Guy, > I need the roofing filter is to get down 30 db as quickly as > possible, without giving up too much of the +/- 250 Hz.
A careful comparison of the 500 Hz 5-pole filter, 400 Hz 8-pole filter, "250 Hz" 8 pole filter and 200 Hz 5 pole filter shows some interesting things ... 1) my "500 Hz" filter is really 470 Hz at - 6dB ... this is consistent (460 - 490 Hz) with measurements reported by others. I intend to check the two other 500 Hz filters received with my second K3/KRX3 when I have some time. 2) the -30 dB bandwidth of my 500 Hz filter (780 Hz) is not significantly different than the -30 dB bandwidth of the 400 Hz filter reported on the Elecraft web site (680 Hz) when cascaded with a DSP filter at 400 or 500 Hz. I set my K3 so the "500 Hz" filter engages at 450 Hz. It is on-line when normalizing the CW filters but is off-line when selecting a composite bandwidth wider than the filter. 3) the 200 Hz five pole filter (209 Hz measured) is narrower than the 250 Hz filter down to at least -40 dB without any any benefit from cascading with DSP. For CW specifically, it's hard to justify the 400/250 Hz pair with their relatively small difference in bandwidth and their significantly greater cost compared to the 500/200 Hz pair. To be an effective alternative to the 400 Hz (450 Hz from Inrad curves, 435 Hz from Elecraft curves) filter, the 250 Hz 8-pole really needs to be held to the nominal 250 Hz bandwidth so that the difference will be useful. Even in a "worst case" (e.g. 40 meters in Europe) scenario, reducing the pre-DSP bandwidth from 450 to 370 Hz is not likely to be significant. > I have used both of these filters for years in my FT1000MP > and am completely familiar with their shape and use in a > contest. The 250 Hz filters in an FT1000MP is an entirely different situation. In the FT-1000D/FT-1000MP, there are two filters - each approximately 350 Hz wide - in cascade yielding a composite 250 Hz (at -6dB) bandwidth. 73, ... Joe, W4TV > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Guy, K2AV > Sent: Sunday, August 31, 2008 10:46 AM > To: elecraft@mailman.qth.net > Subject: Re: [Elecraft] Hearing the effect of narrower roofing filter > > > > > > ..... Maybe > I should have ordered the 200 Hz 5-pole instead, but I got it > into my head that maybe the 8-pole filters were "better" in > some way. I didn't repeat this mistake when I ordered > filters for the subreceiver. > > Dick, K6KR > > I know I will incur some religious wrath from some.... > however here is the OTHER side of that argument. It's not > that I am denying the issues that are raised to question > getting the two 8 pole CW filters (400 and 300 in actuality). > > I am solving a SINGLE happenstance which occurs over, over, > over and over again in contests, so much so that until the K3 > it was in my mind the LIMITING issue in improving 40m CW DX scores. > > I am at a contest station in eastern NC just off Pamlico > Sound. On 40m to Europe we are using a 5 element wire quad > suspended across a 220 foot NE/SW catenary between two towers. > > Broadcast signals above 7.1 routinely peg meters and light > all the lights. > So do some US stations in the NE, and so do some number of > VERY LOUD European stations (VLS). WHAT they are doing to be > so loud is not part of my exposition. They just are very loud. > > At some point in the contest, one has worked all the VLS, > loud, medium and well-antenna'd QRP signals. What remains is > a bewildering and seemingly bottomless pool of stations that > can hear us (QRO on 5 elements) and are trying to work us on > antennas with the gain of a basement floor joist wet noodle > antenna. There are hundreds and hundreds of these. We have > some number of recordings made in Europe of these, and they > are quite weak over there. > > It is guaranteed, only a matter of time, and sometimes > immediate, that a VLS will settle in the next slot above or > below. They may be as close as 350 Hz. The problem now is > working the QRP wet noodle station who is into the noise > without hardware AGC pumping or other effects from a VLS > INSIDE the roofing filter that was set to hear stations who > will call sometimes +/- 200-250 Hz. > > I need the roofing filter is to get down 30 db as quickly as > possible, without giving up too much of the +/- 250 Hz. The > 400 Hz is a good width even for VLS +/- 500 Hz. If a VLS > squeezes me on one side, I only want to give up +/- real > estate in the roofing filter on that side. The procedure is > to drop to 300 hz roofing and DSP and move the RX center 50 > hz away from TX frequency and the VLS. I give up listening so > far on the side of the VLS, but keep the real estate on the > other side. > > It is a matter of the width out to the edge and the largest > db drop per 10 hz in the skirts thereafter. The roofing > filter keeps the VLS from getting into the hardware AGC or > pushing the DSP to the extreme. > > I have used both of these filters for years in my FT1000MP > and am completely familiar with their shape and use in a contest. > > I have heard it said that the 200 5 pole filter will do > better for picking out signals in a very crowded situation, > perhaps so in an extremely tight situation. But thus far I > have been able to go narrow with the 300 8 pole and it is the > DSP handling the work there. > > At some point I would like to be able to tune the center of > the DSP CW selectivity up/down at 25 Hz rate using RIT > *WITHOUT* moving the position of the roofing filter relative > to the band. The point of the roofing filter is to reject > the VLS in the next slot above and below me. Tune the whole > thing up or down to listen to a weak station off-frequency > for whatever reason and you let one of the VLS in under the roof. > > Just the view from the other side of the river... :>) > > > 73, Guy. > > -- > View this message in context: > http://n2.nabble.com/Hearing-the-effect-of-narrower-roofing-fi lter-tp470635p795536.html Sent from the Elecraft mailing list archive at Nabble.com. _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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: Elecraft@mailman.qth.net 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
