Mauri says:
>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.
If we are going to set the signal generator to 50 microvolts pd (which is
-73dBm) and then say that means 50 microvolts at the input to the receiver, that
assumes the receiver input impedance. (Incidentally, this is an argument for
maintaining the older European professional standards of measuring receiver
sensitivity in dB microvolts EMF, (or just microvolts EMF) rather than dBm,
which really comes from the electronic warfare/radar world) Remember, we've
defined 50 microvolts as S9 (without saying EMF or PD! - but PD assumed). Our
50 microvolts is +/-1dB GUARANTEED (there's a difference inherent here,depending
on the confidence level, but that's another argument). So in practice, although
you have 'calibrated' the receiver to give S9 when the generator says 50
microvolts or -73dBm, unless you've measured the actual input impedance, you can
easily get an error of 1 S point from the 50 microvolts. (A 3:1 input resistive
SWR can give a PD between +3.5 and -6dB relative to the matched condition)
Hopefully, difference readings on the same frequency measured at points closely
spaced in time can be calibrated, but the best method for differential
measurements is a calibrated attenuator somewhere in the system.
The split between Europe and the US on using EMF and PD goes back many years.
One argument is that as PD gives a 6dB 'better' figure, marketing forces
prevailed in the US partially because in Europe, marketing was nowhere near as
advanced in the commercial world 60 odd years ago. Where the EMF argument is
really useful is in those areas where non-matched antennas are used, such as
maritime radio, where the standard test antenna source impedance is 10ohms in
series with 250pF for frequencies from 1.6 - 3.8MHz.
So at the end of the day, you don't really know if you have 50 microvolts across
the antenna terminals when the generator said so. Then, just to add to the
confusion, even if the antenna provided a signal EMF of 100microvolts, you don't
know how much of that has got to the receiver - unless you accurately know the
antenna source impedance at that frequency, the receiver input impedance at that
frequency and the feeder loss under those SWR conditions.
Finally, it's normally taken that you can only read an analogue scale to 0.4 of
a division anyway with any accuracy - or, if it's 1 division per S unit, roughly
2.5dB.
And as you're giving a report of ' thanks for the new one you're 5 and 9', it's
all a bit meaningless.
73
Peter G3RZP
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