Function Generator

In the field of electronics engineering, scientists and researchers often require signals that can be fed as an input to various devices which can later test their devices on these signals. Devices like modulators, filters, and analog-to-digital convertors require some input signals and other signals for testing. You will wonder what researchers do to get these signals. The answer to this is Function Generator. These are the devices that can generate different types of signals each representing different functions. These can be trigonometric signals, square functions, or any other necessary function. In this article we will see what is a function generator, we will also see the block diagram of a function generator and understand its working through it.

What is a Function Generator?

Function Generator as the name suggests is an electronic equipment that allows us to generate waveforms corresponding to different functions and also gives us the control over the properties of this function generated by it. By properties, we mean that once a functional waveform has been generated, we can control how much is its amplitude, what is the frequency after which it repeats . Depending on the trigger given the signal generated can be repetitive or not. Function Generator is known to be versatile because of its ability to produce various waveforms with wide frequency range. This is the formal definition of Function Generator

A Function Generator is a scientific device that can generate a variety of different waveforms with a wide range of frequencies.

Block Diagram and Working of Function Generator

Let us understand the block diagram of Function Generator, this will later be used for understanding the working of Function Generator.

Block Diagram of Function Generator

As we observe the circuit ,we see that it consists of a Frequency Control Network which controls the frequency of circuit depending on the current levels in circuit. We can increase or decrease frequency by increasing or decreasing current levels. The current sources are controlled by Frequency control network and the current sources then drive the integrator as shown in the block diagram. Here there are two current sources, namely current source ‘A’ and current source ‘B’ .

Integrator receives a constant supply if current from source A and performs integration on it with time. We can calculate the linear increase in output of integrator over time. So the output of integrator will be

V_out=(-1/C) ∫ i.dt

From this we can see that any variation in current, high or low will directly affect the output voltage which helps in voltage regulation.

Next in the section we observe a voltage comparator and multi-vibrator device which performs the task of triggering a change in the phase of the output voltage corresponding to the last peak level. Any change in phase makes the current supply from Source A to stop and Source B begins to supply power to the integrator. As the current source changes, the direction of current also changes resulting in reverse current. Now the reverse current lowers the output of integrator with time (in proportion) . When current reaches maximum value, the comparator switches the current source beginning to take supply from Source A.

The output of an integrator therefore is a triangular waveform whose frequency is based on current supply from current sources. The output of comparator is a square waveform. The resistance diode in the circuit helps to vary the triangular wave slope with minimal distortion. At the end, the amplifiers help in providing two waveforms which are then observed using oscilloscope.

Function Generator Specifications

Like any other device, function generator has some specifications which are used to determine the overall performance of device. Specification is an important tool that determines the overall design and working of device. We need some parameters in order to define what are the specifications. Depending on the type of generator being used ,the specifications are bound to change.

Here are some different specifications-

Waveforms

Depending on the waveform, we can describe different specifications. Waveforms can be of different type like

• Sine wave: This is generated from triangular wave by adding a pair of back to back diodes. Some specifications of this waveform is higher distortion as compared to sine waves produced by other test instruments
• Triangular wave: The line obtained in this signal will not particularly be a straight line. This means there will be a departure from straight line. If levels are around 90% of waveform amplitude then 99% linearity can be achieved.
• Square wave: It categorizes one important specification which is edge rise and fall time which is significant in logic chips. Synchronous chips needing clock require an edge of specific speed. A function generator can produce rise time and fall time of 100ns between 10 and 90% of the waveform.

Output symmetry: Output symmetry is another important specification. Function generator provides a range over which we can control the symmetry of our output waveform. The average range is 20% – 80% with positive or negative 10% error.

DC offset: This specification is provided by some Function Generators. This allows to control the base voltage level of signal over a given range. Sample range can be around +5V or -5V.

Frequency stability: Every function generator has a different stability range. While analogue instruments are less efficient when it comes to stability as digital ones will use a crystal for the clock in the generator making them more stable. generally the range is 0.1% per hour for analogue function generators, and 500 parts per million for digital Function Generator.

Power requirements: Depending on the type of device, every device has a certain different power need. Depending on the type of function generator its power needs can vary and are mentioned on it. The power required is AC and DC is out of option.

Modulation Techniques in Function Generator

Waves can be modulated with their carrier wave to ensure that they have long-range transmission. It is important to note that modern Function Generator can provide this feature of modulation .Let us see the types of modulation

AM (Amplitude Modulation)

This is the most common type of modulation performed. In the given figure, we have tried to show the setup for performing amplitude modulation and see the waveform generated after amplitude modulation. Observe that in this modulation, the amplitude of the wave is varied in proportion to that of the message signal, which is being transmitted. Function Generator allows us to choose the source of modulation from some other channel without extra circuitry.

Amplitude Modulated Wave

FM (Frequency Modulation)

This method comes into mind when we talk about broadcasting because frequency modulation helps in broadcasting multiple frequencies together .It is widely used for video broadcasting, medical monitoring systems, radar and more. Look at the figure below which shows the 1 kHz sine wave that was frequency modulated using sine wave of 10 Hz.

Frequency modulation

PM (Phase Modulation)

This modulation helps to change the phase of original wave by modulating it with the phase of carrier wave. We can see phase modulation being performed in Wi-Fi, GSM, and satellite broadcasting transmissions. It can performed using different techniques mainly PSK (Phase-shift keying), BPSK (Binary phase-shift keying), QPSK (Quadrature phase-shift keying) and more. Look at a sample phase modulation given below with deviation of 180⁰

Phase modulation

The Function Generator is used for generating different types of waveforms, let us see some of them.

• Sine wave: Sine wave is a significant periodic wave which is a common choice for input signal is communication systems. It is denoted by y=sin(x) and can be used for generating other waveforms. It is a smooth wave that oscillates from 0 to 1 and then from maximum value of 1 to a minimum value of -1 until it begins to repeat itself. It can be generated using an RC network and It is mainly used as current and voltage signal for various generators.

Sine Wave

• Square wave: Square Wave is a a non-sinusoidal waveform which is periodic and whose amplitude oscillates from maximum to minimum with a constant frequency. There are instantaneous transitions rather than gradual transition. This makes square wave suitable choice for digital data transmission. This wave has a duty cycle of 50% due to which its second harmonic is absent.

Square Waveform

• Triangle wave: In a triangular waveform ,signal moves up and down i.e. linearly varies with time. This output is usually generated by an operational amplifier when it works as an integrator. This is a fundamental kind of waveform generated by Function Generator and can be used to generate square pulse as well. The main use of such waveform comes in testing of amplifiers since triangular waveform shows any kind of distortion which other waveforms fail to show. Due to rich harmonics of this kind of pulse, it is used widely in musical instruments. It is used in sound synthesis as it has comparatively less harsh harmonics than other signals which makes it sound good. They are also used in sweep circuits and for testing purposes.

Triangular Wave

• Sawtooth wave: Sawtooth wave can be categorized as some kind of triangular wave in which the rising edges are very sharp as compared to the falling edges which are gradual. The name comes from the shape of the waveform which appears to be a sawtooth. We can generate this waveform by the same method followed for triangular pulse, the only thing to keep in mind is that difference in falling and rising times should be maintained and this can be done by controlling the rate of charge for each element. Similar to triangular wave, sawtooth wave is also used for sound generation. It helps to create sounds with subtractive analog music synthesizers. It is also used in electroencephalogram (EEG) because alpha oscillations of this wave are easier to remove than sine wave.

Sawtooth Waveform

• Pulse wave: Pulse waveform is a special kind of waveform used for dealing with digital data. It is a non-sinusoidal waveform that resembles square wave .The only difference is that it has an offset duty cycle which means that the space ratio is 1:1 . Digital data which consists of bits of 0s and 1s is transmitted through these waves. One most wide-spread application of pulse wave is for analyzation of cardiac output during any major surgery or fluid transport. It is widely used in epidemiological and physiological studies to study the stiffness of arteries and calculate any possible risk.

Pulse Waveform

Types of Function Generators

We categorize the Function Generators depending on the kind of waveform they generate are:

• Analog function generators
• Digital function generators
• Arbitrary waveform generators

Analog Function Generators

As the name suggests analog function generators are specifically used for generating analog signals. Analog signals are continuous function signal in the time domain which can take infinite number of values in a given range. An Analog function generator generates simple waveforms of varying magnitudes and frequencies that repeat over a period of time. These generators use signal generator circuit and an electronic oscillator for generation of signal. Let us see how it works to generate output waveform

Analog function generator

• It mainly consists of an oscillatory circuit which generates fundamental waveforms like sine wave. Operational amplifiers or PLL can be used for implementing this circuit.
• The next part involves waveform shaping which is done using a comparator circuit and a reference voltage for comparison.
• Now a frequency control knob is used in the function generator to control the frequency of signals being generated. This can also be done by using capacitor or resistances. Then ,we change the gain to modify the amplitude of wave.
• Now function selector switch helps us to select the type of waveform we need and then signal goes through a final amplifier for matching. The output is shown.

Digital Function Generators

As the name suggests digital function generators are specifically used for generating digital signals. Digital signals are signals which have discrete values in a certain given range this means they can only take finite number of values. A Digital function generator generates simple waveforms of certain magnitude. These generators use digital technology for generation of signal. They mostly use direct digital synthesis, DDS for this. Let us see how it works to generate output waveform:

Digital Function Generators

• The primary step involved is generation of a digital waveform which is converted to analog format using a digital-to-analog converter (DAC). The quantized values are converted to continuous values.
• The DAC controls the quality of signal by determining the sampling rate and resolution. This is directly linked to the accuracy of waveform.
• The generator allows users to control the properties of wave by allowing them to set them using buttons, knobs, a touchscreen, or software control via a computer.
• Then the random pulse is modulated by Digital function generator by managing frequency/phase of the generated waveform. Frequency sweeping is also performed.
• Synchronization of waveform is done and triggering options can be provided for initiating waveform at certain time instants. Then generated signal is passed through amplifier and shown using screen.

Arbitrary Waveform Generators

This generator has a prefixed list of waveforms that it can generate. In addition to this ,it can generate arbitrary waveforms by controlling the frequency of waveform, the amplitude and offset and basic distortion of waveform. It is popular mainly because of its stability and its great ability to switch almost instantaneously between Voltage levels. It is mainly used in power electronics to generate random signals with high frequency response.

Arbitrary Waveform Generator

Let us see how it works to generate output waveform

• Arbitrary Waveform Generator uses for generating a user defined waveform. Complex waveforms are stored in the memory of AWGs.
• AWGs used interpolation techniques to create a specific and well-defined waveform. This is done by reconstructing the waveform with high fidelity.
• The DAC controls the quality of signal by determining the sampling rate and resolution. This is directly linked to the accuracy of waveform.
• Synchronization of waveform is done and triggering options can be provided for initiating waveform at certain time instants.
• Filters are employed to remove any unnecessary element form signal and the amplification is done for the last time before sending the output pulse on the screen.

Application Of Function Generators

Function Generator are really useful in real-life. Let us understand this with some applications:

• Function Generators are used in laboratories for training and testing purposes due to their ability of generating signals which can be used for testing circuits. DC power supply and even measure the delay margin.
• Function Generators are also used for research and development purposes. One primary example to understand this is their use in R&D labs for research purpose.
• Sometimes they help in optimization of certain devices like they are widely used in automotive units where they perform the task of optimizing different control units especially engines.
• In electronics engineering, function generators are used for repairing and troubleshooting devices this includes PCB . Troubleshooting means figuring out the errors and working on them.
• Function Generators are also included in medical domain for calculating the frequency response of BP machines and for measuring pulse figures. It is often used for troubleshooting devices like ultrasound devices, pacemakers and medical equipment.

Conclusion

We have seen that Function Generator is not only a necessary but an inevitable tool in electronics industry which is used largely for the communication purpose. We have seen how there are different types of function generators , each having a different feature and different requirement for using them .Not only this, we also saw what are the five generic types of waveforms that can be generated using a function generator. These are used depending on the need of the circuit and operation performed. This makes us understand the vital importance of function generators .Advancement in this field is a boon to the society in terms of technological development .Studies are being done to improve the lifetime of these devices.

FAQs on Function Generator

What is meant by frequency response and how does Function Generator measure it?

Frequency Response is a tool for measuring how the amplitude and phase of a system’s output signal change with respect to the input frequency signal. Function Generator measures frequency response by generating a sine wave and analyzing it after passing it through device whose frequency response is to be measured.

Is Function Generator AC or DC?

We see that function generator is used for generating periodic waveforms such as sine, square, or triangular waves. These waveforms are continuously changing in direction and magnitude hence Functional generator is AC.

Who invented Function Generator?

Hewlett-Packard Co. had a function generator in 1951. Robert Brunner designed it for testing, vibration and geophysical studies.