Talking filter Bandwidths
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I have recently replaced my CW Xtal Fiter set and done a realignment using
Spectrogram. I'm now attempting to measure my selected filter bandwidths and using the -6dB points, is this incorrect ? Is -3dB now the preferred or accepted points for BW measurement specs? Why is there some apparent confusion here, any thoughts? 73, Deni K21188 F5VJC / GM3SKN _______________________________________________ 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: Talking filter Bandwidths
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Morning Deni,
The bandwidth of a filter (in whatever form) has always been taken to mean the bandwidth between the upper and lower 3db points of the filter's attenuation vs frequency response, the 0db point being at the frequency of least attenuation in the pass band. One hopes that the passband ripple is much much less than 3db !! Other bandwidths are used, but the dbs of attenuation must always be specified. If the attenuation is not specified, then the 3db bandwidth is used. The bandwidth at 6db attenuation is used together with the bandwidth at 60db attenuation to determine the filter's shape factor, a number >1. Very often the bandwidth at 80db of attenuation is used instead of that at 60db attenuation, but the dbs of attenuation must always be specified. There are good reasons for using the 6db figure, but I won't confuse the issue. 73 Geoff GM4ESD (near Cupar, Fife). ----- Original Message ----- From: "Deni" <[hidden email]> To: <[hidden email]> Sent: Saturday, November 13, 2004 7:45 AM Subject: [Elecraft] Talking filter Bandwidths I have recently replaced my CW Xtal Fiter set and done a realignment using Spectrogram. I'm now attempting to measure my selected filter bandwidths and using the -6dB points, is this incorrect ? Is -3dB now the preferred or accepted points for BW measurement specs? Why is there some apparent confusion here, any thoughts? 73, Deni K21188 F5VJC / GM3SKN _______________________________________________ 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 Deni F5VJC-2
Deni
In the past, the -6 dB number was usually paired with a -60 dB number to calculate the shape factor of the filter. It would be the -60dB bw divided by the -6 dB bw, to give a shape factor somewhere between 1 (vertical sided passband) and maybe 2 at the worst. Not sure where the -3dB comes from, maybe advertising? :-) Makes the filter bandwidths look better. 73, Bob N6Wg _______________________________________________ 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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Just found this in an odd search and thought some others might be interested
in it as well. A solar backpack, they say it provides 2.5 watts, might be an interesting way to keep a small SLA battery Charged, or maybe even the K2 on it's own as well as carry some of the other operating needs in one package Is a tad costly at the 130 dollar mark, but still a neat idea HI HI http://www.eclipsesolargear.com/Backpacks.asp 73 John N5BSD _______________________________________________ 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 n6wg
Bob and all,
The proper point is the 'halfway' point - half the power is -3 dB (10 log 0.5) but half the voltage is -6 dB (20 log 0.5). The -3dB point would be used when plotting power curves, while the -6db point is correct for voltage or current plots. Typical filters usually assume a power curve (-3 dB) points unless stated otherwise. A plot of an IF filter could be either depending on the measurement techniques. We are moving closer to consistancy with the use of dBm in stating power gain (or loss) within equipment like receivers, but I don't think everyone is on board yet. Yes, the advertizing folks sometimes take advantage of differences like this to make their product look better - called 'specsmanship' in some engineering circles. You have to read the really fine print. 73, Don W3FPR ----- Original Message ----- > > Not sure where the -3dB comes from, maybe advertising? :-) Makes > the filter bandwidths look better. > _______________________________________________ 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 John [N5BSD]
I guess backpacking has changed some over the years ... now one carries
a cell phone, PDA, CD player, MP3 player, GPS, and a laptop? Fred K6DGW Auburn CA CM98lw "John ( N5BSD )" wrote: > > Just found this in an odd search and thought some others might be interested > in it as well. > A solar backpack, they say it provides 2.5 watts, might be an interesting > way to keep a small SLA battery > Charged, or maybe even the K2 on it's own as well as carry some of the other > operating needs in one package > > Is a tad costly at the 130 dollar mark, but still a neat idea HI HI > > http://www.eclipsesolargear.com/Backpacks.asp > > 73 > John > N5BSD > > _______________________________________________ > 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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Well you know they say this is the connected generation HI HI
I am thinking that is aimed at kids and young 'adults' around town But would work pretty decent I think to run a small rig and wouldn't be to big Might be wrong will have to see on and gets my paws on it for a bit to know 73 John N5BSD -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Fred Jensen Sent: Saturday, November 13, 2004 3:05 PM To: Elecraft Reflector Subject: Re: [Elecraft] Neat Backpack idea I guess backpacking has changed some over the years ... now one carries a cell phone, PDA, CD player, MP3 player, GPS, and a laptop? Fred K6DGW Auburn CA CM98lw _______________________________________________ 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 Don Wilhelm-3
For a lot of us (I have seen this discussed before) the 3 db points
are fairly useless as a real predictor of performance. The 6 and 60 db points do a much better job, telling where the skirts start, and where they are diving. I will never forget my disappointment in the Collins 500 kHz CW mechanical filter (3 db) for my MP. I just assumed..... Best thing is to get a sweep of the passband. Funny how those are so hard to get for filters unless you're getting them from INRAD's webpage. They have the Collins curves as well. You can compare curves and see yourself. Interesting browsing. You can see from the curves whether the skirts are still going strong and diving past 60 db or whether they're dying and on the way to having a poor ultimate rejection. 73, Guy ----- Original Message ----- From: "W3FPR - Don Wilhelm" <[hidden email]> To: "Bob Tellefsen" <[hidden email]>; "Deni" <[hidden email]>; <[hidden email]> Sent: Saturday, November 13, 2004 1:18 PM Subject: Re: [Elecraft] Talking filter Bandwidths > Bob and all, > > The proper point is the 'halfway' point - half the power is -3 dB > (10 log 0.5) but half the voltage is -6 dB (20 log 0.5). _______________________________________________ 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: Talking filter Bandwidths
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Agree that the 60db/6db shape factor is an essential indicator of a filter's
"selectivity performance". But the filter's rsponse above the 6db points is also very important if you want a good sounding SSB signal, or a Rx's SSB filter that can really dig in a pile-up. If the filter's skirts come up from the stopband, through the 60db and 6db points and continue to 0db at the same rate of attenuation vs frequency, the SSB signal will usually sound "raspy". To avoid this, the "top" of a good SSB filter's response between 0db and 6db (or thereabouts) is designed to be a little rounded, approaching Gaussian, while still meeting the 3db bandwidth required by the signal which is to be passed through. A signal through a 14 crystal SSB filter without the rounded top does not sound good, even with a 3db bandwidth of 2.7kHz. The same applies to Rx CW filters. Very narrow bandwith filters with rounded tops do not "ring" as badly as those with sharp edged tops. Totally agree with you about sweeping filters to obtain their responses. It can be intersting when the stopband is down 90 - 100db, because at no point must the filter be overdriven. These are hombrew filters. I wish that filter manufacturers would specify their filters signal level limits, because too often receiver designers forget that crystal filters especially can be overdriven and introduce distortion. But perhaps if the manufacturers did, some might not sell any filters! 73 Geoff GM4ESD ----- Original Message ----- From: "Guy Olinger, K2AV" <[hidden email]> To: "Deni" <[hidden email]>; <[hidden email]> Sent: Sunday, November 14, 2004 4:53 AM Subject: Re: [Elecraft] Talking filter Bandwidths > For a lot of us (I have seen this discussed before) the 3 db points > are fairly useless as a real predictor of performance. The 6 and 60 db > points do a much better job, telling where the skirts start, and where > they are diving. > > I will never forget my disappointment in the Collins 500 kHz CW > mechanical filter (3 db) for my MP. I just assumed..... > > Best thing is to get a sweep of the passband. Funny how those are so > hard to get for filters unless you're getting them from INRAD's > webpage. They have the Collins curves as well. You can compare curves > and see yourself. Interesting browsing. > > You can see from the curves whether the skirts are still going strong > and diving past 60 db or whether they're dying and on the way to > having a poor ultimate rejection. > > 73, Guy > _______________________________________________ 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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Most of the issues you address are seen in the sweep. The problem that
all the content and implications of the sweep can't be put in a 42 or 45 character item name in a catalog. Where there is a picture in a catalog, the sweep curve would be more meaningful than a picture of the filter itself. But I'm not holding my breath. But IMHO the number one function of a filter is to pass the desired signal and reject all off frequency, nothing in second place. That would make the 6db point the better characterization of the filter, more representative of the beginning of the stop band, IF one MUST be stuck with a single number. And that, precisely, was what was so disappointing about the Collins "500 Hz" mechanical filter, that it was a 610/1630 Hz filter 6/60 when I got around to measuring it (INRAD says 595/1560 on their web page). A very broad 2.6 shape. For CW contest operation, a station up or down the typical 500 Hz, is only reduced by 36 db, as opposed to ~65 db using INRAD's 500 Hz (6 dB) filter. -36 dB not much help copying an S0 or S1 signal 500 Hz away from a 20 over 9 signal. I personally would like to see something like BW 595/2.6 in listings, particularly next to an alternative listed as 510/1.74. One presumes (more than hopefully?) engineering and manufacturing to current standards, that is pass band loss and impedance to specs, and reasonable ripple, and pass band shape for frequency response and delay appropriate to use. In any event, the few dozen characters of a filter description could never address the engineering quality issues you so rightly list. If they did, how would it read? Cat #1234, 455 center 500 Hz bw, poor quality, cheep Cat #3456, 455 center 500 Hz bw, better quality, best buy. Cat #5678, 455 center 500 Hz bw, steep skirts, transparent passband, Rolls Royce, only the rich need apply. Maybe this is one of those food labeling law issues, that all ads have to include certain numbers and you can get slammed for telling lies. As to overdriving, for many rigs that is a roofing filter issue due to their choice of IF's, AND one of the many reasons why the Elecraft K2 design works so well. 73 > Totally agree with you about sweeping filters to obtain their > responses. It > can be intersting when the stopband is down 90 - 100db, because at > no point > must the filter be overdriven. These are hombrew filters. I wish > that filter > manufacturers would specify their filters signal level limits, > because too > often receiver designers forget that crystal filters especially can > be > overdriven and introduce distortion. But perhaps if the > manufacturers did, > some might not sell any filters! > > 73 Geoff GM4ESD _______________________________________________ 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: Talking filter Bandwidths
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Hello again Guy,
Fair comment. As far as the "commercial" issues are concerned I could not agree more. I would add to the wish list the supply of real impedance information especially around the bandpass edge frequencies where the filter's port impedances will gyrate like mad, and not just the nominal impedance. But as you say, how would a few dozen characters in a calalog pass on this information. So the designers had to get samples, at least we did, but we Amateurs can't - usually. I want to go off list to comment on your other points so as not to clutter up the reflector. But one last point before doing so. It is a great pity that the myth exists, IMHO, that double conversion receivers with a high first IF are second runners to single conversion low IF receivers. Perhaps many are because their first IF filters are so wide, and overload can and does occur further down the line. Small bandwidth VHF crystals filters do exist, I have some here, but I suspect that wideband VHF monolithic crystal filters are cheaper for the manufacturers of transceivers etc. However there is no doubt that a properly designed high first IF receiver is far less prone to spurious responses than its low IF cousin, and can have very good strong signal performance. HERESY ! I'll see you off list. 73, Geoff GM4ESD. ----- Original Message ----- From: "Guy Olinger, K2AV" <[hidden email]> To: "Geoffrey Mackenzie-Kennedy" <[hidden email]>; "Elecraft Discussion List" <[hidden email]> Sent: Sunday, November 14, 2004 5:02 PM Subject: Re: [Elecraft] Talking filter Bandwidths > Most of the issues you address are seen in the sweep. The problem that > all the content and implications of the sweep can't be put in a 42 or > 45 character item name in a catalog. Where there is a picture in a > catalog, the sweep curve would be more meaningful than a picture of > the filter itself. But I'm not holding my breath. > > > > I personally would like to see something like BW 595/2.6 in listings, > particularly next to an alternative listed as 510/1.74. > > In any event, the few dozen characters of a > filter description could never address the engineering quality issues > you so rightly list. If they did, how would it read. > > Maybe this is one of those food labeling law issues, that all ads have > to include certain numbers and you can get slammed for telling lies. > > As to overdriving, for many rigs that is a roofing filter issue due to > their choice of IF's, AND one of the many reasons why the Elecraft K2 > design works so well. > > _______________________________________________ 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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