K3: KRX3 question
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Bill
I can't recall seeing that figure published before; where would I look for that figure? David G3UNA > snip > The purpose of the roofing filter is mainly to reduce strong (approximately > S9+30) signals from over-driving the ADC in the DSP. If they do that, then > the DSP determines the final selectivity over its ~100 dB dynamic range. > > 73, Bill > -- ----------------------------------------- Email sent from www.virginmedia.com/email Virus-checked using McAfee(R) Software and scanned for spam _______________________________________________ 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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David I'm not sure which figure you meant, so I'll try to cover all bases: 1. S9+30 limit for over-driving ADC. This can be deduced by the setpoint of the Hardware AGC Mod quoted from Elecraft's mod page below: "This modification raises the hardware AGC threshold from about S9+5 to about S9+25 to +30. The new threshold still protects the DSP's analog-to-digital converter, while providing better receive performance when wider crystal filters are used." 2. 100 dB dynamic range of DSP. I'm not going to search for the exact source but this is often quoted by manufacturers as the practical limit of current DSPs using 24-bit ADCs. Theoretically it should be higher based only on 24-bit ADC resolution but practically it's limited by other factors. All hybird heterodyne/DSP rigs such as Orion, K3, FT-2000/9000 and IC-7700/7800 have about the same limit. I suppose if we get 32-bit ADCs this limit could increase. 3. DSP Shape Factor formula from KK7P: http://www.mail-archive.com/elecraft@mailman.qth.net/msg53271.html Hopefully I covered the figure you were questioning. 73, Bill |
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In reply to this post by Tom Whiteside
Thanks Bill, it was the first 2 that were of interest.
David G3UNA > > > > > > David Cutter wrote: > > > > Bill > > > > I can't recall seeing that figure published before; where would I look for > > that figure? > > > > David > > G3UNA > >> snip > > > > > >> The purpose of the roofing filter is mainly to reduce strong > >> (approximately > >> S9+30) signals from over-driving the ADC in the DSP. If they do that, > >> then > >> the DSP determines the final selectivity over its ~100 dB dynamic range. > >> > >> 73, Bill > > > > > > David I'm not sure which figure you meant, so I'll try to cover all bases: > > 1. S9+30 limit for over-driving ADC. This can be deduced by the setpoint > of the Hardware AGC Mod quoted from Elecraft's mod page below: > > "This modification raises the hardware AGC threshold from about S9+5 to > about S9+25 to +30. The new threshold still protects the DSP's > analog-to-digital converter, while providing better receive performance when > wider crystal filters are used." > > 2. 100 dB dynamic range of DSP. I'm not going to search for the exact > source but this is often quoted by manufacturers as the practical limit of > current DSPs using 24-bit ADCs. Theoretically it should be higher based > only on 24-bit ADC resolution but practically it's limited by other factors. > All hybird heterodyne/DSP rigs such as Orion, K3, FT-2000/9000 and > IC-7700/7800 have about the same limit. I suppose if we get 32-bit ADCs > this limit could increase. > > 3. DSP Shape Factor formula from KK7P: > > http://www.mail-archive.com/elecraft@.../msg53271.html > > Hopefully I covered the figure you were questioning. > > 73, Bill > > > ----------------------------------------- Email sent from www.virginmedia.com/email Virus-checked using McAfee(R) Software and scanned for spam _______________________________________________ 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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> 2. 100 dB dynamic range of DSP. I'm not going to search for the exact
> source but this is often quoted by manufacturers as the practical limit of > current DSPs using 24-bit ADCs. Theoretically it should be higher based > only on 24-bit ADC resolution but practically it's limited by other factors. > All hybird heterodyne/DSP rigs such as Orion, K3, FT-2000/9000 and > IC-7700/7800 have about the same limit. I suppose if we get 32-bit ADCs > this limit could increase. We get about 6 dB per bit of ADC resolution in a perfect ADC. So a 24-bit ADC should give us 144 dB of dynamic range, 144 dB S/N, etc. In practice, we have non-linearity, jitter and other forms of noise that reduce the obtainable accuracy. Pro-audio ADCs work the best. At the time the K3 was designed, the best ADCs provided about 103 to 118 dB of S/N under laboratory conditions. The device we chose is rated at 112 dB. To achieve this requires careful design of all the circuitry that interfaces the ADC to the analog signals. Some ADCs now get S/N in the mid 120-dB range. Use of such devices entails significant additional cost to the radio, but with *very* little benefit. Why? Other important ADC characteristics -- distortion, dynamic range -- limit even these to something in the range of 102 to 108 dB. This is exactly the same as the ADC we use in the K3. Note that these limits are of the ADC itself, not limits of the radio passed on to the ADC. Thanks to its modular construction, the K3 is designed to be able to take advantage of advances in ADC technology. 73, Lyle KK7P _______________________________________________ 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
Bill,
I find the diversity option interesting and I imagine that it will be a very useful feature on topband. In what other situations do you find it useful? I have never tried diversity myself, what would the difference be between a filter matched to the specs provided Elecraft and a set of un-matched filters? 73 de Björn /SM0MDG _______________________________________________ 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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With regard to diversity reception on bands other than 160m --
When 6-meter sporadic-E is very intense and present over a fairly large area between two stations, a phenomenon occurs where the signal will come in alternately at a higher angle and at a lower angle, fading back and forth, on average, every second or two. I've confirmed this by switching back and forth between my high/low yagi with the StackMatch while receiving such a signal. I look forward to that "someday" when I acquire a KRX3 and can try diversity reception on 6 meters. The configuration would be my vertically-stacked 6m yagis each feeding a separate receiver directly. (And of course driven through the StackMatch on transmit, as they are now.) The exact physical model of sporadic-E propagation that is causing this phenomenon is open to some debate, but it is clearly happening -- when the Es is active and complex enough to support multiple-angle paths. Using diversity reception from a two-stack of identical yagis sounds very intriguing, and would certainly work just as well as feedline-combining even when this Es phenomenon isn't occurring strongly. Bill W5VWO DM65 ----- Original Message ----- From: "Björn Mohr" <[hidden email]> To: "Elecraft Reflector" <[hidden email]> Sent: Monday, July 07, 2008 9:10 AM Subject: Re: [Elecraft] K3: KRX3 question > Bill, > > I find the diversity option interesting and I imagine that it will be a very > useful feature on topband. In what other situations do you find it useful? > > I have never tried diversity myself, what would the difference be between a > filter matched to the specs provided Elecraft and a set of un-matched > filters? > > > 73 de Björn /SM0MDG > > > _______________________________________________ > 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 Bjorn
Orion had "quasi" diversity which I've used but it had 2 major problems: 1. There was a constand 2-6 Hz beat between the two signals (which was constant in a given rig but varied from unit to unit) which was very annoying. In true diversity the two RXs are supposed to be phase-locked to exactly the same frequency. 2. Orion's Sub-RX had terrible RF performance, rendering it useless in any sort of large signal environment like contests, large pileups, etc. I've actually never used *true* diversity but am looking forward to it in the K3. The uses I see are: 1. Improvement in weak signal DX copy by feeding two different antennas to the two RXs. 2. Contests where you might want to listen in multiple directions (either azimuth or elevation) simultaneously (i.e. hear signals calling from 2 different geographic directions, copy signals with both high and low angle antennas, or copy signals with both horizontal and vertical polarization antennas). What would be the effect of unmatched filters? You would have a beat frequency, such as I mentioned with Orion above, which would be the difference in the two offsets. Even with identical offsets, you could also have interesting effects if you used different bandwidth filters. This might not necessarily be bad since you could use both a wide bandwidth and a narrow bandwidth simultaneously (somewhat like DUAL PB presently in the K3 but with much more flexibility). It will be fun to experiment with some of these things! 73, Bill |
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Bill
I don't follow how filter offsets degrade oscillator tracking, ie to make a beat frequency. Sorry if I'm being thick. David G3UNA What would be the effect of unmatched filters? You would have a beat frequency, such as I mentioned with Orion above, which would be the difference in the two offsets. Even with identical offsets, you could also have interesting effects if you used different bandwidth filters. This might not necessarily be bad since you could use both a wide bandwidth and a narrow bandwidth simultaneously (somewhat like DUAL PB presently in the K3 but with much more flexibility). It will be fun to experiment with some of these things! 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 |
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With two synthesizers, our software must calculate the proper PLL
divider and reference frequencies for each one. When setting the sub and main to the same RX frequency, if the filter offsets set in the menu are different this can cause small rounding errors in the synthesizer frequency calculation math to place them a couple of Hz apart. (But -not- by the difference in offsets between the filters. We calculate most of that out.) 73, Eric WA6HHQ --- David Cutter wrote: > Bill > > I don't follow how filter offsets degrade oscillator tracking, ie to > make a beat frequency. Sorry if I'm being thick. > > David > G3UNA > > > > What would be the effect of unmatched filters? You would have a beat > frequency, such as I mentioned with Orion above, which would be the > difference in the two offsets. > > Even with identical offsets, you could also have interesting effects > if you > used different bandwidth filters. This might not necessarily be bad > since > you could use both a wide bandwidth and a narrow bandwidth simultaneously > (somewhat like DUAL PB presently in the K3 but with much more > flexibility). > > It will be fun to experiment with some of these things! > > 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 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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Thanks for the input, this is really interesting!
On 2008-07-07 19.23, "Eric Swartz - WA6HHQ, Elecraft" <[hidden email]> wrote: > With two synthesizers, our software must calculate the proper PLL > divider and reference frequencies for each one. When setting the sub and > main to the same RX frequency, if the filter offsets set in the menu are > different this can cause small rounding errors in the synthesizer > frequency calculation math to place them a couple of Hz apart. So basically I could set the compensation value exactly the same for both filters and accept the difference in passband frequency response... whatever effect that will produce. I agree will Bill W4ZV, it will be interesting to play around with diversity. I can already visualize another coax from a second RX antenna entering the building. I though I had that figured out last season with the new antenna switch... the game never ends :) 73 de Björn /SM0MDG _______________________________________________ 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
Bill, > David you may be suggesting that "in theory" but I would pay > *close attention* to what W0YK says. Ed wins many RTTY contests > including several current world records from P49X, and results > sometimes speak louder to me than theory (and I'm an engineer > if that means anything). Ed and I had an extensive off-line discussion concerning the K3 filter options and RTTY. We both made multiple measurements of the composite bandwidth (roofing filter, DSP and dual passband filter) with several settings. The consensus is that the 200 Hz DSP settings work as long as the roofing filter is more than 250 Hz wide. > The 200 Hz crystal filter was measured by Elecraft to be 224 Hz > wide with a shape factor of 4.0, so whether you use a 400, 250, > or 200 Hz roofing filter, that has little bearing on the final > BW when WIDTH is set to 200 as Ed suggested (since all XFILs > are wider than the DSP's BW). This is not the case based on careful measurements. The problem is 'skirt sharpening' when multiple filters are cascaded. Where a single filter might have a 6dB bandwidth of 225 Hz ("200 Hz" roofing fitter), when combined with the 200 Hz DSP filter the effective -6dB bandwidth of the 200 Hz DSP filter and the 200 Hz roofing filter in cascade is 150 Hz! Much of the narrowing is due to the very "humped" (Gaussian) nature of the narrow crystal filters but the best responses for RTTY seem to be with a roofing filter wider than 300 Hz (the "250" Hz/8-pole) and/or keeping the DSP filter wider than 250 Hz with or without the dual passband filter. Note: Ed uses the "250 Hz" filter with his 200 Hz DSP setting. 73, ... Joe, W4TV > -----Original Message----- > From: [hidden email] > [mailto:[hidden email]] On Behalf Of Bill W4ZV > Sent: Monday, July 07, 2008 7:05 AM > To: [hidden email] > Subject: Re: [Elecraft] K3: KRX3 question > > > > > > David Woolley (E.L) wrote: > > > > Ed Muns wrote: > > > >> What DSP bandwidths were you using and where did you have your > >> crystal filters engaging? There is only a 65 Hz bandwidth > difference > >> between the "400" and "250" crystal filters and both are > wider than > >> the what is needed for 170 Hz shift RTTY. The crystal filter > >> function is to protect the DSP > > > > For clarity, 170Hz is less than the minimum required > bandwidth for RTTY. > > That needs to be extended by some multiple (> 1.0) of the > baud rate, > > to avoid excessive truncation of significant sidebands. I > would suggest > > that a filter that was significantly down at 250 Hz would > be introducing > > significant distortion. > > > > David you may be suggesting that "in theory" but I would pay *close > attention* to what W0YK says. Ed wins many RTTY contests > including several > current world records from P49X, and results sometimes speak > louder to me > than theory (and I'm an engineer if that means anything). > > http://www.radio-sport.net/wpxrtty08_post1.htm > > FYI, the DSP filter in the K3 follows the following formula > for shape factor > according to Lyle KK7P: > > Shape Factor = (6 dB BW) / (6 dB BW + 300 Hz) > > At 200 Hz BW, the DSP shape factor computes to 2.5. The 200 > Hz crystal > filter was measured by Elecraft to be 224 Hz wide with a > shape factor of > 4.0, so whether you use a 400, 250, or 200 Hz roofing filter, that has > little bearing on the final BW when WIDTH is set to 200 as Ed > suggested > (since all XFILs are wider than the DSP's BW). > > Many folks seem to have problems confusing the role of > roofing filters for > determining the final bandwidth in the K3. That is simply > not the case. > The purpose of the roofing filter is mainly to reduce strong > (approximately > S9+30) signals from over-driving the ADC in the DSP. If they > do that, then > the DSP determines the final selectivity over its ~100 dB > dynamic range. > > 73, Bill > -- > View this message in context: > http://www.nabble.com/K3%3A-KRX3-question-tp18280281p18314270.html > Sent from the Elecraft mailing list archive at Nabble.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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That's interesting since there isn't a 250 Hz roofing filter. There's a 370 Hz and a 224 Hz, but no 250 Hz. What did you use for the 250 measurement above or is that a calculated result? At any rate, I'll defer to Ed since he's forgotten more about RTTY than I'll ever know, as his contest results demonstrate. ;-) 73, Bill |
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Being tha t it was mentioned that best responses seem to be with a
roofing filter wider than 300Hz and then he mentioned the "250Hz" filter wouldn't you deduce that he meant the 370Hz filter which Elecraft calls 250Hz? I mean maybe I'm jumping to conclusions here but that was pretty clear to me. On Mon, 2008-07-07 at 15:27 -0700, Bill W4ZV wrote: > > > Joe Subich, W4TV-3 wrote: > > > > > > Bill, > > > >> David you may be suggesting that "in theory" but I would pay > >> *close attention* to what W0YK says. Ed wins many RTTY contests > >> including several current world records from P49X, and results > >> sometimes speak louder to me than theory (and I'm an engineer > >> if that means anything). > > > > Ed and I had an extensive off-line discussion concerning the K3 > > filter options and RTTY. We both made multiple measurements of > > the composite bandwidth (roofing filter, DSP and dual passband > > filter) with several settings. The consensus is that the 200 Hz > > DSP settings work as long as the roofing filter is more than 250 > > Hz wide. > > > > That's interesting since there isn't a 250 Hz roofing filter. There's a 370 > Hz and a 224 Hz, but no 250 Hz. What did you use for the 250 measurement > above or is that a calculated result? At any rate, I'll defer to Ed since > he's forgotten more about RTTY than I'll ever know, as his contest results > demonstrate. ;-) > > 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 |
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In reply to this post by Bill W4ZV
KFL3A-250 K3 250 Hz, 8 pole filter
----- Original Message ----- From: "Bill W4ZV" <[hidden email]> > That's interesting since there isn't a 250 Hz roofing filter. There's a > 370 > Hz and a 224 Hz, but no 250 Hz. What did you use for the 250 measurement > above or is that a calculated result? At any rate, I'll defer to Ed since > he's forgotten more about RTTY than I'll ever know, as his contest results > demonstrate. ;-) _______________________________________________ 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
> That's interesting since there isn't a 250 Hz roofing filter. > There's a 370 Hz and a 224 Hz, but no 250 Hz. What did you > use for the 250 measurement above or is that a calculated > result? "250 Hz" is the label (nominal) value for the 8-pole filter. Still the 250 Hz recommendation was based on looking at the slope of filter skirts and attenuation near the edges of the passband of the 200 Hz fitter. There was also some discussion about building some custom 250 or 300 Hz 5 pole filters for testing. Page 9 of the K3 schematic package shows a "250 Hz" design that would probably be 290 Hz wide based on the performance of the 200 Hz filters. 73, ... Joe, W4TV > -----Original Message----- > From: [hidden email] > [mailto:[hidden email]] On Behalf Of Bill W4ZV > Sent: Monday, July 07, 2008 6:28 PM > To: [hidden email] > Subject: RE: [Elecraft] K3: KRX3 question > > > > > > Joe Subich, W4TV-3 wrote: > > > > > > Bill, > > > >> David you may be suggesting that "in theory" but I would pay > >> *close attention* to what W0YK says. Ed wins many RTTY contests > >> including several current world records from P49X, and results > >> sometimes speak louder to me than theory (and I'm an engineer > >> if that means anything). > > > > Ed and I had an extensive off-line discussion concerning the K3 > > filter options and RTTY. We both made multiple measurements of > > the composite bandwidth (roofing filter, DSP and dual passband > > filter) with several settings. The consensus is that the 200 Hz > > DSP settings work as long as the roofing filter is more than 250 > > Hz wide. > > > > That's interesting since there isn't a 250 Hz roofing filter. > There's a 370 > Hz and a 224 Hz, but no 250 Hz. What did you use for the 250 > measurement > above or is that a calculated result? At any rate, I'll > defer to Ed since > he's forgotten more about RTTY than I'll ever know, as his > contest results > demonstrate. ;-) > > 73, Bill > > > -- > View this message in context: > http://www.nabble.com/K3%3A-KRX3-question-tp18280281p18328080.html > Sent from the Elecraft mailing list archive at Nabble.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 Brett Howard
That's a good assumption but not what he said. In other areas of his message, Joe was careful to say "250" (meaning Elecraft's nomenclature) or 370, but the what he actually wrote is: "The consensus is that the 200 Hz DSP settings work as long as the roofing filter is more than 250 Hz wide." I suspect much of this is academic anyway. If you have an extremely strong signal 250 Hz from your center frequency, NO filtering is going to solve issues like the other guy's TX noise, although a narrower XFIL may sometimes help with desense. At least on CW that's the advantage I see with the 200, but also remember that we also have a 50 Hz DSP in our ears which helps on CW (but not on RTTY). 73, Bill |
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Direct quote from the message:
>Much of the narrowing is due to the very "humped" (Gaussian) >nature of the narrow crystal filters but the best responses >for RTTY seem to be with a roofing filter wider than 300 Hz >(the "250" Hz/8-pole) and/or keeping the DSP filter wider than >250 Hz with or without the dual passband filter. Note: Ed uses >the "250 Hz" filter with his 200 Hz DSP setting. He says that the roofing filter needs to be wider than 300 and he thus used the 250Hz filter with a 200Hz DSP filter setting. Its simply put that the roofing filter needs to be wider than 300 and he therefore uses the 8-pole 250 Hz filter which it seems clear you know is 370Hz and therefore fits the build of what is required. Then behind that he has a 200Hz filter at the DSP IF. On Mon, 2008-07-07 at 16:35 -0700, Bill W4ZV wrote: > > > Brett Howard wrote: > > > > Being tha t it was mentioned that best responses seem to be with a > > roofing filter wider than 300Hz and then he mentioned the "250Hz" filter > > wouldn't you deduce that he meant the 370Hz filter which Elecraft calls > > 250Hz? I mean maybe I'm jumping to conclusions here but that was pretty > > clear to me. > > > > That's a good assumption but not what he said. In other areas of his > message, Joe was careful to say "250" (meaning Elecraft's nomenclature) or > 370, but the what he actually wrote is: > > "The consensus is that the 200 Hz DSP settings work as long as the roofing > filter is more than 250 Hz wide." > > I suspect much of this is academic anyway. If you have an extremely strong > signal 250 Hz from your center frequency, NO filtering is going to solve > issues like the other guy's TX noise, although a narrower XFIL may sometimes > help with desense. At least on CW that's the advantage I see with the 200, > but also remember that we also have a 50 Hz DSP in our ears which helps on > CW (but not on RTTY). > > 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 |
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In reply to this post by Joe Subich, W4TV-3
I decided to make my own measurements using an XG1 at 50 uV and the internal dBV meter in the K3 (for 0.1 dB and 1 Hz resolution). These could have some errors due to not being swept narrowband measurements, but they should be consistent which is sufficient for comparison purposes: 1. My "200 XFIL" 6 dB BW is actually 203 Hz (measured by disabling all other XFILs and setting DSP = 500 so there is no cascading effect). 2. My "200 DSP" 6 dB BW measures 187 Hz (measured by enabling only the 500 XFIL so there is no cascading effect). 3. My "250 DSP" measures 210 Hz 4. My "300 DSP" measures 272 Hz. 5. 1 and 2 cascaded measure 157 Hz (i.e. close to the 150 Hz Joe calculated). 6. 1 and 3 cascaded measure 175 Hz. 7. 1 and 4 cascaded measure 190 Hz. Ed W0YK said: "What can make a difference in large RTTY pileups is running the DSP down as low as 200 Hz which rolls off the outer edges of the tones, but eliminates enough of the pileup to sometimes be a net advantage." Ed reported this using DSP = 200 and XFIL = 370 (his "250"). The BW of this combination should effectively be the 187 Hz of the 200 DSP alone since the 370 Hz filter should contribute little attenuation in its center. Using Lyle's formula for DSP shape factor, that's ~487/187 or 2.6. If I select 1 and 4 above, that cascaded combination is 190 Hz BW with a wider shape factor than what Ed used, since it's mainly the XFIL in effect (Elecraft says SF = 4.0 for the 200). I would expect I could copy the same RTTY signal Ed does using the same bandwidth and an even wider shape factor, and it might actually be better for two reasons: 1. It should be to tune because it's shape factor is wider (4.0 for my XFIL alone versus 2.6 for Ed's 200 DSP alone). 2. It has better close-in rejection between the 190 BW edges and the point where the 200 XFIL crosses the 370 XFIL's skirts (which must be well over 400 Hz BW). Incidentally, if my XFIL were 224 Hz as Elecraft measured, all of the above BWs would be about 20 Hz wider (but I'm happy mine is 203 instead of 224!). I'm not sure how critical RTTY tuning is but I believe this demonstrates it's possible to fit a 170 Hz signal inside a 200 XFIL with <6 dB attenuation. It probably requires that you carefully center your 200 XFIL and it may be on the hairy edge of copy (as I believe Ed said) but if RTTY contests are like CW contests, that's what you sometimes need when you want a really narrow filter. I think the bottom line is that I might want both the "200" and "250" if I were a serious RTTY contester, and I would use a set point of 300 for toggling between the two. The latter reminds me...to set a wider DSP BW than the XFIL in my cascaded measurements above, I simply changed my XFIL set point (to 250 or 300 instead of 200). I normally have both my 500 and 200 filter set points a little higher than their actual BW anyway (e.g. 600 and 250). For the 500, this keeps you in that filter when using DUAL PB (which automatically selects whatever XFIL you have set for 600 Hz). One final comment. For those who think the 8-pole filters have zero offsets, that may not be a good assumption, especially for the narrow filters. I was told by someone who has multiple 400 filters that they are actually offset by ~100 Hz. 100 Hz is not a big issue for wider filters but that's significant in a 435 or 370 Hz filter, so you might want to check your narrow filter centers and tweak accordingly (0.01 increment in the menu for each 10 Hz of offset). 73, Bill |
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