Re: [Amps] 10 DB increase

[David Kirkby <david.kirkby@onetel.net>]

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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