A time base generator (also timebase or time base) is a special type of
function generator, an electronic circuit that generates a varying
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
to produce a particular
waveform
In electronics, acoustics, and related fields, the waveform of a signal is the shape of its graph as a function of time, independent of its time and magnitude scales and of any displacement in time.David Crecraft, David Gorham, ''Electronic ...
. Time base generators produce very high frequency
sawtooth wave
The sawtooth wave (or saw wave) is a kind of non-sinusoidal waveform. It is so named based on its resemblance to the teeth of a plain-toothed saw with a zero rake angle. A single sawtooth, or an intermittently triggered sawtooth, is called a ...
s specifically designed to deflect the beam of a
cathode ray tube
A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms ( oscilloscope), pictu ...
(CRT) smoothly across the face of the tube and then return it to its starting position.
Time bases are used by
radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
systems to determine range to a target, by comparing the current location along the time base to the
time of arrival Time of arrival (TOA or ToA) is the absolute time instant when a radio signal emanating from a transmitter reaches a remote receiver.
The time span elapsed since the time of transmission (TOT or ToT) is the ''time of flight'' (TOF or ToF).
Time diff ...
of radio echoes. Analog television systems using CRTs had two time bases, one for deflecting the beam horizontally in a rapid movement, and another pulling it down the screen 60 times per second.
Oscilloscope
An oscilloscope (informally a scope) is a type of electronic test instrument that graphically displays varying electrical voltages as a two-dimensional plot of one or more signals as a function of time. The main purposes are to display repetiti ...
s often have several time bases, but these may be more flexible function generators able to produce many waveforms as well as a simple time base.
Description
![Oscilloscope diagram](https://upload.wikimedia.org/wikipedia/commons/a/a0/Oscilloscope_diagram.svg)
A
cathode ray tube
A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms ( oscilloscope), pictu ...
(CRT) consists of three primary parts, the
electron gun that provides a stream of accelerated
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
s, the
phosphor
A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or vi ...
-covered screen that lights up when the electrons hit it, and the deflection plates that use magnetic or electric fields to deflect the electrons in-flight and allows them to be directed around the screen. It is the ability for the electron stream to be rapidly moved using the deflection plates that allows the CRT to be used to display very rapid signals, like those of a
television signal
Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertising, e ...
or to be used for
radio direction finding
Direction finding (DF), or radio direction finding (RDF), isin accordance with International Telecommunication Union (ITU)defined as radio location that uses the reception of radio waves to determine the direction in which a radio station ...
(see
huff-duff
High-frequency direction finding, usually known by its abbreviation HF/DF or nickname huff-duff, is a type of radio direction finder (RDF) introduced in World War II. High frequency (HF) refers to a radio band that can effectively communicate over ...
).
Many signals of interest vary over time at a very rapid rate, but have an underlying periodic nature. Radio signals, for instance, have a base frequency, the
carrier
Carrier may refer to:
Entertainment
* ''Carrier'' (album), a 2013 album by The Dodos
* ''Carrier'' (board game), a South Pacific World War II board game
* ''Carrier'' (TV series), a ten-part documentary miniseries that aired on PBS in April 20 ...
, which forms the basis for the signal. Sounds are modulated into the carrier by modifying the signal, either in
amplitude (AM),
frequency (FM) or similar techniques. To display such a signal on an oscilloscope for examination, it is desirable to have the electron beam sweep across the screen so that the electron beam cycles at the same frequency as the carrier, or some multiple of that base frequency.
This is the purpose of the time base generator, which is attached to one of the set of deflection plates, normally the X axis, while the amplified output of the radio signal is sent to the other axis, normally Y. The result is a visual re-creation of the original waveform.
Use in radar
A typical
radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
system broadcasts a short pulse of radio signal and then listens for echoes from distant objects. As the signal travels at the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
and has to travel to the target object and back, the distance to the target can be determined by measuring the delay between the broadcast and reception, multiplying the speed of light by that time, and then dividing by two (there and back again). As this process occurs very rapidly, a CRT is used to display the signal and look for the echoes.
In the simplest version of a
radar display
A radar display is an electronic device to present radar data to the operator. The radar system transmits pulses or continuous waves of electromagnetic radiation, a small portion of which backscatter off targets (intended or otherwise) and retur ...
, today known as an "A-scope", a time base generator sweeps the display across the screen so that it reaches one side at the time when the signal has travelled the radar's maximum effective distance. For instance, an
early warning radar
An early-warning radar is any radar system used primarily for the long-range detection of its targets, i.e., allowing defences to be alerted as ''early'' as possible before the intruder reaches its target, giving the air defences the maximum t ...
like
Chain Home
Chain Home, or CH for short, was the codename for the ring of coastal Early Warning radar stations built by the Royal Air Force (RAF) before and during the Second World War to detect and track aircraft. Initially known as RDF, and given the off ...
(CH) might have a maximum range of , a distance that light will travel out and back in 1 millisecond. This would be used with a time base generator that pulls the beam across the CRT once every millisecond, starting the sweep when the broadcast signal ends. Any echoes cause the beam to deflect down (in the case of CH) as it moves across the display.
By measuring the physical location of the "blip" on the CRT, one can determine the range to the target. For instance, if a particular radar has a time base of 1 millisecond, then its maximum range is 150 km. If this is displayed on a four-inch CRT and the blip is measured to be 2 inches from the left side, then the target is 0.5 milliseconds away, or about {{convert, 75, km, miles.
To ensure the blips would line up properly with a mechanical scale, the time base could be adjusted to start its sweep at a certain time. This could be adjusted manually, or automatically trigged by another signal, normally a greatly attenuated version of the broadcast signal.
Later systems modified the time base to include a second signal that periodically produced blips on the display, providing a clock signal that varied with the time base and thus did not need to be aligned. In UK terminology, these were known as ''strobes''.
Use in television
Television signals consist of a series of still images broadcast in sequence, in the
NTSC
The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...
standard such a "frame" is broadcast 30 times a second. Each frame is itself broken down into a series of "lines", 525 in the NTSC standard. If one examines a television broadcast on an oscilloscope, it will appear to be a continual sequence of modulated signals broken up by short periods of "empty" signal. Each modulated portion carries the analog image for a single line.
To display the signal, two time bases are used. One sweeps the beam horizontally from left to right at 15,750 times a second, the time it takes for one line to be sent. A second time base causes the beam to scan down the screen 60 times a second, so that each line appears below the last one drawn and then returns to the top. This causes the entire signal of 525 lines to be drawn down the screen, re-creating a 2-dimensional image.
To ensure the time base began its sweep of the screen at the right time, the signal included several special modulations. With each line there was a brief period, the "front porch" and "back porch" that caused the signal to go negative briefly. This triggered the horizontal time base to start its sweep across the screen, ensuring that the lines started on the left of the display. A much longer but otherwise similar signal, the
vertical blanking interval caused the vertical time base to start, with any lengthy delay causing the time base to trigger.
References
* Anand Kumar
"Time-Base Generators" Pulse And Digital Circuits, PHI Learning, 2008
"NTSC SIGNAL SPECIFICATIONS" National Television System Committee, 1953
Electronic test equipment
de:Zeitbasis