Re: roofing filters - when to use them
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N9VV:
>I have never understood the obsession of the modern CW ops with *narrow* filters. I use my brain and don't seem to miss any QSOs ;-) Ken there are two issues here. You are talking about how the ear/brain processes signals in noise. Indeed there are two different ways people prefer to do that. One is to use the brain's DSP and a wide audio bandwidth. The other is to use a narrow audio bandwidth (i.e. the radio's DSP). I also prefer relatively wide audio bandwidths and use my brain's ~50 Hz filter to separate the signals. The second issue (the real one which roofing filters address) is the generation of spurious products within the receiver itself which can interfere with the desired signal. If you have a IMD mixing product zero beat with the signal you are trying to copy, your ear/brain will not be able to separate them. I'm sure you have heard these products which are produced when two adjacent strong signals mix and you hear the result as phantom bleeps and bloops in your passband. It sounds like CW tones but in some code other than Morse. This second issue is really what roofing filters address...i.e. eliminating the phantom interfering products generated in the receiver itself. This becomes a big deal especially when you have many very strong signals spaced very closely, such as in a contest or a big pileup when trying to copy a weak signal close by. 73, Bill W4ZV _______________________________________________ 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: Re: roofing filters - when to use them
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Thanks Bill. Yes, you are right. I guess my Rx location here in the
Chicago suburbs is so noisy that I rarely have strong adjacent signals ;-) I totally agree with your assessment. It is such a great luxury to be able to choose the filters we need/want in the K3. To me, it is another positive benefit of how Elecraft equipment is DESIGNED BY REAL HAM ops! TU de ken Bill Tippett wrote: > N9VV: > The second issue (the real one which > roofing filters address) _______________________________________________ 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: roofing filters - when to use them
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In reply to this post by Bill W4ZV
Bill Tippett wrote:
> Ken there are two issues here. You are > talking about how the ear/brain processes signals > in noise.... > The second issue (the real one which > roofing filters address) is the generation of > spurious products within the receiver itself > which can interfere with the desired signal. > ... Ken, There is a third, equally important issue, and this is often the dominant one during contests or in pile-ups: Very strong interfering signals within the passband of the roofing filter can de-sense the reciever (or at minimum pump the AGC) by overloading stages after the filter (I.F. amp, second mixer, A-to-D converter and its buffer, DSP, PIN diode attenuators, etc.). I can't count how many times K2 customers have told me that their "other" rig (I won't list them) was useless in the presence of strong signals, while the K2, with its narrow roofing filters, completely eliminated this effect. The K3 also has this advantage. A narrow roofing filter -- compatible with the communications bandwidth required -- will protect "downstream" stages. The K3's shift/width/hicut/locut controls automatically select and properly position the optimal roofing filter. You'll see the FL1-FL5 icons selected as you rotate these controls. 73, Wayne N6KR --- 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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> I can't count how many times K2 customers have told me that their > "other" rig (I won't list them) was useless in the presence of > strong signals, while the K2, with its narrow roofing filters, > completely eliminated this effect. The K3 also has this advantage. > > > 73, > Wayne > N6KR Wayne for reference, what is the width of the roofing filter in a K2? Are we adjusting the roofing filters when we set FL1, FL2 etc in the K2? This may shed more light on the subject for those of us who have a K2. Rich KQ9L _______________________________________________ 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: Re: roofing filters - when to use them
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Administrator
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On May 1, 2007, at 8:29 PM, Richard Lim wrote: >> >> I can't count how many times K2 customers have told me that their >> "other" rig (I won't list them) was useless in the presence of strong >> signals, while the K2, with its narrow roofing filters, completely >> eliminated this effect. The K3 also has this advantage. >> >> >> 73, >> Wayne >> N6KR > > Wayne for reference, what is the width of the roofing filter in a K2? Typically 200 - 2000 Hz (variable). You can get as low as 100 Hz, but with a lot of passband loss. > Are we adjusting the roofing filters when we set FL1, FL2 etc in the > K2? Yes. "Roofing Filter" simply refers to the first very narrow filter in the receiver -- i.e. at the first I.F. In some rigs the first I.F. may be 30-70 MHz, and that's why their roofing filters are limited to very wide bandwidths (3 kHz minimum, typically, and often as wide as 20 kHz). Rigs with such wide roofing filters sometimes have narrower filters at a second or third I.F. (or both). However, some portion of the I.F. circuitry will be exposed to the area in the roofing filter passband, which is why In the K3 the roofing filters *are* the narrow filters, since we down-convert to a low I.F. These narrow roofing filters have several advantages. 73, Wayne N6KR --- 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 |
Re: Re: roofing filters - when to use them
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In reply to this post by Rich Lim
Rich,
The analogy that crops up in my mind is that in the K2 the FL1 ... FL4 bandwidth is both the 'roofing filter' and the ultimate filter - assuming no other filtering is present. If either the KAF2 or the KDSP2 is installed and active, that filter can be the 'ultimate filter' while the FL1 ...FL4 becomes the 'roofing filter'. With the K2 and KDSP2 or KAF2, that ultimate filter is at audio frequencies, but with the K3, the ultimate filter is done at the 15 kHz IF. Yes, with the DSP and KAF2, a very narrow filter can be developed even if the IF filter is set to 2.4 KHz (SSB filter width), but when operating, a strong signal within that 2.4 KHz passband can (and will) activate the receiver AGC and de-sense the receiver. All is well if that strong signal is the desired one, but if it is an undesired one, you may find the desired signal is no longer Q5 copy because the undesired signal has reduced the gain of the receiver. That effect in the K2 is most noticeable when operating digital modes with the filter set to the full SSB filter width - when a strong signal comes on, the signal that you are trying to copy becomes weaker because the strong signal activates the receiver AGC. Switching to a more narrow filter reduces the AGC effect of the strong adjacent channel undesired signal and will allow copy of the desired signal - in this particular case, the K2 filter is the roofing filter and the digital mode software in the computer is acting as the 'ultimate filter'. The K3 is not significantly different in this respect because the analog AGC is developed after the roofing filter and before the DSP filtering. I am told from the FAQ page that the DSP does offer some additional AGC, but I do not have any details. 73, Don W3FPR Richard Lim wrote: >> >> I can't count how many times K2 customers have told me that their >> "other" rig (I won't list them) was useless in the presence of strong >> signals, while the K2, with its narrow roofing filters, completely >> eliminated this effect. The K3 also has this advantage. >> >> >> 73, >> Wayne >> N6KR > > Wayne for reference, what is the width of the roofing filter in a K2? > Are we adjusting the roofing filters when we set FL1, FL2 etc in the K2? > This may shed more light on the subject for those of us who have a K2. > > Rich > KQ9L 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 Rich Lim
> "other" rig (I won't list them) was useless
As soon as I have figured which one of my K2s has been insulted.... hi hi vy 73 de toby -- DD5FZ, 4N6FZ (ex dj7mgq, dg5mgq, dd5fz) K2 #885, K2/100 #3248 K3/100 #??? (< #200) DOK C12, BCC, DL-QRP-AG _______________________________________________ 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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