At 18:51 03/05/2002 +0200, you wrote:
My slide rule says: 2 dBm with 600 ohms = 0.98 V Ueff
and RMS (roots mean square) nevertheless Ueff means or?
I have a Rycom 3136 level meter, with similar scales. There are 3 input
impedance settings - 600, 125, 75 ohms. The dBm scale measures the input
power, with 0dBm = 1mW. It always reads correct, whatever impedance is
selected - so with the same voltage across the input terminals, changing
Rin from 600 to 75 ohms increases the reading by 10log(600/75) = 9dB,
because more power is delivered to the low impedance load for the same
input voltage. It does this trick by having an input transformer with
different taps for the different impedances. However, this also means the
voltage scales are only scaled correctly when 600 ohm Rin is selected - it
reads about 7dB high on 125 ohms, and 9dB high on 75ohms. In addition,
changing the input impedance will usually change the voltage delivered by
the source, unless the source impedance is very low, or the "unterminated
input" setting is used.
The W&G meters I have allow you to select dBm or dBu. dBu is the voltage at
the input with 0dBu defined as 0.775V (=1mW into 600 ohms), irrespective of
the input impedance. So 0dBuV is -117.8dBu, or in 75ohms -108.8dBm.
So I sometimes get totally confused! If in doubt, always use the dBm
measurement, and work out from that and the selected input impedance what
the voltage is. If you want to measure voltage directly, use the 600 ohm
input, and connect a resistor in parallel to get the right input impedance
if necessary.
Cheers, Jim Moritz
73 de M0BMU
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