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 _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
