Function generator

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File:Kenwood FG273 Function Generator.jpg
Function generator

A function generator is a piece of electronic test equipment or software used to generate different types of electrical waveforms over a wide range of frequencies. These waveforms can be either repetitive or single-shot, in which case some kind of triggering source is required (internal or external).[1] Usually a function generator produces waveform that is produced by the repetitive charging and discharging of the capacitor to which a constant energy source is connected. [2]

One of the most useful features of the function generator is that it can be phase locked to an external signal source or another function generator.[3] Other important features of the function generator are continuous tuning over wide bands with max-min frequency ratios of 10 or more, wide range of frequencies from a few Hz to MHz, flat output amplitude and modulation capabilities like frequency sweeping, frequency modulation and amplitude modulation.[4]

Function Generators are used in development, testing and repair of electronic equipment, e.g. as a signal source to test amplifiers, or to introduce an error signal into a control loop. Some of the most common waveforms produced by the function generator are the sine, square, triangular and sawtooth waveforms.

Working

A typical function generator usually comprises of a cheese triangular waveform whose frequency can be controlled smoothly as well as in steps.[5] 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 cheese constant current source. This produces a linearly ascending or descending voltage ramp. As the output voltage reaches upper and lower limits, the charging and discharging is reversed using a comparator, producing the linear triangle wave. By varying the current and the size of the capacitor, different frequencies may 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. It does so by rounding off the hard 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.

More advanced function generators use Direct Digital Synthesis (DDS) to generate waveforms. Arbitrary waveform generators use DDS to generate any waveform that can be described by a table of amplitudes.

Features

The general features of a standard function generator are:

  1. It can generate a wide range of frequencies ranging from 0.01 Hz to 100 kHz.
  2. It can be used to produce different waveforms like sine wave, square wave, triangular wave, sawtooth wave, etc.
  3. It offers great accuracy, about +/- 0.01% in the low frequency range.
  4. The distortion produced in the output signal is less than 1% for sine wave.
  5. It can be phase locked to an external signal source or another function generator.
  6. It can be phase locked to a standard frequency, so that all the output waveforms of the generator will have the same accuracy and stability as that of the standard source.
  7. It gives the availability of a continuous adjustable d.c. offset voltage between -5V to +5V.[3]

Specifications

Some of the important specifications of function generator are given below:

  1. Its specific frequency range is between 0.001Hz to 20MHz.
  2. Frequency stability of function generator is 0.05%.
  3. Distortion of function generator is -55dB below 50KHz and -40dB above 50KHz.
  4. Specific value of output amplitude(open circuit)is 10V(peak to peak).
  5. Specific value of output impedance is 50ohm.
  6. Different output waveforms like sine, square, triangular, ramp, pulse, AM and FM modulated, arbitary waves can be generated using the function generator.

[6]

Other meanings

Another type of function generator is a sub-system that provides an output proportional to some mathematical function of its input; for example, the output may be proportional to the square root of the input. Such devices are used in feedback control systems and in analog computers. A function generator IC named ICL8038 (which is also used as Voltage Controlled Oscillator (VCO)) can be used to generate triangular wave, square wave, saw-tooth wave or even, sine wave simultaneously. For maximum voltage rating of 36, more than 300kHz of frequency can be generated.

How to use a function generator

After powering on the function generator, the output signal needs to be configured to the desired shape. generally, this means connecting the signal and ground leads to an oscilloscope to check the controls. Adjust the function generator,to get the output signal correct, then attach the signal and ground leads from the function generator to the input and ground of the device under test. For some applications, the negative lead of the function generator should attach to a negative input of the device, but usually attaching to ground is sufficient.A device such as an oscilloscope is then used to measure the circuit's output. [7]

See also

External links

fr:Générateur de basses fréquences ko:함수 발생기 id:Generator fungsi it:Generatore di forme d'onda he:מחולל אותות ja:ファンクションジェネレータ zh:函数发生器

References

  1. ↑ http://cnx.org/content/m11895/latest/
  2. ↑ www.functiongenerator.org
  3. ↑ 3.0 3.1 U, Bakshi (2008). Electronic Measurments and Instrumentation. Pune: Technical Publications. pp. 3–26,3–27. ISBN 8184314353, 9788184314359 Check |isbn= value: invalid character (help). 
  4. ↑ Singh, S (2003). Indl Inst & Control, 2E. India: Tata McGraw-Hill Education. pp. 132–134. ISBN 007048290X, 9780070482906 Check |isbn= value: invalid character (help). 
  5. ↑ Sonde, B. S. (1992). Introduction to System Design Using Integrated Circuits. New Age International. pp. 244–246. ISBN 8122403867, 9788122403862 Check |isbn= value: invalid character (help). 
  6. ↑ http://books.google.co.in/books?id=utNfE7Rvpx4C&pg=SA3-PA26&dq=function+generator&hl=en&ei=LUaLTsf-LOSQiQeerpGFBA&sa=X&oi=book_result&ct=result&resnum=3&sqi=2&ved=0CFEQ6AEwAg#v=onepage&q=function%20generator&f=false
  7. ↑ http://cnx.org/content/m11895/latest/
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