Re: Elecraft Digest, Vol 14, Issue 19 --- S-meter & S-units

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Re: Elecraft Digest, Vol 14, Issue 19 --- S-meter & S-units

Wa6bfh
One very important consideration of the use of “S-meters”, and “S-units” is
 remembering that signals are heard as a “Square Law Function” Every time
you  double or lengthen the communications path by twice the distance, the
signal has  fallen to one-fourth, or a 6 dB change!
 
It is good to know how many dB per S-unit your receiver‘s meter indicates.  
This will probably not be a linear consideration -- and don’t believe the old  
wives tale about 6 dB’s per S-unit. It is the rare radio indeed where that is  
true!
 
Even better than using “S-units”, give other stations reports in (ACCURATE)  
dBm! You will need the use of a calibrated signal generator to assess your  
receiver.

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RE: Re: Elecraft Digest, Vol 14, Issue 19 --- S-meter & S-units

Ron D'Eau Claire-2
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!..."

----------------------------------------------

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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