Georges, Although the Hilberling PT-8000 series of transceivers are very expensive, this could be because they have been designed for Military and 'commercial' use and not only for amateurs. I believe that it should be possible to design and market a high performance transceiver using an up-conversion architecture, whose selling price is reasonable for most amateurs.
While I agree with Alan's N1AL comment about the higher density of phase noise generated by the receiver's first LO in an up-conversion scheme, it was possible back in 1994 to devise a LO system whose phase noise density allowed a SFDR3 > 100db at 2 kHz spacing to be obtained from an up-conversion HF receiver, whose IF was at low VHF. However the task was not easy at the time. With regard to roofing filters there is a problem which is often overlooked, and is present in both HF and VHF crystal filters. This is the level of filter generated IMD vs. filter bandwidth. The results I have of tests performed by other people and myself, show that the level of odd order IMD generated by a crystal ladder filter using a given quality of quartz increases as the filter's bandwidth is decreased. This effect can be seen when the two parent signals used during the tests are either inside or outside the filters passband, which places a question mark over the use of very narrow bandwidth roofing filters in a receiver designed for high performance. Again I agree with Alan that the use of amplifiers and mixers before the second IF filter, which have very high dynamic range, should be used to offset the effects of signals passing through a wider bandwidth roofing filter. IMHO these are not very complex circuits, but the amplifiers usually operate at high values of standing current. Comprehensive Gain Distribution calculations are essential, of course, when designing a receiver. I would also suggest that the frontend gain between the antenna input and roofing filter's input should be negative in value to avoid overdriving the roofing filter, provided that a useful receiver Noise Figure is obtained. 73, Geoff LX2AO On Friday, October 12, 2012 at 8:36 AM, Georges Ringotte F6DFZ wrote: > Out of an extremely interesting private mail, I don't have got comments > on this topic, which was much debated in the past. > I post it once again because I think the Elecraft team and Elecraft > aficionados are more apt to discuss this subject than on other reflectors > dealing with down conversion receivers. I added some comments. <snip> > Yes this German made transceiver, built like a measuring instrument, is > very > expensive, but surely it's concept can be used to market simpler rigs > (100W, > 13,8V, only one receiver, no VHF, not so luxurious, with true general > coverage ...) for a more correct price. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
