[1000mp] S Meter in MP

[Tom (xmission)]

I was interested to read this thread about the S-meter in the MP.  For any
of you who are interested, I thought I would pass along the results of some
testing I did on my MP's S-meter recently.  I have always suspected that my
S-meter is quite generous.  So I brought home a Hewlett Packard HP E4432B
calibrated RF signal generator from work to check it out.

From what I understand, the traditionally accepted definition of S9 is a 50
microvolt rms signal at the 50 ohm antenna input to the radio.  A little
algebra shows this to be the same as a signal level of -73 dBm.  And for the
size of an S-unit, I used the common 6dB-per-S-unit assumption.  So, in my
calculations, S8 = -79dBm, S7 = -85dBm, 10dB over S9 = -63 dBm, etc.

My goal was to create a simple graph showing the indicated S-meter reading
on one axis, and the correct S-meter reading on the other axis.  I was a
little disappointed to find that there are lots of things that make huge
differences in the indicated versus true S-meter readings.  The curve was
very different for each band I tested (80, 40, 20, 15, 10), and it was
different depending on whether the preamp (menu setting 8-4) was set to
"tuned" or "flat."  So I ended up with a family of curves on my graph.
Fortunately it didn't change much with the IF bandwidth setting, or that
would have made it even worse.

I would also like to thank the author of the nice little program "S-Meter
Lite" which was very helpful in capturing the S-meter readings for each
condition.  For example, I started out with a signal level of -33dBm (40
over S9), and then decreased in 5 dB steps until I got to S9.  Then I kept
going down, but in 6dB steps to hit S8, S7, etc.  The S-Meter Lite program
captured a little graph of the S-meter setting as I let it pause on each
reading for a few seconds.  Then I entered all the data into an Excel
spreadsheet and made some calculations, and finally the graph, from these
recorded values.

Just for a teaser, here's what I got for a power input of -73dBm (which
should have indicated S9 on all bands, I would assume).

Condition                Indicated on MP

80M Flat                 S9+14dB
80M Tuned             S8.2                 (the best--actually quite good
for indicated levels above S9)
40M Flat                 S9+10dB
40 Tuned                 S9+2.5dB
20M Flat                 S9+15dB
20M Tuned             (same as 20M flat)
15M Flat                 S9+12dB
15M Tuned             (same as 15M flat)
10M Flat                 S9+15dB
10M Tuned              S9+26dB       (the worst--WOW, very generous indeed!)

I also found that the slope of the S-meter curves was not too bad for
signals above about S8 or S9.  What I mean by that is that a 5 dB change in
the input signal level produces about the right amount of change on the
S-meter, etc.  In this upper region, the S-meter is just reading too hot by
so-many dB, as the table above hints.  But at about S7 or S8 the indicated
S-meter readings drop like a rock with modest reductions in input power.
Going from indicated S6 to S2 is only about 6 dB change in input signal
level, but it should be 24 dB to cover 4 S-units.  So the bottom half of the
S-meter is pretty useless, if you ask me.

If anyone would like a copy of the jpg image of the graph, I'd be happy to
email it to you (w7gt@arrl.net).

You know, I was kind of disappointed that the good folks at Yaesu didn't do
a better job on the S-meter in such an expensive radio, and otherwise
fabulous radio.  Obviously the AGC-derived S-meter signal is digitized,
since it can be read via the CAT port by the program S-Meter Lite.  So it
would have been a simple matter to run the digitized S-meter word through a
lookup table and correct it for the various mode settings, so that it
produced a more accurate result.  I don't think it would have cost them a
penny in recurring cost--just a little additional software (OK, maybe a few
bytes of extra ROM) between the segments on the front panel, and the
digitized S-meter value.  But alas...

Does anyone have any different experience?  I only have one radio to
test--mine.  Perhaps yours is different.

Best of 73,
Tom / W7GT

----- Original Message -----
From: "Arie Kleingeld en Marian van Boheemen" <kkk.bbb@12move.nl>
To: <1000mp@mailman.qth.net>
Sent: Saturday, December 15, 2001 7:52 AM
Subject: Re: [1000mp] Main versus Sub Receivers


> Byron,
>
> Yes, this makes sense!
> I noticed a small difference is  the s-meter readings also, but only under
> s8 or so.
> The s-meter of  the main receiver is not to be trusted, try listening to
an
> s8 signal and then swith in the attenuator. Ever 6 dB the s-meter should
> drop 1 s-point. Well, it drops, but never one s-point....... try this! My
> s-meter drops at least 2 or 3 s-points. So dont worry about those
s-meters.
> By setting the if gain to 13, the agc kicks in with weaker sigs, so the
> s-meter will come up on those weaker sigs. When the sigs are above s9,
> difference of a few dBs is not noticable on any of the two meters.
>
> About the pre-amp: the flat amp gives more gain on 40 than the tuned amp.
On
> 40 I can swith off the tuned amp (IPO) and see barely any difference
(maybe
> one or two dB, not more). Can anybody tell me why it is there?
>
> Happy new year everybody.
>
> Arie, PA3AAM
>
>
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