[Elecraft] K2: RXVFO Waveform?

[Jack Smith]

Sverre:

Thanks for the historical FM interference research.

At the suggestion of Bob, N5BZ, I recently made some measurements of my 
K2's sensitivity to responses of the form N*LO +/- 4915 KHz, via a short 
piece of exposed coaxial cable (a BNC-to-binding post adapter at the end 
of the cable), moving the cable end around, outside the K2's enclosure. 
N is an integer representing the harmonic number of the LO signal.

I found responses up to 73 MHz (I stopped testing at that frequency, so 
I am not saying that higher frequency unwanted responses are not 
present). I found the strongest  response when the cable end was near 
the right rear bottom of the enclosure, i.e., near the SO239 antenna 
connector. During the tests,  the only connections to the K2 were the 
power supply and headphones. The antenna input was connected to a 
shorting cap.

I could hear the unwanted signals with a signal generator output as low 
as -20 to -30 dBm. Since the signal generator's radiating element is 
only a binding post adapter, it is perhaps 1" (2.5 cm) long at most.  I 
imagine the  coupling mode is a combination of pickup on the power 
connection, the headphone connection and direct leakage into the enclosure.

The driving signal to a diode mixer of the type used in the K2 will be a 
flat topped as the diodes are driven into conduction during the peak 
excursions of the LO signal, exactly as you have shown with the 
oscilloscope photograph you made.


Jack K8ZOA

Sverre Holm wrote:
> Jim,
>
>   
>> Does this look OK?
>>
>>     
> http://www1.snapfish.com/share/p=55201169346768265/l=239913880/g=15542617/co
> brandOid=1000131/otsc=SYE/otsi=SALB
>
> I think these waveforms are as expected when driving such a nonlinear load
> as the diode mixer. 
>
> Also this correlates very well with the reported interference from strong
> near-by FM stations in 2002 due to harmonic mixer products, see e.g.
>
> http://www.ac6rm.net/mailarchive/html/elecraft-list/2002-05/msg00829.html
>
> There were many reports about this in May 2002, just search for FM in
> http://www.ac6rm.net/mailarchive/html/elecraft-list/2002-05/index.html
>
> and here are some possible explanations:
>
> http://www.ac6rm.net/mailarchive/html/elecraft-list/2002-05/msg00907.html
> http://www.ac6rm.net/mailarchive/html/elecraft-list/2002-06/msg00386.html
> http://www.ac6rm.net/mailarchive/html/elecraft-list/2002-06/msg00528.html
>
>
>
> 73
>
> Sverre
> LA3ZA
>
> Unofficial Guide to K2 Modifications
> http://www.qslnet.de/member/la3za/K2/mod.html
>
>