I took the reasoning a step further since the data for the 1 kHz 8 pole filter is:
Filter 20kHz 10kHz 5kHz 2kHz --- 1 kHz, 8 pole 100+ 100+ 100 94 --- So I ordered the 1 kHz filter since I'm just a ragchew type cw operator. It's nice to be able to run as wide as 800 Hz or so when tuning around, something I couldn't do with the narrower filters - but with numbers like this I don't think I have much to worry about! Bob NW8L On 9/18/07, Bill Tippett <[EMAIL PROTECTED]> wrote: > > > This topic has drifted from "2.7k versus 2.8k" to "2.7k > versus 400 Hz". *Of course* a 4-500 Hz filter is needed for CW > and will have vastly better IMD/BDR than a 2.7k/2.8k. But I > repeat that there will be very little if any difference between a > 400 (actually 435 Hz) 8-pole and 500 ( 565 Hz) 5-pole. In my > opinion, you are really wasting money to buy 8-pole filters for > improved RX performance, unless the 130 Hz BW difference > is important. In practice, this implies the difference in a signal > spaced 435/2 = 218 Hz versus 565/2 = 283 Hz...65 Hz is not > much difference, and I actually prefer the wider BW to catch > more off-frequency callers. From Eric's posted data: > > Filter 20kHz 10kHz 5kHz 2kHz > > 400 Hz, 8 pole 100+ 100+ 100+ 95 > 500 Hz, 5 pole 100+ 100+ 100+ 94 > > Again in my opinion the 250/200 Hz filters are > redundant and unnecessary if you have a 400/500. > They do not improve 2 kHz IMD significantly as > seen below (1 dB difference is meaningless as > that is well within measurement error): > > Filter 20kHz 10kHz 5kHz 2kHz > > 200 Hz, 5 pole 100+ 100+ 100+ 95 > 250 Hz, 8 pole 100+ 100+ 100+ 95 > > You could argue that the 250/200 would be better > for IMD fom extremely close-spaced signals (e.g. > <200 Hz spacing from your TX frequency), but at > that spacing other factors such as the transmitted > signal's phase noise, key clicks, etc. will override > any theoretical IMD advantage (i.e. the IMD becomes > "noise limited" in ARRL terminology). > > Remember also that Passband Tuning can be used > to shift a 400/500 Hz filter if you actually do need > to eliminate a signal spaced at 200-250 Hz from your > TX frequency...not that it would actually do any > good to eliminate phase noise or key clicks. > > 73, Bill W4ZV > > > > _______________________________________________ > 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 > _______________________________________________ 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
