S meter differences from K3 to P3

[Jack Berry]

When operating I see a big difference between the noise floor and signal indications between the K3 meter and P3 display.
For instance, on 80 meters tonight I see a noise floor of around S2-3 on the P3 and a signal peaking just over S9.

At the same time the K3 meter is displaying a noise level of S9 and a signal level of 20 over S9.

Is that a calibration error, a result of settings that I have selected - or just the way things are?

 
God Bless & 73!
Jack - WE5ST
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[k6dgw]

On 10/13/2013 6:22 PM, Jack Berry wrote:

> When operating I see a big difference between the noise floor and
> signal indications between the K3 meter and P3 display. For instance,
> on 80 meters tonight I see a noise floor of around S2-3 on the P3 and
> a signal peaking just over S9.
>
> At the same time the K3 meter is displaying a noise level of S9 and a
> signal level of 20 over S9.
>
> Is that a calibration error, a result of settings that I have
> selected - or just the way things are?

Well Jack, it could be a calibration error, but

1.  Do you have the "K3 IF output" mod installed.  It's one resistor,
and it raises the 8MHz IF output to the P3.

2.  I calibrated my S-Meter with my service monitor, S9 for 50uv in.
Then I adjusted the P3 gain to give me -73dBm on the screen.

3.  The P3 averages signals [you can set the sample size, 2 is the
minimum], and my P3 never shows the same reading as my S-Meter on CW or
SSB except when I'm doing RTTY or some other constant amplitude mode,
and sometimes even those aren't exactly on what I expect them to be.

4.  I don't think it's a problem for you, it's just inherent in the
technology.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2014 Cal QSO Party 4-5 Oct 2014
- www.cqp.org

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[Brian Hunt]

The difference is likely due to any averaging you have dialed in. It was
driving me nuts for a while until I figured out that the K3 S-meter
seems to read the peak value of a signal while the P3 will show the
averaged amplitude.  I programmed "PEAK" onto one of the PN# buttons on
the P3 and when I tap it the two amplitudes are in very good agreement
with the default calibration.  I have a new enough K3 (#4113) that it
has the higher IF output designed for the P3.

73,
Brian, K0DTJ


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[Alan Bloom-2]

There can be several factors at work, but the main one is that the
effective bandwidth of the P3 is less than the K3.  On an SSB signal,
for example, the filter in the K3 passes the entire signal, and that is
what the S-meter reads.  However the P3 divides the signal into a nunber
of frequency points on the display, each of which only receives a small
slice of the total SSB signal.  A similar thing happens with noise.

However on a narrow-band signal like CW, the P3 and the K3 should agree,
assuming the P3 and K3 are calibrated correctly and the K3's S-meter
mode (CONFIG: SMTR MD) is set for "ABS" (so the S meter doesn't change
when you switch the attenuator or preamp).

There is a more complete explanation in the P3 Owner's Manual in the
section "How to Set Up and Interpret the P3 Display."

Alan N1AL


On 10/13/2013 06:22 PM, Jack Berry wrote:

> When operating I see a big difference between the noise floor and signal indications between the K3 meter and P3 display.
> For instance, on 80 meters tonight I see a noise floor of around S2-3 on the P3 and a signal peaking just over S9.
>
> At the same time the K3 meter is displaying a noise level of S9 and a signal level of 20 over S9.
>
> Is that a calibration error, a result of settings that I have selected - or just the way things are?
>
>
> God Bless & 73!
> Jack - WE5ST
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>

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[Jim Brown-10]

On 10/13/2013 6:55 PM, Fred Jensen wrote:
> The P3 averages signals [you can set the sample size, 2 is the
> minimum], and my P3 never shows the same reading as my S-Meter on CW
> or SSB except when I'm doing RTTY or some other constant amplitude
> mode, and sometimes even those aren't exactly on what I expect them to
> be.

Fred has given you excellent advice.  I'll expand on he averaging issue.
I always set up my P3 for the maximum averaging (20, I think), because
it is best at separating signals from noise. But most of us set the
S-meter for peak reading. For this reason, there will always be a lot of
difference between the peak value of the signal and the average (except
for keydown modes like PSK, RTTY, JT65).

The P3 can, however, be set to read peaks, and both displays are useful,
so I assign the toggle for the peak function to a soft key. Once you
have done the calibration that Fred has described (see the manual), the
peak mode of the P3 and the peak mode of the S-meter should agree.

73, Jim K9YC
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[Jim Brown-10]

On 10/13/2013 9:23 PM, Alan Bloom wrote:
> However the P3 divides the signal into a nunber of frequency points on
> the display, each of which only receives a small slice of the total
> SSB signal.  A similar thing happens with noise.

Yes. There's also a setting in the P3 menu to automatically adjust the
vertical gain of the display to compensate for the variable width of
those slices (FFT guys call them "bins").

There's also the uncertainty principle as applied to frequency and
time.  Because frequency and time are the inverse of each other, the
product of frequency resolution and time resolution is a constant. It is
impossible to accurately know BOTH frequency and time. We can adjust for
one or the other, but always at the expense of the other.

73, Jim K9YC
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[WE5ST]

I have completed the IF mod but I'll say it was the ugliest soldering job I ever did. I swore off surface mount work after that first and last experience. I expected it wouldn't work but it did.

There is some good information in many of the responses on how the P3 works and I think that accounts for a good bit of the difference. I'll break out the XG3 and check my calibrations.

Thanks!