[Elecraft] P3 Questions concerning measurement

[Alan Bloom]

Hi Fred,

The amplitude scale on the P3 shows the signal level in dBm at the K3's
antenna connector.  As the K3 attenuator and preamp are turned on or
off, the P3 automatically adjusts its internal gain so the display
always reads the correct signal level.

The reference level and scale can be adjusted from the P3 front panel.
"Reference level" is the level in dBm at the bottom of the display.
"Scale" is the difference in dB between the top and bottom of the
display.  So, for example, if the REF LVL is -120 dBm and the SCALE is
80 dB, then signal levels between -40 dBm and -120 dBm can be seen.

One complicating factor is that the P3 automatically adjusts its DSP
sample rate proportional to the frequency span.  As a consequence, the
noise level drops as you narrow the span.  (That works both for thermal
noise and band noise coming in the antenna.)  There is a MENU option to
have the REF LVL change automatically as you vary the span to keep the
noise level at the same point on the display.  In this way you always
get the best possible dynamic range.

On my "to-do" list is to add a calibration routine, similar to the
S-meter calibration in the K3.  The signal level at the K3's IF output
connector changes somewhat from band to band and from unit to unit.  The
accuracy should be plenty good enough for giving signal reports on the
air, but for those who want to do more accurate measurements it would be
nice to be able to calibrate it.  Even without calibration, the RELATIVE
amplitude accuracy is excellent, limited only by the display resolution.
So if you tell Alex that his signal is 7 dB weaker than Monica's, he
will have no grounds for argument.  :=)

73,

Alan N1AL



On Sun, 2010-05-02 at 13:12 -0700, Fred Atchley wrote:
> Hi all. While waiting for the P3 I have a few questions on setup and
> measurement. Since I have never used LP pan or similar devices I’ll
> need some Elmering. Specifically there are two capabilities listed in
> the Elecraft P3 page that I need help on:
> 
> 1.  Adjustable reference level and amplitude range scaling 
> 
> 2.  User calibration of the display to show absolute signal level in
> dBm 
> 
> Number 1 suggests two parts. It has an “adjustable reference level”.
> Question: WRT what reference: 0 dBm, the bottom of the spectrum
> display baseline or some particular value? The other part adds
> “amplitude range scaling”. Is this a separate function or is part 2
> the conclusion of the process? 
> 
> My second question concerns the use of the absolute signal level in
> dBm. If I am assisting a Ham friend in adjusting his signal, will this
> capability allow me to evaluate such things as his BW and compression?
> I know that I can do that now by ear but will the P3 give me a visual
> clue WRT onset of changes over-the-air? I have an HP Spectrum Analyzer
> that I have used to check my own K3 on- the-bench. So the phrase
> “over-the-air” is the big question.  
> 
> These questions do not pertain to the main reason I am looking forward
> to the P3 (I.e. operating) but rather to the “other” useful
> possibilities. I appreciate the group’s Elmering.
> 
> 73, Fred, AE6IC, K3 #2241, P3 #?
> 
>