Agilent Technologies 54701A, User'S And Service Manual

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Chapter 2: Probing Considerations
Resistive Loading Effects
The measurements from the first three waveforms lead to a rule of thumb:
To minimize signal distortion due to probe resonance, provide a two-to-one, or
greater, difference between the resonant frequency of the probe and the
bandwidth of the signal being measured.
For pulsed data applications, the rise time of a signal can be related to the
bandwidth by using a constant of 0.35 as shown in equation 4. This equation is
derived from a first order RC response.
Equation 4
Bandwidth =
Example
The 1.25-ns edge (waveform 3 in figure 12) equates to a 280-MHz bandwidth.
Bandwidth =
This is approximately half the resonant frequency calculated for the 6-pF
probe with 1/2-inch ground, 580 MHz. Therefore the subdued ringing on
waveform 3 validates the rule of thumb.
As noted before, waveform 4 shows the effect when a low-capacitance probe
measures a high-frequency signal. Because of the low capacitance the
resonant frequency is high. Therefore, there is less chance of the probing
system affecting the measurement of the signal.
0.35
t
r
----------
0.35
t
r
----------
0.35
1.25
- 9
× 10
------------------------ 280MHz = =