K3 RX spurs?

[Ramiro Aceves]

Hi all,

I am going to describe a problem that I have found recently.
Several days ago, I was encouraged by another ham to see aircraft
doppler radio reflections. I wired my K3 to Linrad (www.sm5bsz.com) FFT
waterfall
Linux program. I tuned a spanish broadcast  station on 21610 KHz (REE,
Radio Exterior de España) just to see a 1KHz line on the waterfall. Soon I
discovered aircraft reflections on the screen, but a nasty problem appeard.
Aircraft lines on the waterfall were duplicated, as if the broadcast station
carrier were a mirror. After some nightmares, headaches
and own teories about this behaviour, comenting with other hams, even
thinking that long path signal could
have hit the aircracft (hi), suspecting that my audio card was a bad one
(M-audio Fast Track Pro USB), a final testing with a FT-897 receiver
told me that the spurs were generated inside the K3.
Investigating further in USB, LSB , CW and DATA RX mode, I see that the
problem is not present in USB or LSB modes (very slightly indeed) and is
strongly noticeable in CW and DATA modes.

What do cause these spur signals? What is the diference among CW-DATA vs
USB-LSB modes?

I will appreciate your help.

I tried reproducing the problem without depending on aircrafts
reflections. I used a home made PIC controlled DDS generator as an
"aircraft simulator". It is not a very pure signal cause it has a
builtin 40 MHz clock. I has got 50 Hz spaced spurs, so ignore them on
the pictures below.

You can see the pictures with comments in the following URLs:

http://ea1abz.ure.es/cw.jpeg
http://ea1abz.ure.es/usb-lsb-data.jpeg

Many thanks for your time.

73, Ramiro, EA4NZ (ex-EA1ABZ).



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