Al, thanks for a very informative and helpful discussion. This sort of posting is what makes the Elecraft listserv so valuable.
Lew N6LEW > On Oct 24, 2016, at 10:21 PM, Al Lorona <alor...@sbcglobal.net> wrote: > > Receivers are always ranked by the "2 kHz third order dynamic range", such as > at: http://www.remeeus.eu/hamradio/pa1hr/productreview.pdf but do we really > grasp the meaning of these specs? For instance, the Elecraft K3's (after > synthesizer upgrade) number is 103 dB, good enough to be in the top ten. In > fact, this number is so strong that very few hams will ever be affected by > it. To the best of my knowledge, I have *never* been close to running out of > dynamic range. To understand why, let's put "103 dB" into English. > > Let's say you're on 20 meter CW, operating at 14.050 MHz. You're listening > through your fine Elecraft 500 Hz crystal filter when suddenly, and by > incredible coincidence, two equally strong 49 dB over S9 signals begin > transmitting at the exact same time, one on 14.052 and the other at 14.054 > MHz, exactly 2 kHz and 4 kHz up from where you're listening. With the preamp > off (which is totally believeable on 20 meters with a decent antenna) you > will just barely hear a "ghost signal" right at the noise level... if you > notice it at all. That "ghost" signal is the two-tone, 3rd order intermod > product generated in the K3 receiver by those two hugely strong and perfectly > placed signals. > > Not a very likely scenario, but that's what 103 dB of dynamic range buys you. > > I have assumed a noise floor or MDS of -130 dBm because it's a nice round > number. If your 20 meter noise floor is higher than this, then the two > signals would have to be *even stronger* to hear the intermod come out of the > noise. > > Even if each of those interferers was *60* dB over S9 -- pegging the > S-meter-- the intermod product on 14.050 would still be only S5. Amazing. > This kind of performance begs the question, "How much more dynamic range is > really needed?" and some (like Rob Sherwood) have said that once you're above > 90 dB, you already have enough, at HF at least. > > Perhaps it's time to rank receivers by a different measurement, something > that affects more of us. Looking through the table at the link above we see > another measurement called "2 kHz blocking gain compression" and for the same > K3 it is 143 dB. This is a measurement not of two interfering signals, but a > single interferer just 2 kHz away. Since there's only one signal, it won't > generate a "ghost", but it will reduce the gain of the receiver. ARRL defines > this as the signal level that reduces the gain by 1 dB. One dB is really > small, something like changing your RF Gain knob from the 3:00 o'clock > position to maybe the 2:45 o'clock position. Barely noticeable. Nonetheless, > for our K3 the signal required to do this is about +13 dBm, or 20 milliwatts, > which is probably near the damage level of the receiver! (I'm quite sure that > Wayne has made intercontinental QSOs at 20 mW.) It's a theoretical value that > very, very few hams would ever encounter... only the ones living next door to > a guy running a kilowatt. So this measurement is even less relevant to us. > > Finally, we notice a measurement called "2 kHz reciprocal mixing dynamic > range" -- probably the limiting spec nowadays for top tier receivers. In our > example of the single strong signal, way before reducing the gain of the > receiver, that signal will have another effect: it will mix with the phase > noise of the K3's own local oscillator and deposit that phase noise right > onto your desired frequency of 14.050 MHz. As you're listening there, you > suddenly notice that the noise floor seems to be rising for no apparent > reason. You listen some more, and notice that the noise is following some > kind of CW keying. You glance at your panadapter and notice an enormous > signal just 2 kHz away on 14.052. So there are two culprits: that strong > signal, and the K3 oscillator phase noise. The K3 with upgraded synths has a > spec of "-115 dBc", again near the top of the list, which means that a signal > 2 kHz away and 115 dB above the noise floor will cause the noise floor to > rise by 3 dB. For a K3 n > oise floor of -130 dBm this is -15 dBm, or about 60 dB over S9. The reason I > say this is the limiting factor is because the chance of just one 60 dB over > S9 signal nearby is greater than *two* of them at the right spacing as in our > discussion of 3rd order DR. > > For these reasons, we could start ranking receivers by 2 kHz reciprocal > mixing dynamic range because reciprocal mixing is far more likely to happen > to a larger number of hams. It's not a catastrophic effect, but it's quite > noticeable. There's a problem, however, because sampling receivers don't > follow the classical reciprocal mixing model. We need a measurement that > hasn't been invented yet to compare modern receivers. Maybe we could simulate > the worst-case contest by applying thousands of signals and noise to the > receiver and seeing how much junk is generated to cover up the signal you're > trying to copy at 14.050, something kinda like the noise power ratio test. > But in order to compare apples to apples, the exact same test conditions must > be used on every receiver, regardless of architecture. > > Finally, the general unlikelihood of any of these conditions occurring also > convinces me that other ergonomic factors -- not necessarily measureable-- > should be considered when choosing a receiver. 'Usability' (whatever that > means to you) is underrated. So is 'listenability' -- again, a very > subjective term. For instance, I have come to believe that the AGC system in > a receiver has everything to do with how it "sounds" and explains why > receivers with similar specs can sound so different from each other. This > area needs further study. > > I hope that this helped turn mere numbers into an actual operating reality. > > Al W6LX > ______________________________________________________________ > 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 l...@n6lew.us > Lew Phelps N6LEW Pasadena, CA DM04wd Elecraft K3-10 / KXV144 / XV432 Yaesu FT-7800 l...@n6lew.us www.n6lew.us Generalized Law of Entropy: Sooner or later, everything that has been put together will fall apart. ______________________________________________________________ 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
