[Elecraft] Example - Re: P3 auto REF LVL adjust?

[Mike Markowski]

I'll follow up on my own note because there's nothing as helpful as an 
example:

   1) I move to 17m band with P3 set for +/-100 kHz span
   2) P3 REF LVL knob used to set -122 dBm to match noise level at my QTH

   3) -122 dBm = 6.30957 x 10^-13 mW
   4) Since noise power N = kTB,  (k is Boltzman's constant),
      T = N / (kB) = 6.30957 x 10^-13 / (k x 200 kHz)
      k = 228500 K    <--- recalc when REF LVL knob is twiddled again!

   Now I want to change P3 span to +/-5 kHz, or B = 10 kHz:

   5) N = kTB = (1.380648813 x 10^-23) x 228500 x 10x10^3
        = 3.15479 x 10^-14 mW
        = -135 dBm    <--- new REF LVL setting, send to P3

Easy to add to firmware?  If not, it is easy to add to external 
software.  The only thing not automatic is that the initial manual REF 
LVL setting is required to take into account antenna system noise 
presented to the P3.

I hope other REF LVL twiddlers find this useful.  73,

Mike ab3ap

On 01/01/2012 03:26 PM, Mike Markowski wrote:
> Since the PA temp is available as well as the filter bandwidth in use, I
> wonder if the P3 could have an auto-adjust of REF LVL? It would only
> work within a given band and only after the op has adjusted the REF LVL
> to begin with. But after that it seems that the good ol' N = kTB would
> be a simple enough calc that P3 REF LVL could be auto-adjusted as SPAN
> or filter bw is changed.
>
> I know, talk about being lazy... :-) Happy 2012!
>
> Mike ab3ap