Happy New Year 2009 everyone!
Thanks for your answers so far. Yes, I was aware that Wayne N6KR
designed both the 40A and K2. I was simply amazed to receive an offline
email from him, given the demands on his time! I asked Wayne if I could
share the following:
Wayne wrote that he “optimized the NorCal 40A for best noise figure by
using ideal coupling to a gilbert-cell multiplier (NE602). Normally the
exceptionally low MDS of the '40A (around -143 dBm) would be wasted on
40 m, but with short, low-gain, tuned whips, it could make a difference
because the antenna is very lossy.”
“Contrast that to the average 40-m station, which uses a full-length
40-m dipole, or -- noisier still -- a 40 m ground plane. In this case
the noise floor is probably 15-20 dB higher than the short whip, so the
MDS of the receiver won't matter. At least on 40 m.”
“While the low NF of the NC40A is nice, the NE602 is a very weak mixer,
so it can't be used in the front end of a high-performance rig like the
K2 or K3. Thus these rigs use switching mixers. The K2 uses a
conventional diode mixer and preamp, resulting in an MDS of about 6 dB
worse than the 40A, but about 20 dB better dynamic range. The K3 uses a
custom, low-loss switching mixer and an extremely low-noise preamp and
synthesizer, so its MDS is only a few dB worse than the 40A's. But its
dynamic range is 10-20 dB better than the K2, and thus 30-40 dB better
than the 40A! “
What I am trying to grasp now is -- what is the relationship between the
MDS and the blocking dynamic range and third-order IMD dynamic range in
terms of listening experience? Also, how does the absolute IMD level,
and the difference between IMD level and blocking level translate into
the listening experience? Is a lower relative IMD level a quieter
radio? I have the sense that the depth of the IMD level and the delta
between IMD level and blocking level create a quieter experience? I’m
looking at “QST Product Reviews: A Look Behind The Scenes” QST October
1994, pp. 35 figure 2 found on the Elecraft site. If two radios with
the same absolute BDR and IMD dynamic range but different MDS levels
would one want the lower MDS?
If the 703+ has an IMD level of -50? this is between the 40A -68? and
the K2(-41?)/K3(-34?) both higher, what does this indicate?
Wayne also explained: “Once you have full-size antennas up, MDS will no
longer be the limitation. You'll need better dynamic range “
Can someone help me understand these receiver comparison concepts?
Are these right?
NC40A
-143 mds
? bdr
75 imd
Imd level -68
Blocking level
K2
-136 mds
126 bdr
95 imd
Imd level -41
Blocking level -10
K3
-138 mds
140 bdr
104 imd
Imd level -34
Blocking level -2
703
-141 mds
122 bdr
91 imd
Imd level -50
Blocking level -19
K1
Mds >40A and <K2
?
KX1
Mds >40A and <K2
Best,
Dan Perez
AD1P
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