Measurement Principles
R&S
®
RT-ZD40
38
User Manual 1410.5363.02 ─ 05
5.2.1
Signal Loading for Differential Input Signals
The
presents an equivalent circuit model of an R&S
RT-ZD40 differen-
tial probe. The differential input impedance of the probe is equal to the impedance
between its positive (+) and the negative (-) signal socket. The resulting input
impedance versus frequency is indicated in
Figure 5-6: Magnitude of the differential input impedance of the R&S
RT-ZD40 probe as a
function of frequency
The differential input impedance varies greatly versus the frequency and is
defined by the following values:
●
Differential input resistance R
dm
= 2 R
1
●
Differential input capacitance C
dm
= C
1
+ C
2
/ 2
●
Differential RF resistance R
RF
= 2 R
2
5.2.1.1
Input Resistance R
dm
The input resistance determines the loading of the DUT at DC and very low fre-
quencies (<
100
kHz). A low input resistance can potentially disturb measure-
ments of high-frequency signals as it influences the DC operating point of active
components.This effect is negligible for the majority of applications involving the
R&S
RT-ZD40 probe due to the very high input resistance of the probe (1
MΩ).
Signal Loading of the Input Signal
