[TenTec] Noise reduction of roofing filter?

[Dr. Gerald N. Johnson]

On Wed, 2007-03-07 at 13:39 -0500, Bill Cotter wrote:
> 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.

And it cuts down on how much the peaks of the noise spikes overload
stages.

> 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).

The more you narrow the band going into the DSP, the more you handicap
its noise reduction algorithms.
> 
> 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.
> 

-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer