Measurement Principles
R&S
®
RT-ZS10E/10/20/30
35
User Manual 1410.3531.02 ─ 07
4.2.1.3
RF Resistance
The RF resistance R
RF
determines the minimum input impedance and thus the
maximum loading at high frequencies above 1.0 GHz. Thus, the measurement
result depends on the source impedance of the DUT. The RF resistance prevents
the input voltage from rising immediately to its final value for fast transients.
The initial voltage V
init
depends on R
RF
, see
Figure 4-9
.
4.3
Probing Philosophy
The previous sections explained that probes exert a load on the signal to be mea-
sured and change its characteristic. The signal at the test point where the probe
makes contact (V
in
) is therefore different from the signal that was present before
the probe was connected (V
S
). This effect cannot be avoided and occurs with all
real probes – independent of type and manufacturer.
As a result, there are different opinions which signal is the better output of the
probe:
●
The initial signal that is not loaded by the probe (V
S
), and that corresponds to
the signal at the test point without the probe being connected.
●
The input signal that is loaded with the input impedance of the probe (V
in
) and
that is present between the probe tips.
Both approaches are physically correct and have their individual advantages and
disadvantages. In theory, it is even possible to convert mathematically the two
measurement results into each other, but conversion is a complex transformation
to and from the frequency domain. Probe manufacturers use one or the other of
these two approaches.
Rohde & Schwarz has decided in favor of the user-friendly approach. In our opin-
ion, most users want to know the signal present in the DUT before it was altered
by the influence of the probe. Their goal is to characterize the DUTs, not the
probe.
If measurements are carried out in a 50
Ω environment, the signal displayed on
the oscilloscope's screen is always a direct representation of the unloaded signal
V
S
, see
Figure 4-10
.
Probing Philosophy
