Agilent Technologies 54701A, Руководство пользователя и сервисное руководство

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Chapter 2: Probing Considerations
Resistive Loading Effects
Probe Bandwidth
The bandwidth of the probe is often given much consideration during
purchase, then forgotten while making measurements. Error in
measurements occur when the frequency content (at the -3 dB point) of the
signal being measured approaches or exceeds the bandwidth of the probe.
The probe can be modeled as a low-pass filter for the signal.
Example
If a 700-MHz probe is used to measure a 1-ns signal, the rise time error can be
calculated using equations 4 and 5. For this exercise assume that the oscilloscope
bandwidth is great enough not to contribute any errors.
Equation 5 ,
where
t
r
1 is the rise time of the probe,
t
r
2 is the rise time of the signal.
1. Calculate the rise time of the 700-MHz probe (equation 4).
2. Calculate the rise time of the 1-ns signal as measured by the 700-MHz
probe (equation 5).
The measurement error between the actual signal and what was measured is 12%.
To keep measurement errors less than 6%, use a probe with a band- width three or
more times that of the signal.
3. Calculate the bandwidth of the 1-ns signal (equation 4).
Use a probe with a bandwidth of 1.05 GHz (the rise time is 0.333 ns, equation 4).
4. Calculate the rise time of the 1-ns signal measured by the 1.05-GHz probe
(equation 5).
Now, the measurement error is less than 6%.
t
r
t
r
1 ( )
2
t
r
2 ( )
2
+ =
t
r
0.35
Bandwidth
-----------------------------
0.35
700MHz
---------------------- 0.5ns = = =
t
r
0.5 ( )
2
1.0 ( )
2
+ 1.25 1.12ns = = =
Bandwidth
0.35
1ns
---------- 350MHz = =
t
r
0.333 ( )
2
1.0 ( )
2
+ 1.11 1.054ns = = =