K3 SN0013 IMD observations.

Test conditions - 6.146 kHz two signal source (two XTAL osciallators combined).  35 db over S9 with ATT engaged.

First off - letting the AGC take care of things - this is what I see:

Now - if I back off on the RF gain - and bring up the AF gain - I can get this:

Notice how you can barely see the IMD products - but also notice, the noise floor of the radio has moved up 
20 db due to the RF gain issue I previously documented.  

Also - if you turn off the AGC - the IMD goes away and you end up adjusting the RF gain about where it was
in the above picture.  In other words - backing off the RF gain sufficiently is the same thing as turning off the
AGC and then backing down the RF gain.  I just don't bother with changing the setting.  This also gives you
some protection if a REALLY LOUD signal shows up...  I wish everyone who says they turn off their AGC 
would understand this.

I don't find that I can make the first picture much better with any adjustments of the AGC HLD or THR.  

Now to see how the AGC effects IMD with the RF Gain wide open.  I had to rebuild my IMD test setup,
so we are now on 3760 KHz - about 45 DB over S9 with ATT on.  Here is what I have with the AGC on
and RF gain wide open:

Now with the AGC off and RF gain still open (and AF gain WAY DOWN):

And backing the RF gain back to about 12 O'clock yields this:

Okay - I clearly need some attentuation here.  I am feeding signal into the RX antenna input.  If I switch back to the
SO239 - I still have a S7 signal.   Here is what I see with AGC off and that condition:

Now with AGC on (FAST):

And AGC slow:

My current AGC settings:

AGC THR 002
AGC SLP 000

Changing SLP to 015 gives this with AGC Slow:

and with AGC FAST

not much change.  Going to AGC THR to 008 gives this with FAST

and AGC SLOW

And just for fun - AGC SLOW with the RX ANT back so we are activating HW AGC

Now with 1 uf across C238:

That looks more good.  Let me remove the cap to make sure.

Pretty bad again.  Now let me try 2.1 uF

Just a tad better than the 1.1 Uf case.  

Tree N6TR 03-Dec-2011 and 04-Dec-2011