They are only the same only if input and load resistances are the same.
When these resistances are not equal the 20log Vo/Vi is usually used. An
example would be the gain of an audio interstage amplifier or an Audio power
amplifier which would have perhaps 1k input resistance and perhaps 4 or 8 ohm
load resistance.
73
Bill wa4lav
-----Original Message-----
From: "Radioal" <al.dolgosh@hamradio.org>
To: "hermans" <on4kj@skynet.be>, "'David Kirkby'" <david.kirkby@onetel.net>,
"'Bob Alexander'" <realex@flash.net>
Date: Sat, 5 Feb 2005 18:33:33 -0500
Subject: Re: [Amps] 10 DB increase
Thse are the same formulas - one stated in terms of power and one in terms
of voltage.
Al - K8EUR
----- Original Message -----
From: "hermans" <on4kj@skynet.be>
To: "'David Kirkby'" <david.kirkby@onetel.net>; "'Bob Alexander'"
<realex@flash.net>
Cc: <amps@contesting.com>
Sent: Saturday, February 05, 2005 5:51 PM
Subject: RE : [Amps] 10 DB increase
> Any relation between (G)db = 10.log Po/Pi and (G)db = 20.log Vo/Vi to
> explain the difference between Tx power and Rx S points .....!?
>
> Jos on4kj
>
> -----Message d'origine-----
> De : amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] De
> la part de David Kirkby
> Envoyà : samedi 5 fÃvrier 2005 14:34
> Ã : Bob Alexander
> Cc : amps@contesting.com
> Objet : Re: [Amps] 10 DB increase
>
> 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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>
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