
> > 221 filter, would I experience less atmospheric noise at the
> speaker?
>
>Probably not. The filter tends to integrate the pulse and reduce the
>bandwidth at the expense of stretching it out in time. The
>narrower and
>the sharper cornered the filter frequency response, the worse this is.
>Tentec filters are good at not stretching static compared to vintage
>Collins filters that turn a click to a crash or line noise spikes to a
>continuous noise. The narrow roofing filter will probably reduce the
>noise power but by stretching the pulse it will sound like its been
>increased.
> >
> > (I realize that the roofing filter is normally used to improve
> third order
> > IMD, but am interested in knowing if I would also gain an
> improvement during
> > the summer when the 40 M band typically has high back ground
> noise).
> >
> > My NR in the Omni 6 plus works well, but there are occasions
> where the band
> > is so noisy at my location, I am considering possible QRN noise
> reduction
> > advantages from a roofing filter.
>
>I've always found that reducing the gain before the mixer and
>filter are
>most effective on lightning static reduction. Noise reduction isn't
>tuned to that and noise blankers rarely are a help (and with the many
>strong carriers on 40 meters a noise blanker is of no benefit.
> >
> > Thanks for any and all suggestions.
> >
> > Peter
> > VE1CHS
> >
>
>--
>73, Jerry, K0CQ,
>All content copyright Dr. Gerald N. Johnson, electrical engineer
>
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----------------------------------------------------------
Bill Cotter, N4ALG E-mail: n4alg@qx.net
173 Carolyn Lane Home: (859) 887-5563
Nicholasville, KY 40356-9340 Work: (859) 323-6474
ARRL ARCI #9878 QRPl #1646 QCWA #29393 FISTS #4535
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