PicoVNA 106 6 GHz Vector Network Analyzer
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© 2017 Pico Technology
pv106ug r1
For a transmission line with a short circuit (figure above) the incident signal sees the characteristic
impedance of the line so the scope measures Ei. The incident signal travels along the line to the short circuit
where it is reflected back 180° out of phase. This reflected wave travels back along the line canceling out the
incident wave until it is terminated by the impedance of the source. When the reflected signal reaches the
scope the signal measured by the scope goes to zero as the incident wave has been canceled by the
reflection. The result measured by the scope is a pulse of magnitude Ei and duration that corresponds to the
time it takes the signal to pass down the line to the short and back again. If the velocity of the signal is
known, the length of the line can be calculated:
where
v
is the velocity of the signal in the transmission line,
t
is the measured pulse width and
d
is the length
of the transmission line.
4.7.7.1.2
Example: open-circuited 50 ohm transmission line
Simplified representation of the response of a open line
In the case of the open circuit transmission line (figure above) the reflected signal is in phase with the
incident signal, so the reflected signal combines with the incident signal to produce an output at the scope
that is twice the incident signal. Again, the distance
d
can be calculated if the velocity of the signal is known.
4.7.7.1.3
Example: resistively terminated 50 ohm transmission line
Simplified representation of the response of a resistively terminated line
