[TenTec] Effect of roofing filter selection on Orion II NB

[Grant Youngman]

Played around with the NB a bit this morning, with an S9+20 relatively
constant noise level that had an unusually high line noise component on 75M.
(Fortunately, the line noise has faded as the day has progressed).

Found that DSP NB performance was greatly improved by setting the roofing
filter selection to 20 Khz, and also improved although less so at 6 KHz.
With a 2.4 Khz roofing filter in line, the DSP NR had virtually no effect on
the specific form/content of 75M garbage this morning.  A good bet is that
noise pulse stretching in the narrower roofing filters is enough to affect
DSP NB performance.   Which is a phonomenon not unique to the Orion, and
affects any NB that sits behind a narrow (sometimes even not so narrow)
crystal filter, DSP or not.  Widening the roofing filter selection also
appeared to reduce the amount of distortion imparted on underlying signals.
That's probably because it has a better formed pulse to "blank", and just
does less damage to the signal as a result.

It could be that "the noise blanker on <this rig> or <that rig> works
better" comments is related in part to NOT having narrow roofing filters in
those radios.

The HW NB sits in front the of roofing filters so they have no effect on it.

I frequently have a sharp once per second "pop" caused by some bit of
neighborhood mischief, and either NB has always worked fine on that,
regardless of how the rest of the radio was set.

Grant/NQ5T