[Lowfer] analog vs sdr vlf/lf comparison

[JD]

Excellent report. Doug, with most interesting results.

However, if I might point out one thing that seems to be a discrepancy... if 
I understand your setup correctly, the frequency stability comparison may 
not be entirely fair.  I believe you're comparing drift at 60 kHz in the 
Excalibur Pro against drift at 3060 kHz in the R-75.

Now, that is valid in one sense, because it's the way many people would 
actually be using the two units in the real world.  But so far as checking 
the inherent stability of the two pieces of hardware against each other, 
it's a bit lopsided.  Sixty versus 3060 is rather like comparing apples to 
pumpkins...really big, Guiness record setting pumpkins. ;)

(Practical example: the 100 millihertz drift I experienced during the 
capture of WMS last night, while warming the car interior back up from 45 
degrees F to about 75 degrees, was with the R-5000 tuned directly to 185.3 
kHz.  If I had been going through the upconverter Todd built for me, the 
change would have been just under 2 Hz even if the OCXO held the converter's 
frequency absolutely steady.)

It might be useful to see what would happen if you put the splitter after 
the upconverter, and compared both receivers tuned to the same dial setting.

By the way, I really like those two captures.  It's the first time I've 
actually seen the WWVB "station ID" this way (the blips from the +45 degree 
and -45 degree carrier phase shifts).

Another possible experiment suggests itself, if you haven't already done it: 
I'm making the assumption that the signal from the Clifton Labs antenna 
currently goes to the splitter, and then one output is through the preamp 
and/or attenuator into the Excalibur... while the other output is to an LPF, 
then the upconverter, then the ICOM.  I didn't notice mention of an LPF in 
your post, but I realize it's a necessity with this upconverter.  Anyway, 
what I'm wondering is, have you tried putting the LPF between the antenna 
and the splitter, so that the inputs to both receivers would be filtered? 
I'm curious whether keeping out-of-band signals out of the Excalibur Pro 
would reduce the risk of ADC clipping and enable you to run it with less 
attenuation, and thereby maybe give it an edge in sensitivity for detecting 
weak signals at LF.  Just a thought.

John