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
