Teledyne LeCroy ZS4000 - Manual (Page 18)

ZS4000 High-Impedance, Active Probe
12 923360-00 Rev A
Where C
total
is the combined probe and circuit capacitance and R
total
is combined
circuit and probe resistance.
For a setup where Ct = 0.6 pF and a source resistance is 250 Ω, the measured rise
time will be 330 ps, which will correspond to a bandwidth of 909 MHz, assuming
no inductive loads.
t
rise
= 2.2 x 0.6 x 10
-12
x 250 Ω = 330 ps
Parallel combination of 250 Ω and 1 MΩ is still 250 Ω
Probe input equivalent circuit
To illustrate the effect of capacitive loading at higher frequencies:
At a frequency of 750 MHz the reactance of the 0.6 pF capacitance is 354 Ω, and
at 1.0 GHz the reactance has been lowered to 265 Ω.
If, at a given frequency, the source impedance is large with respect to the input
impedance, a measurable reduction in the output signal amplitude may occur.
where Z
probe
is the probe’s input impedance and Z
source
is the source impedance.
For example: At 750 MHz, where the probe input impedance has reduced to 354
Ω, and a source resistance of 250 Ω the probe output amplitude is reduced to: