[Elecraft] [K3] The Way We Rank Receivers (long)

[Edward R Cole]

Most of this very interesting post is not-relevant to my ham radio 
operating (eme on VHF+).
Comments preceded by asterisk *  inserted below:

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Date: Tue, 25 Oct 2016 05:21:20 +0000 (UTC)
From: Al Lorona <alorona at sbcglobal.net>
To: Elecraft Reflector <elecraft at mailman.qth.net>
Subject: [Elecraft] [K3] The Way We Rank Receivers  (long)

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.
===snip
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.

*I wonder where in the world one sees a 20m noise floor that low when 
antenna is connected?  Only when I lived off the grid running on 
battery power did I see S0 noise on my radio (3.9-KHz with TS180S 
from a dipole).  I doubt that radio had a noise floor that low, but 
maybe.  I see S3/5 noise on my K3 connected to triband yagi, preamp 
off.  That is probably about -110 to -100 dBm noise floor.  My SDR-IQ 
is very good at 28-MHz and displays -132 dBm without an antenna 
connected (190-KHz bandwidth).  But connecting an antenna raises that 
instantly.
===snip
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.

*I can only think of two instances seeing such a strong adjacent signal:
1)  At 310-KHz a GPS-reference station is locate less than a mile 
away and I measure it at -30 dBm on my inverted-L (tuned to 490-KHz) 
with SDR-IQ.
2)  I once measured a high power pager running 158.100 100-foot away 
from my company's 161.325 repeater showing 1/4w on the Bird power 
meter installed on the repeater antenna whenever the pager 
transmitted (repeater was turned off).  That would be +23 
dBm.  Amazingly the repeater Rx survived this (due to duplexer and 
helicoil prefilter).  IMD mixing of the 158.1+161.325 produced a 
horrendous signal on 156.450 (which interfered marine ch.9).
Both examples extremely unlikely to occur for HF hams.
===snip
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.

*this has application for me.  The occurrence of a high power signal 
off frequency mixing with the phase noise of my eme Rx.  This is one 
of the reasons driving a goal for extremely low LO phase noise in eme 
systems.  This is one of the major selling points for me to purchase 
my K3 and upgrade synthesizers.

===snipped the rest (though very interesting proposal)

73, Ed - KL7UW
http://www.kl7uw.com
     "Kits made by KL7UW"
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