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Power Quality Measurements 203

Probe
leads

Differential BNC cable Oscilloscope
probe output

Figure 14.2 Measurement system for making high voltage differential
measurements.

measure the voltage difference between two points in a system. Many
manufacturers make high voltage differential probes that are compat-
ible with oscilloscope inputs. Typical specifications for differential
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probes are
■ Bandwidth: 100 MHz
■ Differential voltage: 5000 V
■ Common-mode voltage: 2000 V
A typical system for making high voltage differential measurements
is shown in Figure 14.2. We see that the output of the differential probe
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is a 50- BNC cable that interfaces with an oscilloscope. The differen-
tial probe has a conversion ratio (millivolts per volt) that brings down
the signal levels to the oscilloscope to a manageable level.

Current Probes
Current probes come in several different types. Hall-effect current
probes (like that used in the Tektronix P6042) detect all the way down
to DC by utilizing the “Hall effect.” Hall devices are semiconductors
that generate an output voltage in response to a magnetic field created
by a current. Hall probes have the advantage that they provide output
at DC. Unfortunately, they are prone to drift and should be calibrated
prior to use.
Other types of current probes use current transformers (CTs). CT cur-
rent probes do not give an output at DC since they are based on trans-
former action. Atypical test setup using a CT and an oscilloscope is shown
in Figure 14.3. CT current probes give a conversion ratio (volts per amp)
when terminated in the proper resistance—in most cases 50 .

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See, for example, the Tektronix P5200 high-voltage differential probe.
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BNC cables are coaxial cables used in instrumentation and video. A BNC cable is char-
acterised by its transmission line characteristic impedance, which is typically 50 or 75