History of Oscilloscope

Hand-drawn oscillograms
Illustration of Joubert's step-by-step method of hand-plotting waveform measurements. [4]

The earliest method of creating an image of a waveform was through a laborious and painstaking process of measuring the voltage or current of a spinning rotor at specific points around the axis of the rotor, and noting the measurements taken with a galvanometer. By slowly advancing around the rotor, a general standing wave can be drawn on graphing paper by recording the degrees of rotation and the meter strength at each position.

This process was first partially automated by Jules François Joubert with his step-by-step method of wave form measurement. This consisted of a special single-contact commutator attached to the shaft of a spinning rotor. The contact point could be moved around the rotor following a precise degree indicator scale and the output appearing on a galvanometer, to be hand-graphed by the technician. [5] This process could only produce a very rough waveform approximation since it was formed over a period of several thousand wave cycles, but it was the first step in the science of waveform imaging.

Automatic paper-drawn oscillograph

Schematic and perspective view of the Hospitalier Ondograph, which used a pen on a paper drum to record a waveform image built up over time, using a synchronous motor drive mechanism and a permanent magnet galvanometer. [6][7]

The first automated oscillographs used a galvanometer to move a pen across a scroll or drum of paper, capturing wave patterns onto a continuously moving scroll. Due to the relatively high-frequency speed of the waveforms compared to the slow reaction time of the mechanical components, the waveform image was not drawn directly but instead built up over a period of time by combining small pieces of many different waveforms, to create an averaged shape.

The device known as the Hospitalier Ondograph was based on this method of wave form measurement. It automatically charged a capacitor from each 100th wave, and discharged the stored energy through a recording galvanometer, with each successive charge of the capacitor being taken from a point a little farther along the wave. [8] (Such wave-form measurements were still averaged over many hundreds of wave cycles but were more accurate than hand-drawn oscillograms.)

Photographic Oscillograph

Top-Left: Duddell moving-coil oscillograph with mirror and two supporting moving coils on each side of it, suspended in an oil bath, Top-Middle: Rotating shutter and moving mirror assembly for placing time-index marks next to the waveform pattern. Top-Right: Moving-film camera for recording the waveform. Bottom: Film recording of sparking across switch contacts, as a high-voltage circuit is disconnected.[9][10][11][12]

In order to permit direct measurement of waveforms it was necessary for the recording device to use a very low-mass measurement system that can move with sufficient speed to match the motion of the actual waves being measured. This was done with the development of the moving-coil oscillograph by William Duddell which in modern times is also referred to as a mirror galvanometer. This reduced the measurement device to a small mirror that could move at high speeds to match the waveform.

To perform a waveform measurement, a photographic slide would be dropped past a window where the light beam emerges, or a continuous roll of motion picture film would be scrolled across the aperture to record the waveform over time. Although the measurements were much more precise than the built-up paper recorders, there was still room for improvement due to having to develop the exposed images before they could be examined.

CRT Invention

Cathode ray tubes (CRTs) were developed in the late 19th century. At that time, the tubes were intended primarily to demonstrate and explore the physics of electrons (then known as cathode rays). Karl Ferdinand Braun invented the CRT oscilloscope as a physics curiosity in 1897, by applying an oscillating signal to electrically charged deflector plates in a phosphor-coated CRT. Applying a reference oscillating signal to the horizontal deflector plates and a test signal to the vertical deflector plates produced transient plots of electrical waveforms on the small phosphor screen. The first dual beam oscilloscope was developed in the late 1930s by the British company A.C.Cossor (later acquired by Raytheon). The CRT was not a true double beam type but used a split beam by placing a third plate between the vertical deflection plates. It was widely used during WWII for the development and servicing of radar equipment. Although extremely useful for examining the performance of pulse circuits it was not calibrated so could not be used as a measuring device. It was, however, useful in producing response curves of IF circuits and consequently a great aid in their accurate alignment.

The triggered oscilloscope

Oscilloscopes became a much more useful tool in 1946 when Howard Vollum and Jack Murdock invented the triggered oscilloscope, Tektronix Model 511. The first oscilloscopes used analog technology in which the electron beam traced on the oscilloscope's screen directly traced the input voltage's waveform. It would start a horizontal trace when the input voltage exceeded an adjustable threshold. Triggering allows stationary display of a repeating waveform, as multiple repetitions of the waveform are drawn over the exact same trace on the phosphor screen -- without triggering, multiple copies of the waveform are drawn in different places, giving an incoherent jumble or a moving image on the screen.

As oscilloscopes have become more powerful over time, enhanced triggering options allow capture and display of more complex waveforms. For example, trigger holdoff is a feature in most modern oscilloscopes that can be used to define a certain period following a trigger during which the oscilloscope will not trigger again. This makes it easier to establish a stable view of a waveform with multiple edges which would otherwise cause another trigger.

Tektronix

Vollum and Murdock went on to found Tektronix, the first manufacturer of calibrated oscilloscopes (which included a graticule on the screen and produced plots with calibrated scales on the axes of the screen). Later developments by Tektronix included the development of multiple-trace oscilloscopes for comparing signals either by time-multiplexing (via chopping or trace alternation) or by the presence of multiple electron guns in the tube. In 1963, Tektronix introduced the Direct View Bistable Storage Tube (DVBST), which allowed observing single pulse waveforms rather than (as previously) only repeating wave forms. Using micro-channel plates, the most-advanced analog oscilloscopes (for example, the Tek 7104 mainframe) could display a visible trace (or allow the photography) of a single-shot event even when running at extremely fast sweep speeds.

Digital oscilloscopes

The first Digital Storage Oscilloscopes (DSO) was invented by Walter LeCroy (who founded the LeCroy Corporation, based in New York, USA) after producing high-speed digitizers for the research center CERN in Switzerland. LeCroy remains one of the three largest manufacturers of oscilloscopes in the world.

Starting in the 1980s, digital oscilloscopes became prevalent. Digital storage oscilloscopes use a fast analog-to-digital converter and memory chips to record and show a digital representation of a waveform, yielding much more flexibility for triggering, analysis, and display than is possible with a classic analog oscilloscope. Unlike its analog predecessor, the digital storage oscilloscope can show pre-trigger events, opening another dimension to the recording of rare or intermittent events and troubleshooting of electronic glitches. As of 2006 most new oscilloscopes (aside from education and a few niche markets) are digital.

Digital scopes rely on effective use of the installed memory and trigger functions: not enough memory and the user will miss the events they want to examine; if the scope has a large memory but does not trigger as desired, the user will have difficulty finding the event.