[Elecraft] Re: Problems with K3 KXV3 IF Output Level?

[Jack Smith]

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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