K3 2.7 k Filter vs. 2.8 k
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>Do any field testers care of offer comments on the 2.7 k filter vs. the 2.8 k? Dick I'm not a beta tester but am familiar with roofing filters because Orion had essentially the same front-end as the K3 (main difference being its 1st IF was at 9.0 MHz instead of 8.2 MHz). N4LCD recently asked a similar question and I did not respond thinking someone else would. They didn't so here goes for both of you. The following are words by George W2VJN of Inrad on page 6 of his excellent article on roofing filters: http://www.qth.com/inrad/roofing-filters.pdf *********************************************************** 5. If 6 poles work so well, why not 8 poles? The most important part of the filter characteristic is from the pass-band on down to about 30 dB on either side of center. Eight poles would provide much better stop-band isolation, but its not required in a roofing filter and would make no noticeable improvement in IMD performance. *********************************************************** Indeed his statement is borne out in the IMD numbers Eric posted previously: http://mailman.qth.net/pipermail/elecraft/2007-September/073442.html Filter 20kHz 10kHz 5kHz 2kHz 2.7 kHz, 5 pole 100+ 98 92 n/a 2.8 kHz, 8 pole 100+ 100 93 n/a The major role of a roofing filter is to prevent adjacent (i.e. unwanted) ~S9+30 signals from entering the IF chain. This means the shape factor of the filter is relatively unimportant beyond about 30 dB down on the filter skirts, so there is little advantage of an 8-pole over a 5-pole filter as far as the receiver is concerned. If your filter eliminates unwanted S9+30 signals from propagating to the DSP stage, then the DSP can provide the actual final bandwidth selectivity (without unwanted IMD products). As you can see in the above measurements, there is essentially no difference in IMD performance (2 dB being well within measurement uncertainty). So, for receiver performance only, I would say there is NO difference in the filters other than the extra cost of the 8-pole. One potential difference has to do with the K3 transmitter. Since it transmits SSB through these same filters, the 8-pole might be preferable since it would more effectively attenuate unwanted products, but I'm sure the 5-pole will meet published specs since it is the standard K3 filter. In other words, if you got an 8-pole, it would make little noticeable difference in the receiver, but it would make your transmitted signal a little cleaner. I hope this helps you. I'm ordering the 5-pole because I don't see the advantages of the 8-pole are worth the cost. 73, Bill _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
Re: K3 2.7 k Filter vs. 2.8 k
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I have a question. The standard IMD test is a two tone test. In a
contest type situation with multiple strong signals in close proximity, wouldn't greater ultimate rejection by the crystal roofing filter be a significant advantage? Mike W5FTD >>Do any field testers care of offer comments on the 2.7 k filter vs. >>the 2.8 > k? > Dick I'm not a beta tester but am familiar with > roofing filters because Orion had essentially the same > front-end as the K3 (main difference being its 1st IF was > at 9.0 MHz instead of 8.2 MHz). N4LCD recently asked a > similar question and I did not respond thinking someone > else would. They didn't so here goes for both of you. > The following are words by George W2VJN of Inrad > on page 6 of his excellent article on roofing filters: > http://www.qth.com/inrad/roofing-filters.pdf > *********************************************************** > 5. If 6 poles work so well, why not 8 poles? > The most important part of the filter > characteristic is from the pass-band on down > to about –30 dB on either side of center. Eight poles would provide much better > stop-band isolation, but it’s not required in a > roofing filter and would make no > noticeable improvement in IMD performance. > *********************************************************** > Indeed his statement is borne out in the IMD numbers > Eric posted previously: > http://mailman.qth.net/pipermail/elecraft/2007-September/073442.html > Filter 20kHz 10kHz 5kHz 2kHz > 2.7 kHz, 5 pole 100+ 98 92 n/a > 2.8 kHz, 8 pole 100+ 100 93 n/a > The major role of a roofing filter is to prevent adjacent > (i.e. _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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Mike,
Not necessarily. As long as the signals within the passband of the roofing filter are not strong enough to overload the DSP, that is sufficient. In other words, the ultimate rejection of the roofing filter is not critical. 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. These are roofing filters and not final filters. If they were final filters, then the ultimate rejection would become important. There is an article on roofing filters in Oct QST that may help to explain. 73, Don W3FPR Corboy-Poteet wrote: > I have a question. The standard IMD test is a two tone test. In a > contest type situation with multiple strong signals in close > proximity, wouldn't greater ultimate rejection by the crystal roofing > filter be a significant advantage? > > _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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In reply to this post by Bill W4ZV
In a message dated 9/18/2007 4:36:17 P.M. Eastern Daylight Time,
[hidden email] 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: [hidden email] 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 |
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[hidden email] wrote:
> 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. The K3 has two AGC loops. One is part of the DSP, so as long as the DSP does not overload, a signal outside the DSP passband but within the crystal filter passband will not cause the AGC to pump. But, if the signal is greater than, I think, S9+30 (30 dB above 50 uv), then it might overload the DSP. So in that case, the analog AGC which is outside of the DSP kicks in to prevent that, and you would notice a gain reduction on the desired signal. This is where you could use a narrower roofing filter. I ordered 2.8, 1.0, and 0.4 KHz filters. I noticed this effect with huge signals by listening with the bandwidth set to 450 Hz, which uses the 1 KHz crystal filter. Moving it to 400 Hz causes the narrower filter to be activated, which eliminates the pumping. -- 73, Vic, K2VCO Fresno CA K3 no. 00007 ('James') http://www.qsl.net/k2vco _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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In reply to this post by Bill W4ZV
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 , [hidden email] sent: >In a message dated 9/18/2007 4:36:17 P.M. Eastern Daylight Time, >[hidden email] 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 > >Craig "Buck" >k4ia >Fredericksburg, Virginia USA > > > > >************************************** See what's new at http://www.aol.com >_______________________________________________ >Elecraft mailing list >Post to: [hidden email] >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: [hidden email] 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 |
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In reply to this post by Buck - k4ia
Craig,
The AGC pumping will not happen if the undesired signal is within the spectrum of filter ultimate rejection. Any filter worthy to be called a filter (even a poor one) will have an ultimate rejection of 60 dB or greater - the K3 filters plots show ultimate attenuation in the 78 dB or greater range. As Vic pointed out in a related post, the hardware AGC only kicks in if the DSP will be overloaded, so as long as the undesired signal is outside the filter passband (by definition, that is the area of ultimate rejection for the filter) the unwanted signal will be severely attenuated and should not activate the hardware AGC. Yes, the K3 is different than the Orion. If OTOH, you are concerned about a signal somewhere on the filter slope (say 20 dB down), then you may encounter an AGC pumping problem, but that has to do with the steepness of the filter slope and is much different than the ultimate rejection that you indicated you were concerned about. 73, Don W3FPR [hidden email] wrote: > In a message dated 9/18/2007 4:36:17 P.M. Eastern Daylight Time, > [hidden email] 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: [hidden email] > 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: [hidden email] 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 |
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