[1000mp] S Meter in MP

[Paul Kiciak]

I ran a test of my MP's S-meter a couple of years ago and
noted similar trends to Tod's & Tom's test results -
reasonable linearity above S9 and lousy below S9. I also
used the somewhat arbitrary 6 dB per S unit convention &
expected to see a 48 dB RF input change to go from S9 down
to S1 indicated, but only saw about half of that.

Furthermore, I found the AGC threshold on my MP on 20m to be

about -108 dBm or about 0.9 uV. In other words, below that
level, the audio output from my MP drops in direct
proportion to its RF input. That's not very good considering

the MP's minimum detectable signal level is some 20 to 30 dB

below that, depending on the Pre-amp setting.

My MP's S-meter, at the time, read S9 on 20m for an RF input

signal level of  -73 dBm, or 50 uV signal. Lots of
granularity though given the number of bars on the S-meter.
That puts my MP's total AGC range at about 95 dB, assuming
it doesn't lose control at S9+60, which I did not test.

Since that test, I read "A High-Performance AGC/IF
Subsystem" in QST, May 1996, by Bill Carver, then K6OLG &
now W7AAZ. After reading Bill's article, at least part of
the reason for the MP's S-meter compression below S9 became
clear.

W7AAZ's IF has an AGC threshold of -131 dBm, when referred
to
the RF input. That's about 23 dB below my MP's AGC
threshold. So there just isn't anything available to drive
my MP's S-meter below -108 dBm. In an ideal world where each

S unit is 6 dB and S9 is 50 uV or -73 dBm, then S1 would be
-121 dBm.

And there's the rub - the MP's AGC system simply runs out of

gas before getting down to a 'real' S1.

Strangely though my MP's S-meter then read S1 at about -97
dBm, so there's a gray area of about 11 dB from -108 to -97
dBm where the AGC voltage is apparently changing, could
affect the S-meter, but does not.

By the way, my S1 measurement is consistent with Doug's
comment on the MP's Technical Supplement setup for 1 segment

on the S-meter. So Yaesu fully intends for S1 to be some
24 dB shy of the S1 ideal.

W7AAZ's IF, with its nearly perfect linear-in-dB AGC voltage

over a 120 dB range, easily displays S1 correctly, as well
as S9+60, and anything in between. I have not heard Bill's
IF yet, but have been told that listening to an AGC system
that handles this wide an input signal range, with good
attack and hang characteristics, quickly becomes addictive.

So, given the state of the art in IF amplifier & AGC
technology, there just doesn't appear to be any technical
reasons for having poorly calibrated S-meters in modern
transceivers.

By the way, Tom, Bill made a comment in his article similar
to yours regarding the apparent inability of high priced
transceivers to properly account for changes in band and
preamps in indicated S-meter readings.

--
73, Paul
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