N8LP wrote: >...Unless you live near a shortwave broadcast station, or > have a high power ham nearby on the same band, you're not likely to > need 130dB BDR anyway. Even in those cases, having 200dB BDR > probably wouldn't help unless there is a LOT of improvement in > transmitter spurious emissions, distortion and phase noise.
This point is brought home all the time here in Albuquerque, where we have to deal with the radiation from ten or more TV/FM analog transmitters line-of-sight on top of Sandia Crest. While all these transmitters except Channel 2 have fundamentals well above 54 MHz (I run a DCI bandpass filter to keep it out of my preamp), the accumulated grunge from the transmitters' perfectly legal low-level spurious emissions and passive mixes are enough to render 6 meters unusable for weak-signal work in the direction of the mountaintop. Since the spurious gunk is actually radiated on hundreds of different frequencies within the 6-meter band -- not to mention the broad-band noise coming from the same mountaintop -- there is little filtering can do about it. Hopefully this will be alleviated to some degree when the last of the analog TV transmitters finally relocate to UHF digital come June (yes, they all elected to stay on the air past February 17th), but at least until then, there is little I can do about it except deploy longer and sharper yagis to minimize the unusable arc of the "dead zone". Ergo, BDR isn't everything. It is still unfortunately necessary to take into account who your RF neighbors are. They may be operating perfectly legally on frequencies well outside your own area of interest, and still pollute the band with spurious RF that the FCC says is within acceptable limits for their class of operation. Bill W5WVO > The highest signals I have seen here, during Field Day when there were > several stations operating within a few miles of me, were <120dB > above the noise floor. Of course, it's very important not to use any > more front end gain than necessary for the band/conditions. > > 73, > Larry N8LP > > > > > > > > > > > Alan Bloom wrote: >> >> I agree this is something amateur equipment manufacturers like >> Elecraft should be keeping their eye on. If Analog Devices or >> someone came out with an under-$100 ADC with performance close to >> the K3, then you could save a lot of money and complexity by going >> to a directly-sampled RF front end architecture. >> >> But a lot of brilliant engineers have been working for many years >> trying to optimize ADC design. I just really have my doubts that >> they are going to make a 15-20 dB breakthrough any time soon. >> >> Al N1AL >> >> >> On Tue, 2009-02-24 at 09:55, Larry Phipps wrote: >>> Not today... but give it a couple years. There is a lot of R&D being >>> poured into this by a number of competing chip manufacturers. Even >>> if the next batch of designs falls a little short, an all digital >>> design with BDR close to the best conventional designs would >>> probably enjoy a very substantial market. >>> >>> 73, >>> Larry N8LP >>> >>> >>> >>> Alan Bloom wrote: >>>> On Mon, 2009-02-23 at 18:40, N8LP wrote: >>>> ... >>>> >>>>> I think the days of receivers with xtal filters are numbered. >>>>> High speed ADCs capable of 140dB dynamic range without xtal >>>>> filtering are on the horizon. A 20-bit ADC with enough processing >>>>> gain would do it. >>>>> >>>> >>>> I don't think you'll find a 20-bit ADC with a high enough sample >>>> rate to digitize the 3-30 MHz HF band (i.e. >65-70 MHz or so). At >>>> least not at a reasonable cost. >>>> >>>> I believe the best suitable, reasonable-cost ADCs available these >>>> days are able to achieve a 500-Hz blocking dynamic range in the >>>> low 120's dB, maybe 15-20 dB worse than the K3. That's >>>> significantly better than the previous generation of ADCs could >>>> achieve, and no doubt someday we'll get even better parts that are >>>> good enough to challenge the traditional superhet/crystal filter >>>> architecture. But I don't believe we're close to that level of >>>> performance today. >>>> >>>> Another issue, of course, is spurious responses. I'm pretty sure >>>> that current ADCs don't have good enough spurious-free dynamic >>>> range to challenge a state-of-the-art receiver like the K3. >>>> >>>> Al N1AL >>>> >>>> >>>> >>>> >> >> ______________________________________________________________ >> 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 ______________________________________________________________ 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
