The following cut is from the K3 FAQ - Elecraft on Roofing Filters. "In order to achieve a K3 blocking dynamic range (desense) in the 140 dB+ range, you -must- use a narrow crystal filter (400 Hz for closer interfering signal spacing) in front of the DSP. We use hardware AGC after the narrow crystal filter and ahead of the DSP to protect the DSP when signals inside the crystal filter exceed a 100 dB dynamic range. If you only use the 2.7 kHz stock filter for CW or data operation you will be significantly desensed once signals within that filter's bandwidth exceed about S9+25. This is before phase noise from the transmitting station becomes a factor. Not uncommon on 40M at night, during a contest or at a multi-op station -- Or every day in major cities. Changing to a 400-500 Hz filter reduces blocking from signals 1-5 kHz away. I've personally confirmed this on the air with my K3 and the other commercial rigs we have here. when I've operated with the K3, or another DSP rig, on CW without using a narrow 400-500 Hz filter ahead of the DSP filtering, I frequently experienced desense (BDR) from nearby signals. Putting in the narrower crystal filter immediately cleaned it up. Using narrow crystal filters ahead of the DSP also reduces AGC pumping from static crashes on 80/160M etc.
This was from Eric's portion of the article. ------------------------- 73, Greg - AB7R Whidbey Island WA NA-065 On Tue Sep 18 17:42 , [EMAIL PROTECTED] sent: >In a message dated 9/18/2007 4:36:17 P.M. Eastern Daylight Time, >[EMAIL PROTECTED] writes: >If the roofing filter is able to knock down the >unwanted signal to a level the DSP can handle, then the DSP will not >overload and the DSP itself will provide great ultimate rejection. > >Well, sorta kinda. If the unwanted signal gets in and pumps the AGC, then >the DSP might knock down the signal but won't be able to do anything about >the >AGC pumping. Then, you won't hear the signal if it is weak. That is the >whole issue of the DSP being in or out of the AGC loop and it generated a >mountain of controversy with the Orion when TT moved the DSP to within the >loop. > >I believe I read the K3 did not follow that mistake but a little >clarification from the gurus who know a whole lot more about this issue would >be helpful. > >Craig "Buck" >k4ia >Fredericksburg, Virginia USA > > > > >************************************** See what's new at http://www.aol.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 _______________________________________________ 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
