Measurement Principles
R&S
®
RT-ZS60
38
User Manual 1418.7342.02 ─ 01
4.3.1.3
RF Resistance R
RF1
and R
RF2
R
RF1
and R
RF2
(summarized R
RF
) determine the input impedance in the frequency
range from 20
MHz to 2
GHz. Due to the constantly high input impedance of
300
Ω
over the whole range, the loading of high-frequency signals in 50
Ω
environ-
ments is very small.
4.3.1.4
Input Capacitance C
in
and Minimum Input Impedance |Z
min
|
The input capacitance C
in
causes the input impedance to decrease for high fre-
quencies above 2
GHz. C
in
is very low - Typically under 300
fF.
The minimum input impedance |Z
min
| mainly depends on the the connection induc-
tance and the connection type. An overview is given in
.
4.4
Probing Philosophy
The previous sections made clear 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.
This has resulted in a difference of opinion as to which of the following two signals
should be output by the probe:
1. 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.
2. The input signal that is additionally loaded with the input impedance of the probe
(V
in
) and that is actually present between the probe tips.
Both approaches are physically correct and have their individual advantages and
disadvantages. In theory, it is even possible to mathematically convert the two
measurement results into each other, but only by performing a complex transfor-
mation to and from the frequency domain. Probe manufacturers will use one or the
other of these two approaches.
Probing Philosophy
