The IMD contributed by inductor cores used in the front-end selective
circuits is often not taken into account either, or for that matter any core
within the signal path. They can bite.
73,
Geoff
GM4ESD
Larry Phipps wrote:
This is a subject which seems to be gaining in importance as receiver
designs surrounding the xtal filter seem to be improving to the point
where the filters are becoming the limiting factor in IMD performance.
73,
Larry N8LP
1. Narrower is not always better (Ten-Tec experience)
2. 8-poles is not always better than 5-poles (per Inrad)
3. Let IMD and BDR measurements be your guide
More evidence below to support waiting for IMD/BDR measurements
before ordering any roofing filters.
73, Bill W4ZV
http://mailman.qth.net/pipermail/1000mp/2007-April/020755.html
There is a fascinating article describing IMD tests on the IC-7800 by
DC4KU
in CQ-DL, August 2005 (in German). In these tests, IP3 at 2 kHz offset
degrades by an astounding 16 dB when switching from the 15 kHz to the 6
kHz
roofing filter. This degradation is due to passive IMD in the filter, and
possibly also to IMD in the filter driver amplifiers caused by mismatch
when
the filter is excited outside its passband. I can send you an
English-language summary of the relevant part privately, if you wish.
<snip>
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