Hello, All, I posted this last Friday, it never appeared, so I send it again. I apologize in advance if the original shows up sometime in the next few weeks, as sometimes happens with Yahoo. ;)
Hello, All, Bob and Alberto's comments on the 'quality' of sound in a DC-SDR provide me to ask a generic question which I have been pondering for a while now. This forum has a lot of talent so I am asking it here first. What was said about the exceptional 'clarity' of signals derived from direct conversion SDRs has been my experience as well, whether the radio was a simple homebrew design, a SoftRock, an SDR14, or even 'The Time Machine'. It has also been true whether I was using Linrad, PowerSDR, Winrad, Rocky, KGKSDR, Spectraview, etc. as the software backend. The 'old-style-radios I used for comparison were FT1000MP Mk V and Elecraft K2 [and before that IC735 and IC970, but these last 2 are not really in the same class at all]. So the sound effect is not limited to one type of hardware or one particular piece of software. I agree that it likely relates to the lack of crystal filters, etc. Unfortunately, in some situations these filters seem to be a 'necessity' and that is the reason for this email/question, as I ponder which course to take as I upgrade my [contest] station here. The problem that has made me unable to go totally SDR, and for which I really wish I had a solution, is that I do weak signal contest work [50 MHz to 24 GHz] but live in an environment where during a [VHF and up] contest I have more than 1 full-legal-power-on-multiple-bands station operating within a few miles of me [one a multi op]. The extremely high RF levels from inband signals from these stations trash the front end of any radio [SDR or not] that I have had available to me that doesn't have a narrow roofing filter [1st IF filter] to greatly attenuate the inband signal [as long as it is outside the roofing filter]. Given that the multiop stations usually have the lower four bands [50-432 MHz] continuously occupied at 1.5 kW output, this is a great problem on these bands. On the bands above 432 MHz it is of course not a problem. My FT1000MP has a 4 kHz roofing filter instead of the stock 12 kHz filter, and the improvement this gives under these contest conditions is tremendous, as long as the strong stations are not within 4-5 kHz or less of where I want to operate. The K2's variable bandwidth crystal filter also does a very good job and before I put the narrow roofing filter into the FT1000MP the K2 ran rings around it in terms of strong signal immunity. The SDR's fold up in this environment with spurious signals, increased noise, etc. I wonder if a hybrid approach such as something with an architecture like the Orion I or II might offer the best of both worlds in terms of performance in the environment described. The Ten-Tec Orion has a conventional front end and what is arguably the best roofing filter arrangement of any commercial conventional radio and follows that with digital processing. Might this be the best way of achieving outstanding, 'bullet-proof' receiver performance? One of the great attractions of the SDR radios from an operational standpoint is the fact that they have wideband displays that from a EMEer or contester's perspective are extremely useful for showing just 'what is going on' away from the operating frequency and on other bands. During a contest I constantly monitor bandscopes on all of 50-432 MHz simultaneously for this reason. The roofing filter approach of minimizing the signal that gets thru to the processing stages seems diametrically opposed to the use of a wideband display and 'doesn't fit' with the direct conversion model. One could achieve perhaps [for a station with my requirement] the best of both worlds with an Orion-type receiver with conventional front end with very good performance and narrow roofing filters, and stealing tiny bits of time to step the LO to sweep the band and generate a bandscope waterfall that way. Or if one designed the radio with the 'usual' 2 receivers, using the second receiver hardware [when not in dual-receive mode] to do the same. Of course with each of these designs, best-possible performance of the RF preamp [if used], First Mixer, and Post-Mixer amp is needed, and these stages must be designed and implemented so that they are not the limiting factors to strong signal performance. So my question is this. Is it possible to design a direct-conversion SDR with wideband display that will under the strong signal conditions that I have described offer the same protection from overload from strong signals while trying to listen to weak [and other strong] ones that architectures such as the Orion II or the modified FT1000MP for example provide? Will the 'best' implementation of a DC SDR always suffer IN THIS REGARD in comparison to the 'best' implementation of a receiver using narrow 'roofing' filters? I ask this question now because [1] I just finished a contest weekend and all of these issues are [too] fresh in my mind, and [2] I am in the process of deciding whether to move for my contest station to [1] multiple Orion II's [operated using N4PY software so all can be operated from a single computer screen] or [2] Multiple SDR-1000/2000/etc, or [3] a combination of the two. It might for example be optimal [for a VHF contest station] to use an Orion for each of the lower 4 bands where strong signal performance is paramount, and then use an SDR for 903 MHz and up. Thanks in advance for your collective wisdom, and 73, Roger Rehr W3SZ Roger Rehr W3SZ http://www.nitehawk.com/w3sz ----- End forwarded message ----- Roger Rehr W3SZ http://www.nitehawk.com/w3sz
