Measurement Principles
R&S
®
RT
‑
ZD10/20/30
47
User Manual 1410.4550.02 ─ 05
6.2.1
Signal Loading for Differential Input Signals
The
presents an equivalent circuit model of an R&S
RT
‑
ZD10/20/30
differential probe. The differential input impedance of the probe is equal to the
impedance between its positive (+) and the negative (-) signal socket. The result-
ing input impedance versus frequency is indicated in
Figure 6-6: Magnitude of the differential input impedance of the R&S
RT
‑
ZD10/20/30 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
6.2.1.1
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
‑
ZD10/20/30 probe due to the very high input resistance of the probe
(1
MΩ).
Signal Loading of the Input Signal
