Dave
The measurement was real, however re-testing gave inconsistent results
and varied depending on which side of the two signals the IMD
measurement was taken. It is possible the 8 pole 400 Hz filter may have
a "blip" in it's response, equally my home made signal source and hybrid
coupler may introduce a strange artefact. However, switching the filters
between 8 pole 400 Hz and 8 pole 2.8 KHz gave a clear spurious signal on
the narrower filter that wasn't present with the wider one.
As others weren't able to reproduce the effect with their K3s, it seemed
to make sense to remove reference to the issue from my web site. I
didn't want to add to the existing debate over which filters to buy!
73 Dave, G4AON
K3/100 #80
----------------
I was perusing the Sherwood Engineering receiver evaluation
data, http://www.sherweng.com/table.html , and I noticed
that the K3 2 KHz dynamic range is reported to be
significantly better with the 200 Hz 5-pole roofing filter
than with the 400 or 500 Hz filter.
Then I came across some IMD data on the K3 Wiki,
http://www.zerobeat.net/mediawiki/index.php/K3_Roofing_Filters ,
which does not show much difference between those filters,
but has an interesting footnote:
"It should be mentioned that in a published review of the
K3, G4AON observed degradation of close-spaced IMD
measurements with the 400 Hz, 8 pole filter: "These figures
are for a 400 Hz bandwidth with the 8 pole 400 Hz roofing
filter, the rather surprising discovery was the dynamic
range improved by almost 10 dB when the 2.8 KHz 8 pole
filter was selected." G4AON's findings haven't yet been
independently
confirmed however."
Any thoughts on resolving these apparent inconsistencies?
Dave Hachadorian, K6LL
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