Maybe I misunderstand but perhaps the gurus here can straighten this out.
The Dynamic Range numbers relate to how strong a nearby signal will produce
IMD distortion. That is the mixing of two signals which appears as unwanted
noise/interference. There is a chart on the Sherwood presentation
http://www.sherweng.com/Dayton_2004/Dayton_2004-Sherwood_files/frame.htm which
suggests a
Dynamic Range of 95 would require the offending signal to be 40 over before it
caused an IMD problem. Even at a Dynamic Range of 70db, the offending signal
would have to be 15 over before there was a problem with IMD.
The roofing filter may make the IMD figure worse but it also cuts down on the
QRM and, very importantly, keeps the AGC circuit from pumping or generating
clicks on its own. This is the primary difference I note with the Orion vs my
Mark V Field. That Field collapses when there is a signal nearby and I don't
think it is because of IMD. You can narrow the filter on the Field to
eliminate the QRM from the nearby signal but the AGC pumps so badly (or is the
front
end overloading?) you can't hear the weaker signal.
I see the primary benefit of the roofing filter as protecting the AGC and
eliminating QRM. If there is a little degradation in the IMD, I can live with
it. How often do you have a 40 over signal right next to you?
Am I on the right track?
Radio k4ia
"Buck"
Fredericksburg, Virginia USA
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