If you want to make accurate RF signal level measurements with an
oscilloscope there are several things to consider.
First, you need to minimize the loading on the circuit by the scope, so
that the operation of the circuit is minimally affected by the
connection of the scope. This is why 10X probes are most often used. A
1X probe may load the circuit with 1 Megohm resistance and many
picofarads capacitive reactance. A 10X probe would be more like 10
Megohms resistive and only a few picofarads.
If you want to compare amplitudes at various frequencies, it is very
important that the probe capacitance is properly compensated to match
the scope input. Otherwise the frequency response will not be flat and
comparisons on different bands ( even if the absolute level measurement
is not important ) will not be accurate.
The bandwidth rating of oscilloscopes is the -3 dB frequency. This means
that using a 50 MHz scope when you measure a constant level sine wave
generator the level indicated by the scope at 50 MHz will be 0.707 of
what it is at lower frequencies. If you want to compare levels in a
transmitter low level driver and power amplifier on various bands 160 -
10 meters, a 50 MHz scope is just barely good enough. A 30 MHz scope is
not good enough, unless you know accurately the scope's response and do
the math to adjust your readings. If you don't need accurate
measurements, but just want to see the level increase through the stages
of the amplifier chain, you can get by with less of a scope.
DE N6KB
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