The history of the oscilloscope reaches back to the first recordings of waveforms with a galvanometer coupled to a mechanical drawing system in the second decade of the 19th century. The invention of the cathode ray tube and vacuum tubes allowed the development of oscilloscopes of the modern form. Vacuum-tube based oscilloscopes were critical to the development of radar during the Second World War, and development of electronic digital computers. The modern day digital oscilloscope is a consequence of multiple generations of development of the oscillograph, cathode-ray tubes, analog oscilloscopes, and digital electronics.
Hand-drawn oscillograms
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 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. 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
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. (Such wave-form measurements were still averaged over many hundreds of wave cycles but were more accurate than hand-drawn oscillograms.)
Tilting mirror
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 reduced the measurement device to a small mirror that could move at high speeds to match the waveform.
The term "oscillograph" was coined by André Blondel in 1893 to refer to his instrument based on the earlier-known mirror galvanometer but adapted to recording high-frequency oscillations. William Duddell also developed a similar instrument few years later.
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.
The term "oscilloscope" was coined in 1907.
In the 1920s, a tiny tilting mirror attached to a diaphragm at the apex of a horn provided good response up to a few kHz, perhaps even 10 kHz. A time base, unsynchronized, was provided by a spinning mirror polygon, and a collimated beam of light from an arc lamp projected the waveform onto the lab wall or a screen.The Science of Musical Sounds, by Dayton Clarence Miller, published about 1924
Even earlier, audio applied to a diaphragm on the gas feed to a flame made the flame height vary, and a spinning mirror polygon gave an early glimpse of waveforms.Late 1800s, possibly? Ganot's Physics, a very popular 19th-C physics text? IIrc, Miller also described this.
Moving-paper oscillographs using UV-sensitive paper and advanced mirror galvanometers provided multi-channel recordings in the mid-20th century. Frequency response was into at least the low audio range.
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. Braun tubes were laboratory apparatus, using a cold-cathode emitter and very high voltages (on the order of 20,000 to 30,000 volts). With only vertical deflection applied to the internal plates, the face of the tube was observed through a rotating mirror to provide a horizontal time base. In 1899 Jonathan Zenneck equipped the cathode ray tube with beam-forming plates and used a magnetic field for sweeping the trace.
Early cathode ray tubes had been applied experimentally to laboratory measurements as early as 1919 but suffered from poor stability of the vacuum and the cathode emitters. The application of a thermionic emitter allowed operating voltage to be dropped to a few hundred volts. Western Electric introduced a commercial tube of this type, which relied on a small amount of gas within the tube to assist in focusing the electron beam.
V. K. Zworykin described a permanently sealed, high-vacuum cathode ray tube with a thermionic emitter in 1931. This stable and reproducible component allowed General Radio to manufacture an oscilloscope that was usable outside a laboratory setting.
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 made 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.
Allen B. Du Mont Labs. made moving-film cameras, in which continuous film motion provided the time base. Horizontal deflection was probably disabled, although a very slow sweep would have spread phosphor wear. CRTs with P11 phosphor were either standard or available.Oscilloscope catalog, Allen B. DuMont Labs, possibly 1949
Long-persistence CRTs, sometimes used in oscilloscopes for displaying slowly changing signals or single-shot events, used a phosphor such as P7, which comprised a double layer. The inner layer fluoresced bright blue from the electron beam, and its light excited a phosphorescent "outer" layer, directly visible inside the envelope (bulb). The latter stored the light, and released it with a yellowish glow with decaying brightness over tens of seconds. This type of phosphor was also used in radar analog PPI CRT displays, which are a graphic decoration (rotating radial light bar) in some TV weather-report scenes.
Sweep circuit
The technology for the horizontal sweep, that portion of the oscilloscope that creates the horizontal time axis, has changed.
Synchronized sweep
Early oscilloscopes used a synchronized sawtooth waveform generator to provide the time axis. The sawtooth would be made by charging a capacitor with a relatively constant current; that would create a rising voltage. The rising voltage would be fed to the horizontal deflection plates to create the sweep. The rising voltage would also be fed to a comparator; when the capacitor reached a certain level, the capacitor would be discharged, the trace would return to the left, and the capacitor (and the sweep) would start another traverse. The operator would adjust the charging current so the sawtooth generator would have a slightly longer period than a multiple of the vertical axis signal. For example, when looking at a 1 kHz sinewave (1 ms period), the operator might adjust the horizontal frequency to a little bit more than 5 ms. When the input signal was absent, the sweep would free run at that frequency.
If the input signal were present, the resulting display would not be stable at the horizontal sweep's free-running frequency because it was not a submultiple of the input (vertical axis) signal. To fix that, the sweep generator would be synchronized by adding a scaled version of the input signal to the sweep generator's comparator. The added signal would cause the comparator to trip a little earlier and thus synchronize it to the input signal. The operator could adjust the sync level; for some designs, the operator could choose the polarity. The KG-635 sync amplifier used a 12AT7 differential amplifier (V5). (id p. 15.) Sync level control would bias the amplifier into cutoff so action would only occur near the end of the sweep; the sync output was a negative pulse to the sweep generator; a diode pulse limiter clamped the sync pulse. (id p. 18.) The sweep generator would turn off (known as blanking) the beam during retrace.KG-635 p. 18 stating, "Retrace blanking is obtained from the plate of V-6A and applied to the cathode of the CRT."
The resulting horizontal sweep speed was uncalibrated because the sweep rate was adjusted by changing slope of the sawtooth generator. The time per division on the display depended upon the sweep's free-running frequency and a horizontal gain control.
A synchronized sweep oscilloscope could not display a non-periodic signal because it could not synchronize the sweep generator to that signal. Horizontal circuits were often AC-coupled.
Triggered sweep
During World War II, a few oscilloscopes used for radar development (and a few laboratory oscilloscopes) had so-called driven sweeps. These sweep circuits remained dormant, with the CRT beam cut off, until a drive pulse from an external device unblanked the CRT and started a constant-speed horizontal trace; the calibrated speed permitted measurement of time intervals. When the sweep was complete, the sweep circuit blanked the CRT (turned off the beam), reset itself, and waited for the next drive pulse. The Dumont 248, a commercially available oscilloscope produced in 1945, had this feature.
Oscilloscopes became a much more useful tool in 1946 when Howard Vollum and Melvin Jack Murdock introduced the Tektronix Model 511 triggered-sweep oscilloscope. Howard Vollum had first seen this technology in use in Germany. The triggered sweep has a circuit that develops the driven sweep's drive pulse from the input signal.
Triggering allows stationary display of a repeating waveform, as multiple repetitions of the waveform are drawn over exactly the same trace on the phosphor screen. A triggered sweep maintains the calibration of sweep speed, making it possible to measure properties of the waveform such as frequency, phase, rise time, and others, that would not otherwise be possible. Furthermore, triggering can occur at varying intervals, so there is no requirement that the input signal be periodic.