The laboratory generator accuracy is expressed in dBm and is relative
to its output power when closed on its nominal load. The receiver S-
meter circuit is not a power meter but only a selective (relatively
narrow band) micro-voltmeter, nothing else.
Checking the voltage of an unterminated 50 Ohm generator with any other
high impedance voltmeter (i.e. an oscilloscope), we find higher
readings than at 50 Ohm but this is not preventing at all the voltage
meter we are usein in beeing an accurate instrument.
A communications receiver S-meter can only pretend to indicate what's
the voltage at the antenna socket and the amount of the possible signal
variations.
The voltage on the antenna terminals is surely the resultant of complex
factors like the antennas (and lines transformed impedances) and the
receiver own impedance, but again there's no impact on the AGC line
derived metering function that stay correct or wrong in any case.
73,
Mauri I4JMY
> ---------- Initial message -----------
>
> From : owner-amps@contesting.com
> To : amps@contesting.com, "'Ian White, G3SEK'"
<g3sek@ifwtech.com>
> Cc :
> Date : Wed, 14 Jun 2000 09:22:01 +0100
> Subject : RE: [AMPS] s meter calibration
>
>
> Question # 1: so what does it tell you as an operator if a signal is
50
> microvolts or 25 microvolts in terms of real, on air operation?
bearing in mind
> that the noise on 80m can easily be S5 and a really good sig on
10metres fine at
> S2? ('You're sounding an S9 signal, but you're only a 2 on the meter')
>
> Question # 2: So how many transceivers have the necessary gain
stability over
> temperature, time and supply voltage for it to be meaningful?
>
> Question # 3: How good is your signal generator accuracy? Hewlett-
Packard as was
> (Agilent now, or genitAl for those with a puckish sense of humour)
rate their
> generators as +/-1dB for the better ones, +/-1.5dB for the others.
(Accuracy,
> NOT resolution). Now that is into an accurate 50ohm lo
s
the input
> return loss of the receiver, and how stable is it with time
temperature et al?
> From a system point of view, how low is the SWR on the antenna?
>
> The result of all this in the end is that the S meter reading is
pretty
> meaningless, and unless the receiver has known gain stability,
including over
> the frequency range, even the relative readings are suspect. That's
why level
> measuring receivers are so pricey.
>
> Which is why I say that the 'S' in 'S meter' stands for suspicious.
>
> 73
>
> Peter G3RZP
>
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