[K3] Filter Cascade Effect
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Thanks to Joe W4TV for recently mentioning the effect that cascading both the DSP and roofing filters can have when the nominal WIDTH setting is near the roofing filter BW. The following is part of a private message I recently sent which may be of interest to others:
I chose my CW filters mainly based on bandwidth. I have 5-poles, which do have the offset issue, but I prefer their BWs. The published specs are: 500 - 565 (mine is 549) 400 - 435 250 - 370 200 - 224 (mine is 203) Another factor I didn't think about until recently is how the cascading effect of the DSP filter can significantly narrow the above. With my 500 and DSP set to 600, the cascaded BW is actually 426 Hz. My 200 with DSP set to 200 is actually 157 Hz. I feel the 500 combination is a little narrow for contest running and it may explain why I'm sometimes surprised to find stations calling me off-frequency that I didn't hear the first time (BTW this was sometimes true with Orion also). I really wish there were a filter around 700 Hz actual BW that would give me 600 cascaded. The 1k is just a little too wide for a heavy duty contest when folks are spaced 500 Hz or closer. Maybe the variable filters will be the answer if they don't have IMD issues due to using varactors. My point is to realize that the cascaded BWs may be 20-25% less than what you think the maximum BWs are from their specs. 73, Bill W4ZV |
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On Fri, 11 Jul 2008 06:08:03 -0700 (PDT), Bill W4ZV wrote:
>My point is to realize that the cascaded BWs may be 20-25% less than what >you think the maximum BWs are from their specs. Right on! BUT -- one good way to deal with this when you already own the narrower filter is to tell the K3 that its bandwidth is wider than it is. For example, I have tell the K3 my 400 Hz 8 pole filter is 500 Hz, and the 1.8 kHz filter is 2.2 kHz. I find that I also want to shift the 1.8 kHz filter down about 150 Hz. Another point -- some have noted that at least some of the roofing filters are actually wider than their nominal value. 73, Jim Brown K9YC _______________________________________________ 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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> -----Original Message-----
> From: [hidden email] > [mailto:[hidden email]] On Behalf Of Jim Brown > > Right on! BUT -- one good way to deal with this when you > already own the narrower filter is to tell the K3 that its > bandwidth is wider than it is. For example, I have tell the > K3 my 400 Hz 8 pole filter is 500 Hz, and the 1.8 kHz filter > is 2.2 kHz. I find that I also want to shift the 1.8 kHz > filter down about 150 Hz. Maybe the K3 software should adjust its DSP bandwidth so that you get a smooth variation of the cascaded bandwidth as you turn the knob. When the narrower filter engages, widen the DSP a bit. There's no way to make it perfect, but it has to be better than having the user fudge the filter bandwidths. 73, Carl WS7L _______________________________________________ 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 Jim Brown-10
> Right on! BUT -- one good way to deal with this when you
> already own the narrower filter is to tell the K3 that its > bandwidth is wider than it is. For example, I have tell the > K3 my 400 Hz 8 pole filter is 500 Hz, and the 1.8 kHz filter > is 2.2 kHz. I find that I also want to shift the 1.8 kHz > filter down about 150 Hz. I don't think this is a "good way" at all. Doing this destroys the smooth consistently incrementing operation of the WIDTH control. Your actual bandwidth goes from 600 to 550 to 400, then stays there until the DSP catches up at 400. If anything, I'd go the other direction so that the crystal filter doesn't engage until the DSP is inside it at a narrower bandwidth. However, another reason some people do what you suggest is to modify the passband shape since they prefer the rounder passband shape of the crystal filter ("softer sound", they say). I still think that is a negligible benefit against creating the WIDTH control anomaly. I agree with Bill that I'd much rather have my roofing filters just outside the DSP bandwidths I commonly use. That's why I think it is prudent for K3 owners to forgo additional crystal filters until they've operated the K3 and found the bandwidths most important to them. Then, if they really need nearby strong signal protection for the DSP, they can select wider crystal filters for their favorite DSP bandwidths. > Another point -- some have noted that at least some of the > roofing filters are actually wider than their nominal value. By design, INRAD makes the actual -6 dB bandwidth greater than the filter name. See the actual analyzer plots on the Elecraft web site. 73, Ed - W0YK _______________________________________________ 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 Jim Brown-10
Been there and done that... DSP Measured Cascaded (with 549 Hz XFIL) 500 482 432 600 522 426 (my usual setting...real but anomalous result) 700 608 457 800 700 492 1000 890 526 As Ed W0YK said, you can get away with a *little* of this but I can assure you there are very serious consequences if you tried to use DSP 1000 above to stretch the 549 Hz XFIL to it's full BW (and even then you only get to 526). I don't want to elaborate but speak from personal experience! The real solution IMHO is about a 750 Hz XFIL which will net about 600 Hz when cascaded with the DSP. I had a 1k and got rid of it because it's simply too wide for serious contest use where it will typically span 200-1200 Hz at normal PITCH settings. For example, if you have 450 Hz PITCH set, that S9+30 guy 500 Hz up (at 950 Hz) will come roaring through your 1k XFIL and de-sense you. As I said before, maybe the variable filters will work...or maybe Inrad will do a 750 Hz filter. 73, Bill |
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In reply to this post by Ed Muns, W0YK
> > Another point -- some have noted that at least some of the > > roofing filters are actually wider than their nominal value. > > By design, INRAD makes the actual -6 dB bandwidth greater > than the filter name. See the actual analyzer plots on the > Elecraft web site. This is a throwback to the practice by Kenwood and Yaesu in the first transceivers with cascaded (8 MHz/455 KHz) filters. The filters were specified based on the effective bandwidth of the cascaded pair. For example, the "Standard" Yaesu "2.4 KHz" filters were actually 2.6 KHz wide (note the original model numbers were XF-ffff-262-xx) - the cascaded pair resulted in an effective bandwidth of 2.4KHz. The same effect can be observed with Inrad's filters. Looking at the 250 Hz pair for the TS-940, the 8.83 MHz filter is 367 Hz wide at -6dB but it is down about 2dB +/- 125 Hz from the center. when this is combined with a 455 KHz filter that is 265 Hz wide at -6db and 250 Hz wide at -4 dB, the effective bandwidth is slightly less than 250 Hz. 73, ... Joe, W4TV _______________________________________________ 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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