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Example
Equation 5
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.
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.
2
2
t
=
(
t
1
)
+
(
t
2
)
,
r
r
r
where
t
1 is the rise time of the probe,
r
t
2 is the rise time of the signal.
r
1. Calculate the rise time of the 700-MHz probe (equation 4).
0.35
t
=
---------------------------- -
=
--------------------- -
r
Bandwidth
700MHz
2. Calculate the rise time of the 1-ns signal as measured by the 700-MHz
probe (equation 5).
2
t
=
(
0.5
)
+
(
1.0
)
r
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).
0.35
Bandwidth
=
--------- -
1ns
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).
2
t
=
(
0.333
)
+
(
1.0
r
Now, the measurement error is less than 6%.
0.35
=
0.5ns
2
=
1.25
=
1.12ns
=
350MHz
2
)
=
1.11
=
1.054ns
Chapter 2: Probing Considerations
Resistive Loading Effects
31
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