Whether a filter is a roofing filter or not isn't really anything to do with
where in the receive chain the filter is located or what it's performance
specifications are. It is to do with the role it has to perform.
The purpose of a roofing filter is to reduce the exposure of receiver stages
to strong signals which would be likely to overdrive them causing IM
products to develop or blocking to occur. The best place for a roofing
filter is at the antenna socket though for most purposes this is not a
practical proposition. The most common place for a roofing filter to be
located is in the first i/f.
When is a roofing filter not a roofing filter? Well, that's when it
contributes to defining system b/w. So taking the Omni VI as an example:
Are the narrow filters at 9 MHz roofing filters or not?
Well, that depends. If you are using a 250 Hz filter in the 6 MHz i/f and a
500 Hz filter in the 9 MHz i/f then the 500Hz 9 MHz i/f filter is a roofing
filter.
BUT still using the same 500Hz filter in the 9MHz i/f, you switch to a 500Hz
filter at 6 MHz. That filter at 9 MHz now no longer performs a roofing
function but is instead contributing significantly to the useable b/w of the
radio.
Whether the 9MHz filters in an Omni VI are roofing filters or not depends
upon how you are using them.
At least that's the way I understand it.
Bob, 5B4AGN
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|