Lyle:
As we have discussed, the K3 does not follow the normal
commercial/military practice of providing a 1:1 relationship between the
IF sample and the signal level at the antenna port. Since the K3 has a
switchable pre-amp, the 1:1 relationship would presumably be with the
pre-amp off.
As I understand it, there's something like a 15 dB difference between
the signal level at the antenna input port and the IF sample out. I'll
have a better feel for this when my K3 arrives (ordered in May 2007; has
been waiting for the 2nd receiver, maybe shipped in August 2008), but
I've had reported to me a couple of measurements in that range.
The J310 used to provide the K3's IF sample has a voltage divider on the
input that knocks down the signal level from the sample point. In
addition, the J310 in source follower mode has an output impedance in
the 100-200 ohm range, depending on idle bias current. (Output impedance
is 1/gm and gm varies with bias/idle current.) Thus, if connected to a
50 ohm device, such as a spectrum analyzer or a receiver, there's an
additional loss in the 10 dB range.
Now, a Softrock or Larry's LP-PAN has enough sensitivity to overcome
most or all of this loss, but it's still a design error.
(The IF sample can also be amplified by an external amplifier such as my
Z10000-U. If configured in high Z input mode, the J310's output
impedance becomes a non-issue.)
Jack K8ZOA
www.cliftonlaboratories.com
Lyle Johnson wrote:
...There is just no tracking of levels at the I F
out with what is fed into the antenna input of the receiver.
If that were true, LP-PAN, SDR-14 and SDR0IQ could not be used as
panadaptors. But they can and are being used, so one can only
conclude that the output at the IF jack most assuredly follows the
signal at the input.
Note that there are a mulititude of signals at the IF output connector
since this is the unfiltered output from the mixer post-amplifier.
These signals must be accounted for in any device which attaches to
the IF output jack. See < URL:http://www.telepostinc.com/LP-PAN.html
> and scroll about halfway down the page to "L.O. leakage /
isolation:" for details
The lower the impedance of the attached load, the lower the gain (or
increased loss) there will be at the IF output. We're talking
microvolt levels here, not hundreds of millivolts.
73,
Lyle KK7P
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