While I agree with you David and Julian's point about shipping costs etc, I
would suggest that in this case the fundamental questions are whether or not
the filter's specification does include IIP3 minimum limits and what are
they. Because, and understandably so, the dynamic range data published by
the RSGB, ARRL and others reflects the preformance of the receiver as a
whole and not of its filters as stand alone components, I have asked INRAD
whether or not they do include IIP3 in their filter specifications or test
for IIP3 as part of their QC.
Multiple signal dynamic range testing is commonly practiced by some when
developing high performance receivers, and can better unearth the sort of
IMD problems that often arise during a contest or during a 'no-split' pile
up than two signal tests. Many filters fall apart in terms of generated IMD
when subjected to multiple signal tests, and I suspect that many people (I
certainly used to!) think that the IMD products are real signals causing
QRM. The problem gets significantly worse of course if the IF cascade's
dynamic range is 'poor'.
There is no doubt in my mind that if a receiver is expected to dig out weak
signals among close spaced 60-65 db over S9 BC stations, every last db of
close in dynamic range does help. BC transmitter phase noise is not the
problem that one might expect it to be.
73,
Geoff
GM4ESD
David Cutter wrote on Saturday, June 21, 2008 11:05 AM
Well, the radio seems ok, but there will be occasions, such as operating
near to very strong in-band signals when it may perform less than
satisfactorily and it is hard to identify what is happening. There may
well be artefacts that are disturbing and only heard on odd occasions such
as this and reduce the effectiveness of the radio in a contest for
instance, and it is difficult to reproduce those conditions on the bench,
ie multiple very strong signals near a weak one. I know someone who
receives 55-60dB over 9 bc signals on 40m and the K3 probably cannot cope
with this.
I think the point about making standardised measurements is that they are
standard, but most of the time you wouldn't notice if you were compliant
or not. In days of old when assessing military radios returned for
service, it was established that >60% failed the spec and were not
reported by the operator for those failures. So, I think I would like to
know that my radio is compliant with its spec then I can look elsewhere
when a problem occurs.
David
G3UNA
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