The History and Technology of Oscilloscopes an Overview of Its Primary Characteristics and Working Principles

PHOTOS: NASA GLENN RESEARCH CENTER (NASA-GRC) & GRID PATTERN- © DIGITAL VISION The History and Technology of Oscilloscopes An overview of its primary characteristics and working principles scilloscopes are one of the main tools for analyz- ing principle is the same there are two main types of oscillo- ing electrical signals. The primary information scopes: analog and digital. obtained from the waveform of the signal is The aim of this article is to provide an overview of the Ovisualization of its amplitude variation over main characteristics of the different types of oscilloscopes time. Oscilloscopes are excellent tools for testing, debug- and correlate their evolution with the development of the ging, and troubleshooting because they can easily detect underlying technologies they incorporate. waveforms and demonstrate if the electrical components or circuit modules J. Miguel Dias Pereira Introduction are working properly. Oscilloscopes André-Eugène Blondel was a French also provide support during the design physicist who was born on 28 August of new electronic circuits. In addition to electrical signals, 1863. He is known as the inventor of the electromagnetic other physical or chemical quantities can be measured by oscillograph, a device that enabled the observation of alter- using different probes that have been developed into an nating signals. The first oscillographs traced an ink record appropriate transducer. on a moving paper chart with a pen arm attached to a mov- Even if the basic philosophy of every oscilloscope’s working coil. As a consequence of the working principle based December 2006 IEEE Instrumentation & Measurement Magazine 27 1094-6969/06/$20.00©2006IEEE on a set of mechanical devices, the first oscillographs had a ◗ visualization of the signal waveform in the display unit very low bandwidth in the range of 10–19 Hz. ◗ the ability to measure and analyze the electrical signal The first evolution of these instruments came with the and to store or print the measurement results. development of light-beam oscillographs. In these instru- The hardware block diagram includes typically five func- ments, there was still a moving coil but this coil was attached tional blocks: to a mirror and a light beam was reflected onto a moving ◗ vertical channel photographic film. With these instruments, the mechanical ◗ horizontal channel bandwidth restrictions were a little bit reduced and the band- ◗ time basis width increased to 500 Hz. ◗ trigger Some years later, in 1897, Karl Ferdinand Braun invented ◗ display unit. the cathode ray tube (CRT). The British company A.C. Acquisition of the electrical signal is performed by the Cossor (later acquired by Raytheon) designed the first dual vertical channel of the oscilloscope that contains the electri- beam oscilloscope in the late 1930s. It applied an oscillating cal interface circuits and amplifiers. They are used to get the reference signal to horizontal deflector plates and the input correct amplitude of the signals that are delivered to the hor- measured signal to the vertical deflector plates. Images of izontal deflector plates of the CRT. transient electrical signals were then obtained on a small The horizontal channel generates a signal that is applied phosphor screen. to the vertical deflector plates of the CRT. This signal has a In 1946, Howard C. Vollum and Jack Murdock invented saw-toothed waveform when the instrument is to provide the the triggered oscilloscope that synchronizes the graphical temporal representation of the acquired input signal (Y) or it representation of repetitive signal waveforms. Since then, has an arbitrary waveform from the external input (X), when and especially after the Tektronix [1] foundation, the majority the oscilloscope is used in the X-Y representation mode. of oscilloscope manufacturers [2]–[8] have technically The oscilloscope time basis unit contains the circuits that improved their products. Bandwidth and accuracy have con- generate the saw-toothed waveform, which provide the hori- tinuously increased, first with analog oscilloscopes and later zontal sweep of the CRT electronic beam. The time basis also with digital sampling oscilloscopes that enable measurement provides a blanking pulse to extinguish the electronic beam of bandwidths in the range of tens of gigahertz. between sweep intervals, during which the waveform is dis- Oscilloscopes became an essential instrument to support played. Without the blanking pulse, the return of the elec- technological development in all engineering areas. Digital tronic beam, from the right edge to the left edge of the technology associated with digital phosphor oscilloscopes display, would be visible by the user. enables the measurement of statistical data (e.g., jitter) that The trigger unit contains a set of circuits that generates were unavailable some years ago. Now oscilloscopes enable the timing signals to synchronize the start of sweep with many more functions than a simple representation of time timing pulses generated from the input signal (internal varying signals; digital signal processing techniques are trigger) or from an external signal (external trigger). This adding new functionalities of spectrum and logic analyzers triggering function is essential to achieve a stable image in to modern oscilloscopes. the display unit. Without triggering, multiple copies of A few words must also be dedicated to the role of oscillo- the waveform are drawn in different places on the dis- scopes in teaching activities. It is difficult to find a more play, giving an incoherent jumble or a moving image on complete instrument for didactic purposes. Analog and digi- the screen. tal versions of oscilloscopes are by themselves a complete As an example, Figure 1 represents a periodic input sig- study case for several electrical engineering subjects includ- nal (y), the sweep signal (sw), and the waveform displayed ing signal conditioning, analog-to-digital conversion, analog in the CRT unit. The trigger threshold has zero amplitude signal processing, digital signal processing, and communica- and a positive edge-trigger set-up and the hold-off time is tion protocols (e.g., RS232, USB, GPIB and Ethernet). equal to the signal period (T). The sweep signal has a period Modern oscilloscopes can also be connected to a network for three times the input signal period (TH = 3T) and the sweep printing, file sharing, Internet access, and advanced commu- speed is equal to T/5 s/div, assuming a display unit with nication functions like sending e-mails triggered by pro- the typical ten divisions in the horizontal time axis. grammed events. The synchronization between input and sweep signals, implemented by the trigger circuits, is essential to obtain a Oscilloscope Functional Blocks stable image on the screen, which means multiple sweeps To simplify the description, I have chosen to explain a classical with the same waveform. The synchronization is still analog oscilloscope with a vector display unit based on a CRT. obtained as long as the input signal is repetitive, not neces- Basically, an oscilloscope performs the following main functions: sarily periodic, and has a minimum update rate. ◗ acquisition of the input electrical signal The oscilloscope display unit was initially a CRT where ◗ signal conditioning (attenuation/amplification) the waveforms become visible due to the impact of the elec- ◗ synchronization tasks that provide a stable representa- tronic beam on a fluorescent and phosphorescent coating tion of the input signal material. Currently, the CRT display units are being 28 IEEE Instrumentation & Measurement Magazine December 2006 replaced by the thin film transistor liquid crystal display output trigger pulse that must define the start of sweeps (TFT LCD) [9]. These displays can achieve high brightness at accurately is obtained from the output of a derivative circuit. low drive voltages and current densities, which result in The CRT is a special kind of vacuum tube that contains more compact units with a lower power consumption. an electron gun, a set of vertical and horizontal deflector plates (mentioned previously), several electronic lenses, Oscilloscope Types anodes, and a display internally coated with a fluorescent Oscilloscopes can be either analog or digital. There are still a and phosphorescent coating material. Figure 2 represents a large number of analog oscilloscopes in use, but they are simplified version of the hardware block diagram of an ana- being gradually replaced by digital oscilloscopes. Much like log oscilloscope. PCs, the cost of digital oscilloscopes is dropping, and they Figure 3 is an old model of a didactic oscilloscope from are using the latest, low-cost, electronic developments in Siemens [10] that has its electrical schematic diagram displayed components. Equivalent time-sampling techniques are used on the front panel. This laboratory oscilloscope provides easy in the sampling oscilloscope to extend the bandwidth when- access to multiple internal signals. It is possible to simultane- ever repetitive and stable high frequency signals are mea- ously display the external input signal and multiple test-point sured. Digital phosphor oscilloscopes enable the signals of the main internal circuits of the oscilloscope. representation of an electrical signal in three dimensions, Typically the bandwidth of an analog oscilloscopes is in the time, amplitude, and amplitude over time, using an almost hundreds of megahertz and the main limitation is the CRT dis- real-time screen

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