roofing filter confusion

[Dick Green WC1M]

After reading this reflector for a while, I'm confused by all the discussion
about 8-pole and 5-pole roofing filters. Or, perhaps some of you are
confused about the way roofing filters work. Here's a good article that
describes how roofing filters work and why more poles might not be better:

 

http://www.dxstore.com/download/inrad_roofing_filters.pdf

 

Some of the comments I've seen on the reflector suggest that owners are
buying narrow roofing filters with steep skirts in order to improve
selectivity. But that's not the purpose of roofing filters. Roofing filters
are there to improve third order dynamic range. They don't need very steep
skirts because ultimate selectivity is determined primarily by the IF DSP
algorithms. This is good, because compared with crystal filters, DSP filters
are able to provide steeper skirts with lower loss and little or no ringing.

 

The main problem with using narrow 8-pole roofing filters is that there's
more insertion loss, and hence less receiver sensitivity. The Orion has a
famous flaw where it switches in an amplifier to compensate for losses in
the 500 Hz and 250 Hz roofing filters, but the gain is too high and the
dynamic range goes all to heck. The K3 designers are to be applauded for
allowing the user to configure the amount of additional gain for each filter
to compensate for the loss.

 

Hopefully, variable gain will compensate for losses with no other distorting
effects. But even if it does, what's to be gained by using an 8-pole filter
in the first place? Can it be demonstrated that the 8-pole filters improve
selectivity beyond what the IF DSP does? Have tests been run to determine
specific gain settings for each filter offered by Elecraft so dynamic range
won't be compromised?

 

73, Dick WC1M

 

 

 

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