[Elecraft] P3 S meter variance

[Bill Conkling]

I just re-read my manual (rev D7), and it says on pg 51 for s-meter cal if
SMTR MD = NOR, set PREAMP ON.  If SMTR MD = ABS, set PREAMP OFF.

...bc  nr4c

-----Original Message-----
From: Don Wilhelm [mailto:w3fpr at embarqmail.com] 
Sent: Sunday, January 16, 2011 12:43 AM
To: alan at elecraft.com
Cc: elecraft at mailman.qth.net
Subject: Re: [Elecraft] P3 S meter variance

  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
>
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