On 4/26/2016 10:15 PM, Robert Vargas-KP4Y via Elecraft wrote:
Just that you know, K9CT has one of the finest contesting station in
> the US. He has switched to direct-sampling SDRs (Flex-6700). Just ask > him if this architecture is 5 years away from being competitive.
While I do not know for sure, I suspect K9CT is using bandpass filters and stubs to prevent inter-station interference in his multi-multi operation just as he did when he used conventional transceivers. With properly designed bandpass filters, stubs and the preselectors in his 6700s he's not likely to have a severe overload issue unless he happens to be close to some high power AM stations (on 160 meters) and even then indications are that a good highpass filter and 10 to 15 dB of attenuation will resolve the problems with a 16 bit ADC. I have not heard any reports of large multi-multi stations switching to direct sampling transceivers in areas of Europe where the 40 meter SW broadcast was a problem for many decades. The point is that direct conversion receivers need either good filters *ahead of the ADC*, attenuation or both to prevent problems. When one uses a Flex 6300 *which has no preselector*, Flex 6500 on multiple bands (which bypasses the preselector), an ELAD DUO which has no preselector, etc. one must make a choice between sensitivity and dynamic range. The ARRL lab tests show that, Sherwood's tests show that, Adam Farson's noise power ratio testing shows that, and other reviewers hint at the problem but they simply don't understand direct sampling well enough to recognize its limits when they see them. One needs to understand the nature of direct sampling receivers and the way they react to overload to recognize the behavior but the test data is unequivocal. 16 bit ADCs are probably 4 to 6 bits "short" of being able to handle worst case signal levels while simultaneously providing maximum sensitivity (MDS of -135 to -140 dBm in 500 Hz) and fully broad band performance (multiple slice receivers on multiple amateur bands). 73, ... Joe, W4TV ______________________________________________________________ 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 Message delivered to arch...@mail-archive.com
