[Elecraft] P3 S meter variance

[Alan Bloom]

Don,

> The manual suggests that you calibrate the S-meter with the preamp ON.  
> If you do that, and set the S-meter to ABS, you will find that the 
> S-meter response is about 2 S-units lower than what you may expect.

Interesting.  I assumed that in ABS mode it didn't matter if the preamp
was on or not.  I sheepishly admit I have never bothered to calibrate my
K3 S meter.  No excuse - I have an HP8656B signal generator sitting
right next to the K3 - I just never got around to it.  I'd do it right
now but it's time to go to bed.  :=)

> The K3 has re-vitalized the old Collins 50 
> uV equal S-9 "standard".  That standard is not a standard at all,

Actually IARU Region 1 has published Technical Recommendation R.1 which
specifies S9 = -73 dBm (50 uV into 50 ohms) and 6 dB per S-unit.
http://en.wikipedia.org/wiki/S_meter#IARU_Region_1_Technical_Recommendation_R.1

But you're right, that recommendation is more honored in the breach than
the observance.  (Gosh, quoting Shakespeare on the reflector - what are
we coming to? :=)

Alan N1AL



On Sun, 2011-01-16 at 00:43 -0500, Don Wilhelm wrote:
> Alan,
> 
> I might add about the K3 CONFIG: SMTR MD = ABS, that the calibration 
> procedure indicated in the manual may (or may not) be the "proper" thing 
> to do.
> The manual suggests that you calibrate the S-meter with the preamp ON.  
> If you do that, and set the S-meter to ABS, you will find that the 
> S-meter response is about 2 S-units lower than what you may expect.  I 
> prefer to do the calibration with the S-meter off, and then the response 
> in SMTR ABS is more what I would expect.
> 
> Since everyone's expectations do not necessarily match mine (obviously, 
> I disagree with Wayne on this item), you can take your pick - do it the 
> way the manual says, or do it like I have.  The choice is entirely 
> yours.  Like all things S-meter reading related, there is a great 
> variation between receivers,  The K3 has re-vitalized the old Collins 50 
> uV equal S-9 "standard".  That standard is not a standard at all, and 
> many transceivers do it differently, but with the K3, you do have a 
> choice depending on how you want to set it up.  If you cannot decide how 
> you want it, go with the manual instructions - they work and work quite 
> well.  If you deviate, know why you are doing that deviation and know it 
> is for your own situation, others may or may not make the same choices.
> 
> 73,
> Don W3FPR
> 
> On 1/16/2011 12:20 AM, Alan Bloom wrote:
> > There are a couple reasons why the K3's S meter and the P3 display may
> > show a different level.
> >
> > As Don mentioned, the P3 will generally show a much lower level than the
> > K3 on noise.  It's because the effective bandwidth of the P3 is
> > generally only one display pixel, about 1/450 of the span.  Less
> > bandwidth means less noise power in each display point.  The same thing
> > happens to some extent with an SSB signal.
> >
> > Another issue is that the S meter reading on the K3 may change when you
> > turn the preamp or attenuator on or off.  To prevent that happening, set
> > CONFIG:SMTR MD to "ABS".
> >
> > If you do that and calibrate both the K3 S meter and the P3 amplitude,
> > then the two should closely agree when measuring a steady CW carrier.
> >
> > Alan N1AL
> >