Measurement Principles
R&S
®
RT-ZD40
31
User Manual 1410.5363.02 ─ 05
5
Measurement Principles
The R&S
RT-ZD40 differential probe provides an electrical connection between
the DUT and the oscilloscope. The probe transfers the voltage of the electrical
signal tapped off the DUT to the oscilloscope, where it is displayed graphically.
Although a probe has a wide variety of specifications, these specifications can be
grouped into two classes of basic requirements:
●
High signal integrity of the transferred signal:
With an ideal probe, the output signal that is transferred to the base unit is
identical to the input signal between the probe tips, and signal integrity is
extremely high. Every real probe, however, transfers the input signal in altered
form. A good probe causes only minimum alterations.
How the probe can fulfill this requirement is mainly determined by its band-
width and CMRR.
●
Low loading of the input signal:
Every probe is a load for the signal to be measured. The signal to be mea-
sured changes when the probe is connected. A good probe causes only a
minimum change to the signal, so that the function of the DUT is not adversely
affected.
How the probe can fulfill this requirement is mainly determined by its input
impedance.
The parameters of a probe are usually specified for a minimally short connection
between the probe and the DUT. With longer connections, the connection induc-
tance has a significant effect on the measurement.
The high-frequency behavior of differential probes is typically characterized in the
following environment. The probe is connected to a differential 100 Ω line that is
fed by a source with 100 Ω internal impedance and that is terminated into 100 Ω.
The voltages at both inputs of the probe are always oppositely equal. The
shows the equivalent circuit model of a probe that is connected to the
DUT.
