[Elecraft] K3 Rx IMD

[Ramiro Aceves]

Thanks Guy for the explanations about IMD measurements. I need a signal 
generator to do better tests and get rid of the need of receiving a 
broadcast carrier. I was thinking in building a simple Xtal oscillator 
and some attenuators.

73, Ramiro. EA4NZ




Guy Olinger K2AV escribió:
> You do know that you cannot do valid IMD sweep measurements with AGC on?
> 
> First thing I have to do for any measurements is turn off the AGC and 
> set fixed levels.  Particularly very fast config of fast AGC won't 
> work.  It won't be valid for absolute measurements, but the bandwidth 
> needs to be at least four times the spacing of the two to prevent AGC 
> severely warping the content. And then you will need to provide your 
> single tone sweep selectivity EXTERNAL to the K3.  Your fast AGC is 
> pulling up the IMD tones by changing the RX gain during the sweep.
> 
> 73, Guy.
> 
> On Wed, Nov 24, 2010 at 3:07 PM, Ramiro Aceves <ea1abz at gmail.com 
> <mailto:ea1abz at gmail.com>> wrote:
> 
>     Hello all
> 
>     As David suggested we continue the discussion on the list. I have done
>     some new poor man tests, using a local AM carrier on Medium Wave at 917
>     KHz. For generating the other tone, I am using the same home made DDS
>     generator as before. At this lower frequencies the signal generated by
>     the DDS is much more cleaner.  The antenna is a 6.5m long dipole on the
>     balcony and an open wire feeder as in the earlier tests, but using only
>     one half of the dipole by just directly connecting one open wire end to
>     the K3 antenna connector. The reasons of doing that is that my T match
>     tunner does not work below 1.8 MHz. This eliminates the posibility of
>     IMD products produced on the tuner toroids. It does not remove, just in
>     case, other no-linearities found everywhere as suggested by (sorry for
>     my poor english, I hope you understand me).
> 
>     10 Hz spacing, ACG-F:
> 
>     http://ea1abz.ure.es/acg-F.png
> 
> 
>     Now with AGC-S, notice the strong reduction of IMD:
> 
>     http://ea1abz.ure.es/agc-S.png
> 
> 
> 
>     Now see the AGS-S to AGC-S-soft transition on the middle of the capture:
> 
>     http://ea1abz.ure.es/agc-S_vs_agc-soft.png
> 
> 
> 
>     Now see the IMD at 100Hz spacing, AGC-F to AGC-S transition at the
>     middle of the screen:
> 
>     http://ea1abz.ure.es/100Hz.png
> 
> 
>     Sorry for the poor noise floor, is the Madrid band noise received by the
>     antenna. Hi.
> 
>     I see that the IMD problem is worse with small carrier distances. At 100
>     Hz spacing they are much more weaker and not problematic.
> 
> 
>     73, Ramiro.
> 
>     EA4NZ
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