Nice explanation Al. It's nice to know that the K3 ranks high on the charts, but I agree that 'usability' and 'listenability' are underrated. Before I purchased my K3, I went to AES in Wickliffe and listened to several receivers. I liked the sound of the Kenwoods, but I liked the 'modularity' of the Elecraft K3. I was also influenced by my conversations with Eric and Wayne at Dayton.
73, Art WB8ENE On Tue, Oct 25, 2016 at 1:21 AM, 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 valvetb...@gmail.com > ______________________________________________________________ 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
