Re: K3 Rx IMD

Posted by Guy, K2AV on Nov 24, 2010; 9:11pm
URL: http://elecraft.85.s1.nabble.com/Re-K3-Rx-IMD-tp5768777p5772073.html

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 <[hidden email]> 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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