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Re: [Amps] 10 DB increase

To: Bob Alexander <realex@flash.net>
Subject: Re: [Amps] 10 DB increase
From: David Kirkby <david.kirkby@onetel.net>
Date: Sat, 05 Feb 2005 14:34:01 +0000
List-post: <mailto:amps@contesting.com>
Bob Alexander wrote:

I went back to the archive to find the original question.
S-meters do not measure received signal level accurately because they do not
measure the signal but rather the level of the AGC voltage required to
maintain
a constant audio level output.

Most S-neters are adjusted to provide an S9 indication with a specific input
signal
level at the antenna terminals...usually around 50 uV.  A 10 dB decrease in
this signal
leaves 15.8 uV at the antenna while a 10 dB increase results in 158 uV at
the antenna.
Remember we are talking voltage not power.

The AGC/ S-meter system is different in every receiver. Even two identical
receivers may require a slightly different change in voltage to provide the
same
degree of AGC action.


With a modern receiver, it would be relatively easy for the manfacturer to correct for this. An A/D to measure the AGC voltage and a D/A to drive the meter, with a trivual bit of processing in between. In fact, it would not even need a processor, as an EPROM or other non-volatile bit of memory could do it using a lookup table.

Whether you would consider it worth while is another matter, but given all the bells and whistles on modern equipment, perhaps a calibrated S-meter would be less useless than some of the other things.



G8WRB

Please check out http://www.g8wrb.org/ of if you live in Essex http://www.southminster-branch-line.org.uk/



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