Posted by
Ron D'Eau Claire-2 on
Jun 16, 2005; 9:56pm
URL: http://elecraft.85.s1.nabble.com/Re-Elecraft-Digest-Vol-14-Issue-19-S-meter-S-units-tp379453p379454.html
WA6BFH wrote:
"don't believe the old wives tale about 6 dB's per S-unit. It is the rare
radio indeed where that is true!..."
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Back when the "old wife" was a YL 6 dB/S-unit was largely true. I tested a
number of receivers from the 40's and 50's including my HRO-5 and several
Hallicrafters units of the period and the S-meter calibration tracked within
a dB of 6 dB/S-unit across the range.
Those old receivers simply monitored the AGC voltage produced by rectifying
some of the signal at the "second detector" and used the d-c level produced
to bias the i.f./r.f. amplifier tubes to reduce their gain. As a result the
AGC could be used only on amplitude modulated phone. The AGC had to be
turned off for SSB or CW because the BFO signal would be rectified and would
shut down the gain of the receiver, rendering it 'deaf'.
Newer rigs are something else. When SSB became the dominant voice mode on
HF, things started to change with more exotic AGC systems that could handle
both CW and SSB. Many rigs use extra amplifiers in the AGC. Some rigs, like
the Elecraft K2, use a completely separate and different i.f. from the main
signal i.f. to drive the AGC detector. Having a dedicated AGC i.f. avoids
getting the BFO signal into the AGC rectifier and rendering the receiver
deaf.
I've never bothered to measure any "modern" receivers to see how close they
track, but I'd not be surprised to find a large range of behavior. On the
other hand, that behavior may be more controllable.
Ron AC7AC
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