Re: Re:AGC THR and noise

Posted by Bill W4ZV on Dec 04, 2008; 11:47am
URL: http://elecraft.85.s1.nabble.com/Re-AGC-THR-and-noise-tp1604984p1613204.html

Steven.Zabarnick wrote

Oops, mistakenly sent this with the wrong subject line.

Jack's measurements of the K3 AGC on the link listed below are quite
interesting. As my noise level on the low bands is usually at least S3 or
4, his plots indicate that the noise (S3 is approx -90 dBm) is either in
the linear range of the AGC, or just barely in the software AGC range.
Don't we really want the band noise in the  flat part of the AGC curve,
down near -130 dBm? Would operating with the ATT on and/or the RF gain
turned down help move the band noise to this part of the curve, resulting
in a  higher S/N ratio for signals just above the background noise?

Yes.  I normally operate on 160 either with ATT ON, or sometimes OFF and RF Gain backed off to 12 o'clock.  As long as you can hear band noise when you connect and disconnect the antenna, you have sufficient RX gain and are not wasting your dynamic range by setting the K3's internal lower limit below the external noise floor.

I also normally have my AF Gain set to 9-10 o'clock (using Extreme Isolation EX-29 headphones and CONFIG:AF GAIN set to LO).  Then I adjust ATT and/or RF Gain for a comfortable listening level.  Using the S-meter indication for noise, you can visually back off RF Gain until the S-meter just stops flickering.  Then you can hear the weakest signals in noise and still have the upper end of the K3's dynamic range available to help with extremely strong signals.

73,  Bill  W4ZV

P.S.  Someone previously asked me about comfort of the EX-29 headphones.  I used them continuously for ~15 hours at a time with no discomfort this past weekend.  They do fit snugly but not uncomfortably.