[TenTec] Noise reduction of roofing filter?

[Dr. Gerald N. Johnson]

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