Al,
As a CW op, I consider the 2 KHz away figures interesting not really
relevant.
I want to know how well a receiver is able to separate a weak signal
from strong signals 50-100 Hz away. It would be interesting to speculate
how such a measurement would be done. Let the RX use whatever analog or
digital tricks it can to achieve the above.
Your idea of simulating a contest with a hundreds of signal injected at
various random frequencies to gauge RX performance has merit.
Have you overlooked the MM, FD and DXpedition RX uses which in fact push
the dynamic range and mixing limits today? There are MM stations which
operate two transmitters on the same band.
73 de Brian/K3KO
On 10/25/2016 5:21 AM, Al Lorona 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
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