12 Measurementsand signal electronics
Quite a number of measuring instruments are required in power electronics in order to
characterise the semiconductor components and they are also used in the application,
for example tomonitor that it is functioning properly and gather data for the control. The
following chapters cover the measuring instruments that are commonly used,
accessories and, where applicable, the signal electronics that may be required.
12.2 Digital storageoscilloscope (DSO)
Based on the principle of Braun tubes
, analogue oscilloscopes have long been a
standard measuring instrument for displaying the chronological behaviour of electric
currents and voltages. An analogue oscilloscope comprises a cathode ray tube, beam
focusing, deflection units for the electron beam, amplifiers and a screen.
The incandescent cathode of the cathode ray tube emits free-floating electrons, which
are accelerated towards the screen by the voltage applied and narrowed to a beam in
the focusing unit. As it runs through the deflection sections, the beam is deflected
towards X and/or Y, depending on the signal applied. Then, when they fall on the
luminance layer of the screen, the electrons leave a luminous spot.
Deflection in the X-axis represents the timebase of the signal and is realised by a time-
proportional, periodic voltage (saw tooth voltage). An image of the signal itself is formed
by deflection in theY-axis. To achieve this, it is fed into a signal amplifier that generates
the voltages necessary for thedeflection, in proportion to the signal course.
The principles of the design of an analogueoscilloscope are shown i
The advantage of an analogue oscilloscope is that it maps the signal to be measured
directly. One serious disadvantage that must be mentioned is that mapping one-off
events requires great effort. This is why DSO have now all but replaced analogue
oscilloscopes. DSO offer many advantages besides simple recording and editing of
individual and regular signals, because they digitalise and store the signal. The following
functions are particularly noteworthy:
Integration ofmathematical functions
Signals are shown in colour
Export ofmeasurement to data processing software
The Braun tube (also known as the cathode ray tube) was developed by German physicist Karl Ferdinand
Braun (1850 - 1918) in1897.