More on S-meter and RF gain

[Pete Smith N4ZR]

Experimenting with the RF gain control, and with the RX equalizer.  I
have discovered that the setting of the equalizer to emphasize CW beat
notes and de-emphasize the highs makes a tremendous difference in the
perceived noise level and strength of the audio on CW.  One thing that
puzzles me, though - when I turn down the RF gain, the S meter reading
on a signal being received increases substantially.  For example, I just
worked E74Y, whose very good signal was only S8 with the RF gain all the
way up, but when I turned it back to roughly 1 o'clock, the peaks were
S9 +10.  Does this make sense?

--
73, Pete N4ZR

The World Contest Station Database, updated daily at www.conteststations.com
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[gm3sek]

Pete Smith wrote:

>Experimenting with the RF gain control, and with the RX equalizer.  I
>have discovered that the setting of the equalizer to emphasize CW beat
>notes and de-emphasize the highs makes a tremendous difference in the
>perceived noise level and strength of the audio on CW.  One thing that
>puzzles me, though - when I turn down the RF gain, the S meter reading
>on a signal being received increases substantially.  For example, I just
>worked E74Y, whose very good signal was only S8 with the RF gain all the
>way up, but when I turned it back to roughly 1 o'clock, the peaks were
>S9 +10.  Does this make sense?
>

Yes... in a sense :-)

The behavior of the K3's S-meter in response to the "RF Gain" control is
emulating traditional hardware receivers. What you now see in the K3 is
essentially the same as you've been seeing for many years in the large
majority of traditional receivers.

In traditional hardware receivers, the S-meter is actually reading the
voltage on the AGC line. Stronger signals produce more AGC voltage,
which deflects the meter more while simultaneously reducing the RF/IF
gain to keep the audio signal level fairly constant.

The manual "RF Gain" control functions by applying a permanent
negative-going DC voltage to the AGC line. This has the same effect as a
signal-derived AGC voltage - it reduces the RF/IF gain and causes the
S-meter reading to rise. The only difference is that the S-meter reading
rises to a steady value, and will not fall back when signals go away. If
a signal is strong enough to generate *more* AGC voltage than you have
already applied through the "RF Gain" control, then the S-meter will
rise - but it will never fall back below the baseline level that you
have set.

In other words, that steady baseline meter reading does *not* represent
the strength of any weak signals. It's better to think of that steady
reading as the "AGC threshold", below which the S-meter readings are
simply not available. True signal strength readings are only available
for signals that are strong enough to deflect the meter *above* that
threshold.

The vast majority of traditional receivers behave like this, and the K3
emulates that behavior quite faithfully.

The only major difference is that the K3 doesn't have any
gain-controlled RF stages. Both the manual "RF Gain" and the AGC are
implemented in DSP at the 15kHz IF, with backup from the hardware AGC
loop in the 8.215kHz IF stage. The only gain controls that genuinely
change the levels at the RF signal frequency are the ATT and PREAMP
buttons.

Bottom line: don't ever expect an "RF Gain" control to do literally what
the label says. You never could... and you still can't.



--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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