Question

Time Scaling on Oscilloscope

and function generator to triangular wave

Answer 1

A time base generator, or timebase, is a special type of function generator, an electronic circuit that generates a varying voltage to produce a particular waveform. Time base generators produce very high frequency sawtooth waves specifically designed to deflect the beam in cathode ray tube (CRT) smoothly across the face of the tube and then return it to its starting position.

Time bases are used by radar systems to determine range to a target, by comparing the current location along the time base to the time of arrival 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. Oscilloscopes 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.

DescriptionEdit
A typical oscilloscope with a time base controlled on the top dial, and the amplification of the signal on the bottom dial.

A cathode ray tube (CRT) consists of three primary parts, the electron gun that provides a stream of accelerated electrons, the phosphor-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 or to be used for radio direction finding (see huff-duff).The electrons are fired from cathode.Cathode is heated up with power supply and electrons as fired through them.The electrons flow is controlled by control grid.It is type of regulator for flow of electrons.

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, 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 radarEdit

A typical radar 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 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, dividing 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, 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 like Chain Home (CH) might have a maximum range of 150 kilometres (93 mi), 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 75 kilometres (47 mi).

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 televisionEdit

Television signals consist of a series of still images broadcast in sequence, in the NTSCstandard 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.

Function generator

A simple analog function generator, circa 1990
A DDS function generator
Sine, square, triangle, and sawtoothwaveforms

A function generator is usually a piece of electronic test equipment or software used to generate different types of electrical waveforms over a wide range of frequencies. Some of the most common waveforms produced by the function generator are the sine wave , square wave, triangular wave and sawtooth shapes. These waveforms can be either repetitive or single-shot (which requires an internal or external trigger source).[1]Integrated circuits used to generate waveforms may also be described as function generator ICs.

In addition to producing sine waves, function generators may typically produce other repetitive waveforms including sawtooth and triangular waveforms, square waves, and pulses. Another feature included on many function generators is the ability to add a DC offset.

Although function generators cover both audio and RF frequencies, they are usually not suitable for applications that need low distortion or stable frequency signals. When those traits are required, other signal generators would be more appropriate.

Some function generators can be phase-locked to an external signal source (which may be a frequency reference) or another function generator.[2]

Function generators are used in the development, test and repair of electronic equipment. For example, they may be used as a signal source to test amplifiers or to introduce an error signal into a control loop. Function generators are primarily used for working with analog circuits, related pulse generators are primarily used for working with digital circuits.

Electronic instrumentsEdit

WorkingEdit

Simple function generators usually generate triangular waveform whose frequency can be controlled smoothly as well as in steps.[3]This triangular wave is used as the basis for all of its other outputs. The triangular wave is generated by repeatedly charging and discharging a capacitor from a constant current source. This produces a linearlyascending and descending voltage ramp. As the output voltage reaches upper or lower limits, the charging or discharging is reversed using a comparator, producing the linear triangle wave. By varying the current and the size of the capacitor, different frequenciesmay be obtained. Sawtooth waves can be produced by charging the capacitor slowly, using a current, but using a diode over the current source to discharge quickly - the polarity of the diode changes the polarity of the resulting sawtooth, i.e. slow rise and fast fall, or fast rise and slow fall.

A 50% duty cycle square wave is easily obtained by noting whether the capacitor is being charged or discharged, which is reflected in the current switching comparator output. Other duty cycles (theoretically from 0% to 100%) can be obtained by using a comparator and the sawtooth or triangle signal. Most function generators also contain a non-linear diode shaping circuit that can convert the triangle wave into a reasonably accurate sine wave by rounding off the corners of the triangle wave in a process similar to clipping in audio systems.

A typical function generator can provide frequencies up to 20 MHz. RF generators for higher frequencies are not function generators in the strict sense since they typically produce pure or modulated sine signals only.

Function generators, like most signal generators, may also contain an attenuator, various means of modulating the output waveform, and often the ability to automatically and repetitively "sweep" the frequency of the output waveform (by means of a voltage-controlled oscillator) between two operator-determined limits. This capability makes it very easy to evaluate the frequency response of a given electronic circuit.

Some function generators can also generate white or pink noise.[citation needed]

More advanced function generators are called arbitrary waveform generators (AWG). They use direct digital synthesis (DDS) techniques to generate any waveform that can be described by a table of amplitudes.

SpecificationsEdit

Typical specifications for a general-purpose function generator are:

Produces sine, square, triangular, sawtooth (ramp), and pulse output. Arbitrary waveform generators can produce waves of any shape.[2]
It can generate a wide range of frequencies. For example, the Tektronix FG 502 (ca 1974) covers 0.1 Hz to 11 MHz.[4]
Frequency stability of 0.1 percent per hour for analog generators[4] or 500 ppm for a digital generator.
Maximum sinewave distortion of about 1% (accuracy of diode shaping network) for analog generators.[5] Arbitrary waveform generators may have distortion less than -55 dB below 50 kHz and less than -40 dBabove 50 kHz.
Some function generators can be phase locked to an external signal source, which may be a frequency reference or another function generator.
Amplitude modulation (AM), frequency modulation (FM), or phase modulation (PM) may be supported.
Output amplitude up to 10 V peak-to-peak.
Amplitude can be modified, usually by a calibrated attenuator with decade steps and continuous adjustment within each decade.
Some generators provide a DC offset voltage, e.g. adjustable between -5V to +5V.[2]
An output impedance of 50 ?.

SoftwareEdit

A completely different approach to function generation is to use software instructions to generate a waveform, with provision for output. For example, a general-purpose digital computer can be used to generate the waveform; if frequency range and amplitude are acceptable, the sound card fitted to most computers can be used to output the generated wave.

Circuit elementsEdit

Waveform generatorEdit

An electronic circuit element used for generating waveforms within other apparatus that can be used in communications and instrumentation circuits, and also in a function generator instrument. Examples are the Exar XR2206[6] and the Intersil ICL8038[7]integrated circuits, which can generate sine, square, triangle, ramp, and pulse waveforms at a voltage-controllable frequency.

Function generatorEdit

An electronic circuit element that provides an output proportional to some mathematical function (such as the square root) of its input; such devices are used in feedback controlsystems and in analog computers. Examples are the Raytheon QK329 square-law tube[8]and the Intersil ICL8048 Log/Antilog Amplifier.[9]

Mechanical function generatorsEdit

Mechanical function generators are linkages, cam-follower mechanisms or non-circular gears, designed to reproduce different types of functions, either periodic (like sine or cosine functions), or single-shot (logarithm, parabolic, tangent functions etc.). [10]

Measurement instruments like pressure gauges, altimeters and barometers include linkage-type function generators as linearization means. Before the advent of digital computers, mechanical function generators were used in the construction of gun fire control systems, and mechanical calculators.

Four-bar function generator of the function Log(u) for 1 < u < 10.

Rocker-slider function generator of the function Log(u) for 1 < u < 10.

Slider-rocker function generator of the function Tan(u) for 0 < u < 45°.