Data Sheet

Proteus VSM
233
Now to add the probes. We shall add both the voltage probes, OUT and U1(POS IP). In a
frequency graph, the two y-axes (left and right) have special meanings. The left y-axis is used to
display the magnitude of the probed signal, and the right y-axis the phase. In order to see both,
we must add the probes to both sides of the graph. Tag and drag the OUT probe onto the left of
the graph, then again drag the robe onto the right. Each trace has a separate colour as normal,
but they both have the same name. Now tag and drag the U1(POS IP) probe onto the left side
of the graph only.
Magnitude and phase values must both be specified with respect to some reference quantity. In
Proteus this is done by specifying a Reference Generator. A reference generator always has an
output of 0dB (1 volt) at 0°. Any existing generator may be specified as the reference generator.
All of the other generators in the circuit are ignored in a frequency analysis. To specify the IN
generator as the reference in our circuit, simply tag and drag it onto the graph, as if you were
adding it as a probe. Proteus assumes that, because it is a generator, you are adding it as the
reference generator, and prints a message on status line confirming this. Make sure you have
done this, or the simulation will not work correctly.