Maria Montero

History of the oscilloscope: visualizing with light, the CRT, the modern oscilloscope

In this third part of our series, we cover the most recent history of the oscilloscope to modern times.

Found in almost every workshop or lab where electrons are used, many can take for granted everything that happened for the oscilloscope to exist. The journey is interesting, and it led to the culmination of many accidental discoveries and strange observations.

Here is the third and final part of a series on the history of the oscilloscope, describing the evolution of the oscillograph using light, the invention of the cathode ray tube, to the form we know today.

You can see the first two parts of this series here:

Visualizing with light: photographic and mirror waveforms

In Part 2, we learned about the progression towards automation of part of the waveform development process with the Hospitalier Ondograph, invented by French physicist Édouard Hospitalier. While this invention managed to speed up part of the process, there was still a lot of work involved, it still lacked precision and was limited by the frequency of the signal. This was also an indirect measure of the waveform being measured and was not occurring in real time.

One step closer to accurate real-time waveform measurement came with the invention of the moving coil oscillogram. Invented by the English engineer William Duddell in the early 1900s, the device worked by rotating a mirror that was suspended in oil and exposed to the magnetic fields of the measured current. The mirror would rotate according to the direction and intensity of this current, and a beam of light would be reflected off a photographic slide to record the beam. This made it possible to record higher frequency signals.

There was still a barrier to providing real-time measurements, as photographs had to be developed, but they simplified the process significantly.

Invention of the cathode ray tube and commercial adoption.

The development and subsequent commercialization of the cathode ray tube (CRT) would lead to the first initial oscilloscopes. The CRT oscilloscope began as an experimental instrument invented by the German physicist Karl Ferdinand Braun in the late 19th century, and it provided information on the behavior of electrons. The charged baffle plates would deflect the electron beams onto a phosphor coated surface, giving immediate visual viewing.

The concept would be further developed and used in commercial equipment in 1932 by AC Cossor, a British company that would eventually become Raytheon. The CRT oscilloscope was widely used during World War II, and variations were made using different phosphorescent materials. This material had an impact on the intensity of the displayed signal and the time it took to fade. Some materials would be better suited for single-shot or low-frequency waveforms, which would fade more slowly and be visible longer, for example.

These CRT scopes were initially not very accurate for measurement, but they were excellent at providing information on electrical system behaviors.

AC’s 1035 MKIII Harvester CRT Display Oscilloscope, released in 1950. Image courtesy of Richard Sears.

Frequency trigger and time trigger

In the 1930s, another company gained significant momentum in the development of oscilloscopes. DuMont began selling the DuMont 164 oscilloscope in 1939, and invented the first frequency-triggered sweep oscilloscope, the Model 224-A.

Shortly after the war, in 1946, Tektronix, Inc. was founded. One of the four founders, Howard Vollum, invented the first time-triggered oscilloscope known as the Tektronix Type 511. Vollum was motivated to develop a device that could provide measurements quantitative. The CRT-based, calibrated 511 would weigh 65 pounds, draw 180 W, and sell for $ 795 in 1947 (equivalent to just under $ 10,000 in 2019). It had a frequency range of 10 Hz-10 MHz.

Tektronix would continue to innovate in the oscilloscope domain, going from a 12-employee company in 1947 to 359 employees in 1952.

Tektronix type 511 oscilloscope. Image courtesy of W140.

Solid state to digital

Between the rest of the 1940s and 1970s, several other companies would benefit from the market as well, such as Hewlett-Packard and LeCroy. Hewlett-Packard would introduce the HP 1200A in 1969, the first fully solid-state oscilloscope with a 500 KHz bandwidth.