Hi Doug,
> Can someone explain to me why the above/below results
> for these two filters are so disparate at these narrow
> spacings?
Let me speculate...
you might want to read the description by PA3AKE, especially the
chapters on his development of his roofers:
<http://www.xs4all.nl/~martein/pa3ake/hmode/index.html>
He talks about, among other things, how important clean surfaces are for
good IMD performance in crystals filters.
---------------------------------------------------------------------
Quartz and IMD
The surface finish plays a big role in the linearity of the
quartz. Particles (micro dust) polluting the surface of the
quartz are known to cause IMD. To make things considerably
worse, the IMD they cause is not following 3rd order law by
any means.
(from the PA3AKE website)
---------------------------------------------------------------------
Sherwood also mentions this in the information (in one of the audio
files, if I remember correctly) available at his web site.
If one of the crystals in the filter is "dusty" then you ought to see an
immediate difference.
My 2700Hz filter, if our numbers (DD5FZ & DK4YJ) are really correct, is
better than the ARRL filter but the difference is not major. I assume
this is due to normal variance which occurs during the production
process (and minor differences in the calibration of the measurement
equipment).
Also the 8 pole 2800Hz filter is likely to have a bit more attenuation
in the passband than a 5 pole filter, which may have a minor affect on
the amount of IMD in the post filter IF amps and second mixer. All of
this could combine to explain the differences at 1kHz and 2kHz spacing.
On the other hand, at 20kHz both filters are almost identical, so a
"dusty" quartz crystal may not be part of the reason after all.
> review suggests that the observed asymmetry is quite unusual
For many filter topologies the asymmetry is perfectly normal. Not to
forget that in a design such as the K3, it's the first 20dB to 30dB of
stop band attenuation which are really important. The final total
selectivity comes from the DSP filter and not from the roofing filter.
This said, I am playing with idea of rolling my own roofing filters for
my K3 for three reasons. Firstly, can a couple dBs be tickled out by
using extremely good crystals (expensive!)? Secondly, it should be
educational. And most importantly, HAMs just wanna have fun.
One thing the ARRL report does show, and I agree with this after having
a 2700Hz filter in my K3 for about a year now, the five pole 2700Hz
filter is plenty good. An 8 pole filter (i.e. the 2800Hz filter) does
not appear to be necessary.
vy 73 de toby
--
DD5FZ (ex 4n6fz, dj7mgq, dg5mgq, dd5fz)
K2 #885, K2/100 #3248, K3/100 #67
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