Re: [Elecraft] [K3] Questions about ARRL review
Hi Doug, Can someone explain to me why the above/below results for these two filters are so disparate at these narrow spacings? Let me speculate... you might want to read the description by PA3AKE, especially the chapters on his development of his roofers: http://www.xs4all.nl/~martein/pa3ake/hmode/index.html He talks about, among other things, how important clean surfaces are for good IMD performance in crystals filters. - Quartz and IMD The surface finish plays a big role in the linearity of the quartz. Particles (micro dust) polluting the surface of the quartz are known to cause IMD. To make things considerably worse, the IMD they cause is not following 3rd order law by any means. (from the PA3AKE website) - Sherwood also mentions this in the information (in one of the audio files, if I remember correctly) available at his web site. If one of the crystals in the filter is dusty then you ought to see an immediate difference. My 2700Hz filter, if our numbers (DD5FZ DK4YJ) are really correct, is better than the ARRL filter but the difference is not major. I assume this is due to normal variance which occurs during the production process (and minor differences in the calibration of the measurement equipment). Also the 8 pole 2800Hz filter is likely to have a bit more attenuation in the passband than a 5 pole filter, which may have a minor affect on the amount of IMD in the post filter IF amps and second mixer. All of this could combine to explain the differences at 1kHz and 2kHz spacing. On the other hand, at 20kHz both filters are almost identical, so a dusty quartz crystal may not be part of the reason after all. review suggests that the observed asymmetry is quite unusual For many filter topologies the asymmetry is perfectly normal. Not to forget that in a design such as the K3, it's the first 20dB to 30dB of stop band attenuation which are really important. The final total selectivity comes from the DSP filter and not from the roofing filter. This said, I am playing with idea of rolling my own roofing filters for my K3 for three reasons. Firstly, can a couple dBs be tickled out by using extremely good crystals (expensive!)? Secondly, it should be educational. And most importantly, HAMs just wanna have fun. One thing the ARRL report does show, and I agree with this after having a 2700Hz filter in my K3 for about a year now, the five pole 2700Hz filter is plenty good. An 8 pole filter (i.e. the 2800Hz filter) does not appear to be necessary. vy 73 de toby -- DD5FZ (ex 4n6fz, dj7mgq, dg5mgq, dd5fz) K2 #885, K2/100 #3248, K3/100 #67 ___ 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
Re: [Elecraft] [K3] Questions about ARRL review
Toby Deinhardt wrote on Thursday, November 27, 2008 at 9:17 AM: you might want to read the description by PA3AKE, especially the chapters on his development of his roofers: http://www.xs4all.nl/~martein/pa3ake/hmode/index.html He talks about, among other things, how important clean surfaces are for good IMD performance in crystals filters. Toby, I'll second that, I am using the same 9MHz crystals that Martein uses. Not to forget that in a design such as the K3, it's the first 20dB to 30dB of stop band attenuation which are really important. Maybe, assuming that the IMD performance of the roofer and ALL stages which follow the roofer is good enough at those spacings. This said, I am playing with idea of rolling my own roofing filters for my K3 for three reasons. Firstly, can a couple dBs be tickled out by using extremely good crystals (expensive!)? Secondly, it should be educational. And most importantly, HAMs just wanna have fun. Something to bear in mind is that the passband insertion loss of a quadrature type roofer using good crystals is less than that of most roofing filters of the same bandwidth, which will increase signal levels at the IF's input all other things being equal. The difference can be as much as 6 - 8db in the case of a 500 Hz roofer. Also if you do build these filters, include a shield so that the input - output hybrids do not see one another and forget about using disc ceramic capacitors - bad for IMD! 73, Geoff GM4ESD ___ 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
Re: [Elecraft] [K3] Questions about ARRL review
Hi Geoff, Not to forget that in a design such as the K3, it's the first 20dB to 30dB of stop band attenuation which are really important. Maybe, assuming that the IMD performance of the roofer and ALL stages which follow the roofer is good enough at those spacings. It also depends on whether you are talking about signals within the pass band of the roofing filter or signals in the stop band. I would argue that within the roofing pass band the K3 does have weaknesses, but if the roofing filter bandwidth is close to the DSP bandwidth, then these become secondary as other aspects such as TX and RX phase noise will start to become limiting factors, especially at narrow bandwidths. Something to bear in mind is that the passband insertion loss of a quadrature type roofer using good crystals is less than that of most roofing filters of the same bandwidth, which will increase signal levels at the IF's input all other things being equal. The difference can be as much as 6 - 8db in the case of a 500 Hz roofer. Which could be taken care of by using a 6dB resistive pad, should the lower loss become a problem. I rather doubt that a quadrature type will fit into the space which Elecraft has alloted per filter. It could be the largest challenge if I decide to go the path which you and Martein have. disc ceramic capacitors - bad for IMD! Full Ack. very 73 de toby ___ 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
Re: [Elecraft] [K3] Questions about ARRL review
Table 2 of the ARRL review finally answered my questions about the relative performance of the 2700 and 2800 Hz filters at 1 Hz and 2 Hz spacing. The results are very impressive. Can someone explain to me why the above/below results for these two filters are so disparate at these narrow spacings? There are really two questions here. The first question has to do with the skirt selectivity of the roofing filter. If you look at the larger values of attenuation (134 vs 115, or 113 vs 93) the result is mostly due to the difference in shape factor of the roofing filter. Expressed differently, an 8-pole filter is likely to have a steeper slope in its selectivity curve than a 5-pole filter. The second question has to do with the difference between the above and below values for a given filter when the signal spacing is less than the nominal width of the filter. This asymmetry is due to the way in which the K3 aligns the roofing and DSP filter passbands. The signal is not necessarily centered in the roofing filter passband; rather, the signal is shifted towards an edge to maximize the use of the roofing filter's skirt selectivity. Why is this important? Consider the case in which you are using a wider filter in a crowded band while operating CW. A huge signal appears very close by, perhaps 1 or 2 kHz away. The use of CW REV or CW may make a considerable difference on the impact of that signal on the receiver. If the passbands were centered, this tool would be less effective. And if huge signal are on both sides? Time to get a narrower roofing filter! 73 and Happy Thanksgiving, Lyle KK7P ___ 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
Re: [Elecraft] [K3] Questions about ARRL review
Hi Lyle, If you look at the larger values of attenuation (134 vs 115, or 113 vs 93) the result is mostly due to the difference in shape factor of the roofing filter. Expressed differently, an 8-pole filter is likely to have a steeper slope in its selectivity curve than a 5-pole filter. But on the other hand, AFAIK, an 8 pole filter will tend to be more likely to create higher distortion of the Nutzsignal due to group and phase delay, ringing, etc. at each pole. So more poles is not always better than less poles. vy 73 de toby ___ 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
Re: [Elecraft] [K3] Questions about ARRL review
Hi lyle the List, ... higher distortion of the Nutzsignal due ... das Nutzsignal = the wanted signal Sorry about the German which kinda snuck in. vy 73 de toby ___ 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
Re: [Elecraft] [K3] Questions about ARRL review
Toby Deinhardt wrote on Thursday, November 27, 2008 3:33 PM: Hi Toby, It also depends on whether you are talking about signals within the pass band of the roofing filter or signals in the stop band. I would argue that within the roofing pass band the K3 does have weaknesses, but if the roofing filter bandwidth is close to the DSP bandwidth, then these become secondary as other aspects such as TX and RX phase noise will start to become limiting factors, especially at narrow bandwidths. Putting aside the K3 and thinking in the context of receivers in general, if you plot the Input IP3 of a ladder crystal filter vs frequency you will find that in most cases the value of its IIP3 has not reached a maximum untill the frequency is well into the upper and lower stopbands. For example the IIP3 of the 500Hz quadrature ladder filter that I use starts to decrease from + 55dbm at delta 6kHz from centre passband frequency, to a minimum of + 34 dbm at delta 500Hz, stays between +34dbm and +38dbm until reaching the opposite delta 600 Hz, then increases back to near +54 dbm at delta 10kHz before settling. Now if the plot of a filter's IIP3 is superimposed onto the plot of its frequency response it then becomes possible to determine relatively quickly, in the context of Gain Distribution, what is the level of two or more signals appearing in the skirt and stopband regions and their positions that will result in noticeable IMD products generated by the filter appearing in the passband. This is why I said maybe with ladder filters in mind, and is one of the reasons why three and four tone IMD tests are used by some. I agree with your comment about RX LO phase noise which must be suitably low if one objective is good in-passband performance, which also requires a bullet proof IF. Surprisingly TX phase noise has not been an issue here, even when digging out a SSB DX station sitting between very strong 40m BC stations. Something to bear in mind is that the passband insertion loss of a quadrature type roofer using good crystals is less than that of most roofing filters of the same bandwidth, which will increase signal levels at the IF's input all other things being equal. The difference can be as much as 6 - 8db in the case of a 500 Hz roofer. Which could be taken care of by using a 6dB resistive pad, should the lower loss become a problem. A pity to lose any potential improvement in overall noise figure by doing that, especially on the higher bands! 73, Geoff GM4ESD ___ 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
Re: [Elecraft] [K3] Questions about ARRL review
Hi Geoff, Which could be taken care of by using a 6dB resistive pad, should the lower loss become a problem. A pity to lose any potential improvement in overall noise figure by doing that, especially on the higher bands! Yep. It would be nice to avoid the resistive pad but any gain on the high bands could be detrimental on the lower. A problem if your two favorite shortwave bands are 160m and 10m. Of course front end attenuation is only a button push away... Putting aside the K3 and thinking in the context of receivers in... Thanks for pointing this out. vy 73 de toby ___ 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] [K3] Questions about ARRL review
Table 2 of the ARRL review finally answered my questions about the relative performance of the 2700 and 2800 Hz filters at 1 Hz and 2 Hz spacing. The results are very impressive. Can someone explain to me why the above/below results for these two filters are so disparate at these narrow spacings? The review suggests that the observed asymmetry is quite unusual. Is the performance of a radio most accurately described by the larger or the smaller of the above/below values? Is the asymmetry due to a design problem in the roofing filters or something in the K3 circuitry? If the asymmetry could be corrected by a change in roofing filter design, would the above/below values both become similar to the average of the two values? For instance, would the 94/81 IMD values at 1 kH spacing for a redesigned 2800 Hz filter approach 87.5/87.5, which would be a truly spectacular result for a radio with a single 2800 Hz roofing filter. Doug N0HH (K3/10 #1213) ___ 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
Re: [Elecraft] [K3] Questions about ARRL review
Doug, I have never seen a crystal filter that is truly symmetrical. That is just the facts of life. The real question is, just how much asymmetry is the user willing to tolerate. Since this is a roofing filter, and not the ultimate filter for the K3, I tend to believe that the 5 pole filter is adequate for my purposes. YMMV. 73, Don W3FPR Douglas G. Bonett wrote: Table 2 of the ARRL review finally answered my questions about the relative performance of the 2700 and 2800 Hz filters at 1 Hz and 2 Hz spacing. The results are very impressive. Can someone explain to me why the above/below results for these two filters are so disparate at these narrow spacings? The review suggests that the observed asymmetry is quite unusual. Is the performance of a radio most accurately described by the larger or the smaller of the above/below values? Is the asymmetry due to a design problem in the roofing filters or something in the K3 circuitry? If the asymmetry could be corrected by a change in roofing filter design, would the above/below values both become similar to the average of the two values? For instance, would the 94/81 IMD values at 1 kH spacing for a redesigned 2800 Hz filter approach 87.5/87.5, which would be a truly spectacular result for a radio with a single 2800 Hz roofing filter. ___ 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
