Teledyne LeCroy HFP2500 - Manual (Page 27)

Operator’s Manual
922253-00 Rev A 21
For a setup where C
t
= 0.7 pF and a source resistance is 250 Ω, the measured rise time will be 0.385
ns, which will correspond to a bandwidth of 909 MHz, assuming no inductive loads.
( t
rise
=2.2 x 0.7 X 10
-12
x 250 Ω = 0.385 ns )
( parallel combination of 250 Ω and 100 kΩ is still 250 Ω )
Figure 4, Probe Input Equivalent Circuit
To illustrate the effect of capacitive loading at higher frequencies:
At a frequency of 851 MHz the reactance of the 0.7 pF capacitance is 267 Ω, and at 1.5 GHz the
reactance has been lowered to 152 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 851 MHz, where the probe input impedance has reduced to 267 Ω, and a source
resistance of 250 Ω the probe output amplitude is reduced to: