K3 Design
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After seeing the QST ads for the OMNI VI+ for so many
years, where they showed the noise floor rise to cover up small signals inside your passband by larger signals outside of the passband - and knowing that the OMNI VI+ is a ham bands only rig, it seemed to indicate that any of the radios that offered General Coverage in the main receiver suffered that fault. The numbers seemed to prove that K2, OMNI VI+, Drake R4C, Corsair II, all ham band only have the best numbers as compared to the very best broadband designs - most often from Japan. The Orion only seemed to prove that one more time, the main transceiver is ham bands only with the weaker broadband second receiver for general coverage. That's a long way to go to provide ham plus general coverage unless those ham band only strips are fundamentally important. According to the early K3 info, the transmitter is broadband, the only limitation is where they place the bandpass filters and in theory they could arrange that any way they want. It would be interesting to know what the real design issues are and (no offense to Elecraft), how they were the first to find the best of both worlds. _______________________________________________ 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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I'll guess.
The way my other rigs get general coverage is to up convert to an IF that's higher than all the frequencies the radio supports. In the ICOM HF rigs I've owned (761, 775, 7800), that's around 70 mHz. At that high IF, it's difficult (expensive or unobtainium) to purchase a tight mode-specific filter (like 500 Hz wide with steep skirts for CW). So a relatively broad "roofing filter" is applied at the first IF and further selectivity is applied at the 2nd and/or 3rd intermediate frequencies (or DSP). Although the K3 sports general coverage, I bet it gets a little touchy around the first IF frequency, somewhere in the 8-9 mHz range, where the roofing filters can be quite sharp and are available at reasonable cost. By "a little touchy" I mean that the rig's performance there might be different, or at the very least there is some clever engineering to deal with that range. Dick, K6KR -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Don Rasmussen Sent: Sunday, July 15, 2007 3:41 PM To: Elecraft Subject: [Elecraft] K3 Design After seeing the QST ads for the OMNI VI+ for so many years, where they showed the noise floor rise to cover up small signals inside your passband by larger signals outside of the passband - and knowing that the OMNI VI+ is a ham bands only rig, it seemed to indicate that any of the radios that offered General Coverage in the main receiver suffered that fault. The numbers seemed to prove that K2, OMNI VI+, Drake R4C, Corsair II, all ham band only have the best numbers as compared to the very best broadband designs - most often from Japan. The Orion only seemed to prove that one more time, the main transceiver is ham bands only with the weaker broadband second receiver for general coverage. That's a long way to go to provide ham plus general coverage unless those ham band only strips are fundamentally important. According to the early K3 info, the transmitter is broadband, the only limitation is where they place the bandpass filters and in theory they could arrange that any way they want. It would be interesting to know what the real design issues are and (no offense to Elecraft), how they were the first to find the best of both worlds. _______________________________________________ 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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It looks to me like the "general coverage" of the K3 is like the Collins
S-line and Drake A/B/C line radios were. They perform most anywhere except at and around IF frequencies. John N1JM -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Dick Dievendorff Sent: Sunday, July 15, 2007 3:55 PM To: 'Don Rasmussen'; 'Elecraft' Subject: RE: [Elecraft] K3 Design I'll guess. The way my other rigs get general coverage is to up convert to an IF that's higher than all the frequencies the radio supports. In the ICOM HF rigs I've owned (761, 775, 7800), that's around 70 mHz. At that high IF, it's difficult (expensive or unobtainium) to purchase a tight mode-specific filter (like 500 Hz wide with steep skirts for CW). So a relatively broad "roofing filter" is applied at the first IF and further selectivity is applied at the 2nd and/or 3rd intermediate frequencies (or DSP). Although the K3 sports general coverage, I bet it gets a little touchy around the first IF frequency, somewhere in the 8-9 mHz range, where the roofing filters can be quite sharp and are available at reasonable cost. By "a little touchy" I mean that the rig's performance there might be different, or at the very least there is some clever engineering to deal with that range. Dick, K6KR -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Don Rasmussen Sent: Sunday, July 15, 2007 3:41 PM To: Elecraft Subject: [Elecraft] K3 Design After seeing the QST ads for the OMNI VI+ for so many years, where they showed the noise floor rise to cover up small signals inside your passband by larger signals outside of the passband - and knowing that the OMNI VI+ is a ham bands only rig, it seemed to indicate that any of the radios that offered General Coverage in the main receiver suffered that fault. The numbers seemed to prove that K2, OMNI VI+, Drake R4C, Corsair II, all ham band only have the best numbers as compared to the very best broadband designs - most often from Japan. The Orion only seemed to prove that one more time, the main transceiver is ham bands only with the weaker broadband second receiver for general coverage. That's a long way to go to provide ham plus general coverage unless those ham band only strips are fundamentally important. According to the early K3 info, the transmitter is broadband, the only limitation is where they place the bandpass filters and in theory they could arrange that any way they want. It would be interesting to know what the real design issues are and (no offense to Elecraft), how they were the first to find the best of both worlds. _______________________________________________ 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 _______________________________________________ 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 Don Rasmussen
Don,
You bring up some valid considerations. When I first got into ham radio back in the mid-50s, the big selling points for any receiver were sensitivity and selectivity. Not much mention of IMD, dynamic range, signal to noise ratio, etc. The evolution of ham receivers regarding the number of IF conversions is quite interesting to me. As of the mid-1950s, the best selling point for a receiver seemed to me to be the number of IF conversions. If it was not a triple conversion superhet, it did not get much attention unless the brand was Colins - that was the only practical way to provide selectivity at a reasonable cost. Yes, the Collins mechanical filter provided selectivity at an IF of 455 kHz, but many receivers converted down to 50 to 100 kHz because sharp LC tuned IF stages were possible at that low frequency. I myself bought into that camp and being a ham without funds to buy the latest and greatest, I built my own version of the HBR-16 receiver using 85 kHz IF transformers salvaged from the BC-453 surplus receivers. It used plug-in coils for band changes (and yes I built general coverage coils), but it did provide a good selective receiver - I even had a Q-Multiplier in it operating at 85 kHz for super selectivity when needed - it worked great, but I have no idea what the IMD and Dynamic Range numbers were for it - they were likely poor by today's standards. Then there was an article in QST that caught my attention - asking 'What is Wrong with Our Present Receivers. That article (among other things) seemed to indicate that the multi-conversion receivers were NOT the best way to go. In fact, if I can generalize, the receiver with the fewest number of mixers will perform better - given all other conditions remain the same - simply because of the reduced number of mixer products. While many ham transceivers used the technique of converting to a high IF to enable general coverage and then down-converting from there, the K2 (and a few others) came along with a single conversion design. The K2 performance showed that a single conversion design could perform very well in a ham-band only receiver. The stage by stage gain and noise is controlled to provide a good dynamic range and IMD measurements. The K3 adds two things to the K2 philosophy (in terms of IF conversions) and that is to convert to 15 kHz rather than direct to audio to allow the DSP to 'do its thing' at that 15 kHz input; and general coverage has been added, but not by upconverting to 70 MHz and then down again, but by providing a local oscillator that can tune almost continuously along with a set of 'general coverage' bandpass filters. Certainly the performance numbers outside the ham bands are not likely to be as good as the ham-band only numbers (the bandpass filters are wider), but it is quite possible to achieve general coverage without adding conversion (mixer) stages - of course the receive capability at the IF frequency is a slight problem, so there is a gap in general coverage near the IF frequency. All in all, I believe this is the 'best of both worlds' - the number of IF conversions is minimized and general coverage is available with only a small band where full coverage is not possible. 73, Don W3FPR Don Rasmussen wrote: > > The numbers seemed to prove that K2, OMNI VI+, Drake > R4C, Corsair II, all ham band only have the best > numbers as compared to the very best broadband designs > - most often from Japan. > _______________________________________________ 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 Don Rasmussen
I wonder if, with today's computer control and other
fancy tricks like switched in band pass filters, wide range vfo gizmos, etc, if it would be possible to build a single conversion receiver with a 455 KHz IF frequency. With good filters and DSP, could it work? Brett N2DTS _______________________________________________ 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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> I wonder if, with today's computer control and other > fancy tricks like switched in band pass filters, wide range > vfo gizmos, etc, if it would be possible to build a single conversion > receiver with a 455 KHz IF frequency. > > With good filters and DSP, could it work? The DSP can't help for the image rejection. 73 Val LZ1VB _______________________________________________ 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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Val [hidden email] wrote:
Brett N2DTS -- >> I wonder if, with today's computer control and other >> fancy tricks like switched in band pass filters, wide range >> vfo gizmos, etc, if it would be possible to build a single conversion >> receiver with a 455 KHz IF frequency. >> >> With good filters and DSP, could it work? Val -- > The DSP can't help for the image rejection. > > 73 Val LZ1VB ------------------------------------------------------------ An Image Reject mixer might be useful in this case Val, although I have not looked at the specific "numbers" for an IF of 455 kHz. If high performance is required, for sure large cored coils would be required in the preselector to avoid IMD being generated there, e.g. T-68-X size. Also "strong" mixers and all of the following stages including the filters and DSP must be able to handle big signals both within and outside of the IF filter passband. Another option might be direct conversion using an Image Reject mixer feeding audio DSP, but you would be hard pushed I think to get more than 50 - 60 db unwanted sideband suppression from the mixer and associated audio networks themselves, but no other images of consequence would have to be dealt with - assuming a clean LO and strong mixers. Collins used this approach to cover the 20 to 2000 MHz range in their 95S-1A receiver produced some years ago. Perhaps the DSP wizards know of a way to improve on the unwanted sideband suppression. 73, Geoff GM4ESD _______________________________________________ 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 gazdzinski-2
-----Original Message-----
From: Brett gazdzinski <[hidden email]> >I wonder if, with today's computer control and other >fancy tricks like switched in band pass filters, wide range >vfo gizmos, etc, if it would be possible to build a single conversion >receiver with a 455 KHz IF frequency. >With good filters and DSP, could it work? You're looking at the wrong end of the receiver. The main problem with a single-conversion HF receiver using a 455 kHz IF is image response. Most of what you have described, such as VFO gizmos, computer control and DSP, will not help that problem. With really good front-end filtering, such a receiver will have adequate image rejection up to about 40 meters. Maybe even as high as 30 meters with lots of high Q tuned circuits. But that's about the limit, if you want image rejection of more than 30-40 dB. If you're willing to give up the higher HF bands, or put up with images, there's no reason such a receiver couldn't be built. Images are the main reason amateur HF receivers moved away from the single-conversion-455-kHz-IF design more than a half century ago. This was after receivers like the HRO and BC-342 had used two tuned RF stages ahead of the first mixer. W3FPR mentioned the 1957 article "What's Wrong With Our Present Receivers?" by W1DX, and I highly recommend it today. It explains the classic problems of the high performance HF superhet rx and the tradeoffs in design. The technology may have changed but the basic problems remain. All Elecraft rx designs are direct descendants of the ideas presented in that 1957 article. The two main advances in the intervening 50 years (besides the use of sandstate instead of hollowstate) are the use of a lownoise lowdrift synthesized LO, and improvements in the back end of the rx like IF DSP. --- A much-overlooked article was in QST in 1971, called "An Experimental Receiver for 75 Meter DX Work" by W1KLK. This rx used a high selectivity front end, lownoise/lowgain RF amplifier, high dynamic range mixer, and single conversion to 455 kHz. But it only covered 80/75 meters. All receiver designs are tradeoffs - no exceptions, including even the K3. The question is what you trade off. For example, the K3 won't do a very good job receiving around its first IF (8.215 MHz). But there's no ham band there. If it means better performance on other frequencies, lower cost and less complexity, I'll take that tradeoff! 73 de Jim, N2EY ________________________________________________________________________ AOL now offers free email to everyone. Find out more about what's free from AOL at AOL.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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