[Elecraft] K3 AGC and Apparent Noise and Signal Distortion

[Lyle Johnson]

> 20m ambient noise here on my C31XR at 76 feet is around 1 uV here for
> SSB bandwidths, which IS engaging the AGC because it is S3.
>
> Use ATT before you reduce RF gain, as it provides the best headroom in
> the overall circuit. 

With an MDS around -136 dBm and 1 uV being -107 dBm, the noise is about 
30 dB over MDS.  Adding even 20 dB of input attenuation wouldn't harm 
weak signal reception and would provide 20 dB more headroom for nearby 
strong signals before they started to bother you.

The K3 has a 10 dB input attenuator.  However, you can use a trick to 
get a 10/20/30 dB input attenuator if you have a KXV3 or KXV3A and 
aren't using a separate Rx antenna.  Just insert a 20 dB attenuator 
between the RX ANT IN and RX ANT OUT jacks on the KXV3/A panel.

If you need 10 db ATT, use the ATT button. If you need 20 dB, remove the 
10 dB ATT and select RX ANT.  For 30 dB select both RX ANT and ATT.  You 
still can use PRE to get +10 dB, so you now have 40 dB of front end gain 
control in 10 dB steps to allow you to match the front end sensitivity 
of the K3 to current band/noise conditions.

The downside is that the S Meter will read 20 dB lower when you have RX 
ANT engaged since the DSP doesn't know about the attenuator.

Enjoy!

73,

Lyle KK7P