originally cathode-ray oscilloscopes (CRO) have been employed (Binclerl914;
Gabor 1927; Krug 1927). At that time. the electromagnetic capability (BMC) was not
a problem because the available CRO were designed for HV measurements and thus
not equipped with sensitive amplifiers for vertical and horizontal defiection. In the
1940s, additionally. peak voltmeters have been employed. commonly designed by
the HV laboratory personnel itself using vacuum tabes for rectifying the fast LI
signal. Digital :recorders. originally referred to as "digitiz.ers". entered in HV measuring
technique in the early 1980s (Malewski et al. 1982). After the initial EMC
problems have su~essfully been solved (Strauss 1983 and 2003; Steiner 2011);
digital recorders are nowadays exclusively used for recording and processing LI and
SI test voltages (Fig. 7.55). Due to the recent achievements in digital signal processing
(DSP), computerized digital recorders are also increasingly employed not
only for LI and SI test voltage measurements but also for AC and DC voltage
measurements as well as fm measuring composite and combined test voltages.
A photograph of a stand-alone device operating in connection with an IPC is
shown in Fig. 7.56. and a simplified block diagram of a digital recorder is shown
in Fig. 7 .57. The main units are the voltage attenuator at the input followed by a
low-noise amplifier, an analogue-digital converter, a memory unit and an industrial
computer. A micro-controller serves for the adjustment of the input sensitivity
as well as for controlling the various units for signal processing, acquisition and
data storage. The industrial computer runs with a specific software package
capable for both the acquisition of the stored raw data and the visualization of the
time-dependent input signal. Simultaneously, the relevant impulse parameters are