[Elecraft] start without ANY extra roofing filters....
I wrote: Bottom line: 1. Narrower is not always better (Ten-Tec experience) 2. 8-poles is not always better than 5-poles (per Inrad) 3. Let IMD and BDR measurements be your guide More evidence below to support waiting for IMD/BDR measurements before ordering any roofing filters. 73, Bill W4ZV http://mailman.qth.net/pipermail/1000mp/2007-April/020755.html There is a fascinating article describing IMD tests on the IC-7800 by DC4KU in CQ-DL, August 2005 (in German). In these tests, IP3 at 2 kHz offset degrades by an astounding 16 dB when switching from the 15 kHz to the 6 kHz roofing filter. This degradation is due to passive IMD in the filter, and possibly also to IMD in the filter driver amplifiers caused by mismatch when the filter is excited outside its passband. I can send you an English-language summary of the relevant part privately, if you wish. It is highly significant that professional receivers manufactured by the likes of RS, Rockwell-Collins, Racal and Harris have a single roofing filter. This filter is typically 12 to 16 kHz wide, to pass multi-channel ISB, VFT (multiplexed teletype) and high-speed crypto, all of which have extremely stringent in-band IMD requirements. To quote a British engineer who used to design shipboard HF receivers for the Royal Navy: The up-converting architecture, with a roofing filter at a first IF above the highest RF frequency, allows the designer to limit the bandwidth presented to the first IF chain and second mixer. The bandwidth of this filter is a trade-off. Its 3 dB BW must be sufficient to pass the widest emission the receiver is required to handle, but not so narrow that IMD and temperature-drift effects in the filter become a concern. Cheers for now, 73, Adam VA7OJ/AB4OJ ___ 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] start without ANY extra roofing filters....
Well done, Bill. This again shows the importance of real life measurements... and why I didn't order any filters until more is known about them. Anybody interested in the subject of filter design must read the article by list member Jack, K8ZOA in the current QEX. It gives a lot of valuable xtal filter design insight, and has a page of excellent references at the end for those who wish to read more on the subject. This is a complex subject, but as Jack points out, proper characterization of the crystals and rigorous attention to detail can produce accurate models and repeatable designs. Jack touched on drive level dependency in his article. Perhaps he can focus in on the effects of xtal nonlinearity as it affects IMD for a future piece (not trying to create work for you Jack ;-) This is a subject which seems to be gaining in importance as receiver designs surrounding the xtal filter seem to be improving to the point where the filters are becoming the limiting factor in IMD performance. 73, Larry N8LP Bill Tippett wrote: I wrote: Bottom line: 1. Narrower is not always better (Ten-Tec experience) 2. 8-poles is not always better than 5-poles (per Inrad) 3. Let IMD and BDR measurements be your guide More evidence below to support waiting for IMD/BDR measurements before ordering any roofing filters. 73, Bill W4ZV http://mailman.qth.net/pipermail/1000mp/2007-April/020755.html There is a fascinating article describing IMD tests on the IC-7800 by DC4KU in CQ-DL, August 2005 (in German). In these tests, IP3 at 2 kHz offset degrades by an astounding 16 dB when switching from the 15 kHz to the 6 kHz roofing filter. This degradation is due to passive IMD in the filter, and possibly also to IMD in the filter driver amplifiers caused by mismatch when the filter is excited outside its passband. I can send you an English-language summary of the relevant part privately, if you wish. It is highly significant that professional receivers manufactured by the likes of RS, Rockwell-Collins, Racal and Harris have a single roofing filter. This filter is typically 12 to 16 kHz wide, to pass multi-channel ISB, VFT (multiplexed teletype) and high-speed crypto, all of which have extremely stringent in-band IMD requirements. To quote a British engineer who used to design shipboard HF receivers for the Royal Navy: The up-converting architecture, with a roofing filter at a first IF above the highest RF frequency, allows the designer to limit the bandwidth presented to the first IF chain and second mixer. The bandwidth of this filter is a trade-off. Its 3 dB BW must be sufficient to pass the widest emission the receiver is required to handle, but not so narrow that IMD and temperature-drift effects in the filter become a concern. Cheers for now, 73, Adam VA7OJ/AB4OJ ___ 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
Re: [Elecraft] start without ANY extra roofing filters....
This I have noticed during my own IP3 measurements on my FT-1000D, not as much as 16dB but around 3 dB. This was with the INRAD roofing filter and 2 kHz offset. in any case this was enough for me not to use the filter. I´m eagerly waiting for measurement figures on the K3. Beats me why they can´t be presented, that I don´t understand at all, after all it´s no rocket sience. This will also help people select the roofing filter. Maybe I just have to by one and measure myself, then atleast I know it´s done right. /SM2EKM --- Bill Tippett wrote: I wrote: Bottom line: 1. Narrower is not always better (Ten-Tec experience) 2. 8-poles is not always better than 5-poles (per Inrad) 3. Let IMD and BDR measurements be your guide More evidence below to support waiting for IMD/BDR measurements before ordering any roofing filters. 73, Bill W4ZV http://mailman.qth.net/pipermail/1000mp/2007-April/020755.html There is a fascinating article describing IMD tests on the IC-7800 by DC4KU in CQ-DL, August 2005 (in German). In these tests, IP3 at 2 kHz offset degrades by an astounding 16 dB when switching from the 15 kHz to the 6 kHz roofing filter. This degradation is due to passive IMD in the filter, and possibly also to IMD in the filter driver amplifiers caused by mismatch when the filter is excited outside its passband. I can send you an English-language summary of the relevant part privately, if you wish. It is highly significant that professional receivers manufactured by the likes of RS, Rockwell-Collins, Racal and Harris have a single roofing filter. This filter is typically 12 to 16 kHz wide, to pass multi-channel ISB, VFT (multiplexed teletype) and high-speed crypto, all of which have extremely stringent in-band IMD requirements. To quote a British engineer who used to design shipboard HF receivers for the Royal Navy: The up-converting architecture, with a roofing filter at a first IF above the highest RF frequency, allows the designer to limit the bandwidth presented to the first IF chain and second mixer. The bandwidth of this filter is a trade-off. Its 3 dB BW must be sufficient to pass the widest emission the receiver is required to handle, but not so narrow that IMD and temperature-drift effects in the filter become a concern. Cheers for now, 73, Adam VA7OJ/AB4OJ ___ 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] start without ANY extra roofing filters....
Hat tip to Larry ... now all I need is a commission on every QEX sold ... Crystals are non-linear and their motional parameters are, to some degree or other, a function of drive voltage. Since a filter's loss is a function of its motional parameters, the corollary to that is intermodulation can and will be caused by what we think of as purely passive elements such as crystals. (This phenomenon is also found in ferrite cores and powdered iron core inductors, as they have a non-linear B versus H curve.) There are examples of receivers that have as the limiting IP3 crystal filter intermodulation. See Experimental Methods in RF Design for a discussion of Wes Hayward's observation of crystal filter IMD when building a receiver featured in that book. It's devilishly hard to measure crystal filter IMD, however, for a variety of reasons. This is why a filter with fewer elements (poles) can, in some circumstances, yield a better IP3 than a filter with more poles, as counter-intuitive as that might seem. Whilst the filter with more poles will keep more trash out of later receiver stages, small changes to the motional parameters of the crystals that make up the filter with more poles will have a greater effect on the filter's transfer function than for a filter with the same crystals but fewer poles. Thus, although later stages are better protected from undesired signals, that very protection itself causes intermodulation interference. That's why a high performance receiver must be designed in a holistic fashion. Jack K8ZOA Larry Phipps wrote: Well done, Bill. This again shows the importance of real life measurements... and why I didn't order any filters until more is known about them. Anybody interested in the subject of filter design must read the article by list member Jack, K8ZOA in the current QEX. It gives a lot of valuable xtal filter design insight, and has a page of excellent references at the end for those who wish to read more on the subject. This is a complex subject, but as Jack points out, proper characterization of the crystals and rigorous attention to detail can produce accurate models and repeatable designs. Jack touched on drive level dependency in his article. Perhaps he can focus in on the effects of xtal nonlinearity as it affects IMD for a future piece (not trying to create work for you Jack ;-) This is a subject which seems to be gaining in importance as receiver designs surrounding the xtal filter seem to be improving to the point where the filters are becoming the limiting factor in IMD performance. 73, Larry N8LP Bill Tippett wrote: I wrote: Bottom line: 1. Narrower is not always better (Ten-Tec experience) 2. 8-poles is not always better than 5-poles (per Inrad) 3. Let IMD and BDR measurements be your guide More evidence below to support waiting for IMD/BDR measurements before ordering any roofing filters. 73, Bill W4ZV http://mailman.qth.net/pipermail/1000mp/2007-April/020755.html There is a fascinating article describing IMD tests on the IC-7800 by DC4KU in CQ-DL, August 2005 (in German). In these tests, IP3 at 2 kHz offset degrades by an astounding 16 dB when switching from the 15 kHz to the 6 kHz roofing filter. This degradation is due to passive IMD in the filter, and possibly also to IMD in the filter driver amplifiers caused by mismatch when the filter is excited outside its passband. I can send you an English-language summary of the relevant part privately, if you wish. It is highly significant that professional receivers manufactured by the likes of RS, Rockwell-Collins, Racal and Harris have a single roofing filter. This filter is typically 12 to 16 kHz wide, to pass multi-channel ISB, VFT (multiplexed teletype) and high-speed crypto, all of which have extremely stringent in-band IMD requirements. To quote a British engineer who used to design shipboard HF receivers for the Royal Navy: The up-converting architecture, with a roofing filter at a first IF above the highest RF frequency, allows the designer to limit the bandwidth presented to the first IF chain and second mixer. The bandwidth of this filter is a trade-off. Its 3 dB BW must be sufficient to pass the widest emission the receiver is required to handle, but not so narrow that IMD and temperature-drift effects in the filter become a concern. Cheers for now, 73, Adam VA7OJ/AB4OJ ___ 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
Re: [Elecraft] start without ANY extra roofing filters....
The IMD contributed by inductor cores used in the front-end selective circuits is often not taken into account either, or for that matter any core within the signal path. They can bite. 73, Geoff GM4ESD Larry Phipps wrote: This is a subject which seems to be gaining in importance as receiver designs surrounding the xtal filter seem to be improving to the point where the filters are becoming the limiting factor in IMD performance. 73, Larry N8LP 1. Narrower is not always better (Ten-Tec experience) 2. 8-poles is not always better than 5-poles (per Inrad) 3. Let IMD and BDR measurements be your guide More evidence below to support waiting for IMD/BDR measurements before ordering any roofing filters. 73, Bill W4ZV http://mailman.qth.net/pipermail/1000mp/2007-April/020755.html There is a fascinating article describing IMD tests on the IC-7800 by DC4KU in CQ-DL, August 2005 (in German). In these tests, IP3 at 2 kHz offset degrades by an astounding 16 dB when switching from the 15 kHz to the 6 kHz roofing filter. This degradation is due to passive IMD in the filter, and possibly also to IMD in the filter driver amplifiers caused by mismatch when the filter is excited outside its passband. I can send you an English-language summary of the relevant part privately, if you wish. snip ___ 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] start without ANY extra roofing filters....
Adam (VA7OJ/AB4OJ) wrote: It is highly significant that professional receivers manufactured by the likes of RS, Rockwell-Collins, Racal and Harris have a single roofing filter. This filter is typically 12 to 16 kHz wide, to pass multi-channel ISB, VFT (multiplexed teletype) and high-speed crypto, all of which have extremely stringent in-band IMD requirements. To quote a British engineer who used to design shipboard HF receivers for the Royal Navy: Adam I wouldn't get too hung up on commercial/military designs, while not wishing to start a thread running off at a tangent... a friend has a well maintained Racal RA1792 military receiver which is hopeless compared to even moderately good amateur gear such as his Kenwood TS-850, I appreciate the 1792 wasn't one of the better Racal receivers but they weren't cheap. A listening comparison - even under non contest conditions quickly leads to the Racal on/off switch. Add the generally poor sensitivity of those designs, weight and cost and there's no way they will ever compete with a K2 or K3. I don't come across many hams using ex military/commercial gear in preference to amateur gear... At least not those I work on QRP CW. 73 Dave K1, K2 and soon K3/100 (on order) ___ 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] start without ANY extra roofing filters....
Dave G4AON wrote: Adam a friend has a well maintained Racal RA1792 military receiver which is hopeless compared to even moderately good amateur gear such as his Kenwood TS-850, I appreciate the 1792 wasn't one of the better Racal receivers but they weren't cheap. If things in the Mother Country are anything close to how they are over here in the Colonies, nothing the military buys is cheap, regardless of how well they work ... or don't :-) Fred K6DGW - Northern California Contest Club - CU in the 2007 CQP Oct 6-7 - www.cqp.org ___ 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] start without ANY extra roofing filters....
W3FPR said: The real facts are: 1) I will order my initial K3 with only the stock 2.7 kHz filter. followed later by My best recommendation at this time: Understand the consequences of depending on a wide roofing filter while making your decisions - if you choose only the wide filter, live with it for a while. First, for the majority of readers here, I fully agree with the approach of starting out without anything extra (roofing filters). Second, I really have to chuckle at the entire RELATIVELY of these discussions (not directed to Don, whom I hold in high esteem). Other than Orion, or home brew/post-market mod, you simply can't get a NARROWER roofing filter in any double/triple/quad conversion ham radio on the market than 3.0kc. A 2.7 kc roofing filter is not wide compared to what else is out there...quite the opposite, it is THE MOST NARROW (exceptions cited above). You are STARTING OUT NARROWER than everything else! Calling a 2.7kc roofing filter wide is a big misnomer (at least for me). As an example, the highly respected (by many, not me) Icom 756 Pro III has a 15.0kc wide roofing filter. Now THAT IS what I call wide. http://www.icomamerica.com/products/amateur/756proIII/ Excluding Europe and Japan, the odds of a non-contester, non-top band-DXer needing anything narrower than the stock 2.7 for ssb/cw is remote, at best. I dare say that even most contesters won't need anything narrower...only the upper tier who want to battle it out with the best. The number of times a contester will actually NEED a 500 hz roofing filter is very small (comparing the number of QSO's where it is needed to the total number of QSO they make during a contest in which they are top-tier). Of course, there is no accounting for those who simply want it even if they never need it. Price elasticity (for you economics buffs) for a item perceived to carry PRESTIGE can have no known bounds. :-) de Doug KR2Q ___ 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] start without ANY extra roofing filters....
It's deja vu all over again (from May 1). 73, Bill W4ZV http://mailman.qth.net/pipermail/elecraft/2007-May/067147.html K0WA: How many roofing filters do you need in a K3? I would suggest one for SSB and one for CW. That's basically correct, although it depends on how you plan to operate (i.e. serious low band DXing or contests versus casual operating). Remember that a roofing filter's purpose is to prevent IMD and BDR products from passing through the IF chain to the final DSP stage where the final filtering is done. That stage will have variable filter bandwidths over a wide range (every 50 Hz if I recall correctly). The DSP stage really does the heavy lifting after the roofing filter ensures that unwanted products are eliminated at the first IF. If you do casual operating where adjacent signals are seldom above S9+30, it is very unlikely you need more than the stock 5-pole 2.7 kHz filter. Remember that (excluding Orion) few modern rigs use roofing filters narrower than 3 kHz (this includes the IC-7800, FTDX9000, FT-2000 and most of the Inrad roofing filter add-ons for older rigs (Ten-Tec's Omni VI excluded which can be fitted with a 600 Hz). Since the filters have not yet been fully defined or characterized, I would be tempted not to order ANY optional filters at this point. As ARRL discovered in their Orion tests, narrower is not always better. This resulted in Ten-Tec redesigning the Orion II with ONLY 4-pole roofing filters, instead of using the older 8-pole 500 Hz and 6-pole 250 Hz filters used in the original Orion. By characterized above, I mean published IMD and BDR specs for each filter. As Ten-Tec has discovered with Orion, roofing filters are somewhat black magic and there is no substitute for actual measurements before making a decision about which may be best. A good discussion of roofing filters by George W2VJN of Inrad is here: http://www.qth.com/inrad/roofing-filters.pdfhttp://www.qth.com/inrad/roofing-filters.pdf Filling K3 with filters as Toby suggested is both unnecessary and expensive IMHO. I think he is forgetting that there is very good DSP filtering in the final DSP IF that is already doing what he is attempting to do with a wide range of roofing filters. If you are a serious contester, likely to operate in an environment of S9+30 signals spaced every 500 Hz on 160 meters, then you probably do need a narrow filter like the 500 Hz or 400 Hz. Going narrower is questionable IMHO because: 1. You would not hear off-frequency callers with narrower bandwidths (I've found even 500 Hz is too narrow in some cases. 2. At 500 Hz signal spacing, which a 500 Hz roofing filter will handle very well (i.e. +/- 250 Hz excludes the +/- 500 Hz interfering products), you are probably already limited by issues like transmitted phase noise, key clicks, etc. which will override even a perfect receiver with infinitely good IMD and BDR performance. Even a perfect receiver cannot prevent an adjacent TX signal's defects! Regarding 5-pole versus 8-pole, note W2VJN's comment from page 6 of the article above: 5. If 6 poles work so well, why not 8 poles? The most important part of the filter characteristic is from the pass-band on down to about 30 dB on either side of center. Eight poles would provide much better stop-band isolation, but its not required in a roofing filter and would make no noticeable improvement in IMD performance. Bottom line: 1. Narrower is not always better (Ten-Tec experience) 2. 8-poles is not always better than 5-poles (per Inrad) 3. Let IMD and BDR measurements be your guide 73, Bill W4ZV -- * Previous message: http://mailman.qth.net/pipermail/elecraft/2007-May/067152.html[Elecraft] CW/DATA mod r ___ 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
