Function Generator

 In electrical and electronics engineering, 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). Another feature included on many function generators is the ability to add a DC offset. Integrated circuits used to generate waveforms may also be described as function generator ICs.

In particular it can be made to become a sine wave generator, square wave generator, and triangular wave generator. Also a function generator may be able to vary the characteristics of the waveforms, changing the length of the pulse, i.e. the mark space ratio, or the ramps of the different edges of triangular or sawtooth waveforms.

The function generator is only be able to create the waveforms that are built in to the function generator. It cannot be programmed to create additional waveforms - an arbitrary waveform generator, AWG is required for this. Apart from just generating the waveforms themselves, this type of test instrument has the capability to add a DC offset to the signal. This can be very useful in a number of testing applications.

Typically function generators are only able to operate at relatively low frequencies, some only operating to frequencies of around 100kHz, although more costly test instruments can operate at higher frequencies, up to 20 or 30MHz. It is also interesting to note that many oscilloscopes now include a function generator - this can be included quite easily in many designs, and therefore manufacturers believe this will give their product additional functionality and appeal.

Principles of Operation

Simple function generators usually generate triangular waveform whose frequency can be controlled smoothly as well as in steps. 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 linearly ascending 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 frequencies may be obtained. Sawtooth waves can be produced by charging the capacitor slowly with low 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.

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.
• It can generate a wide range of frequencies. For example, the Tektronix FG 502 (ca 1974) covers 0.1 Hz to 11 MHz.
• Frequency stability of 0.1 percent per hour for analog generators or 500 ppm for a digital generator.
• Maximum sinewave distortion of about 1% (accuracy of diode shaping network) for analog generators. Arbitrary waveform generators may have distortion less than -55 dB below 50 kHz and less than -40 dB above 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.
• An output impedance of 50 Ω.

Function generator capabilities

Function generators, named for their ability to produce various functions or waveforms, are essential tools capable of generating a range of repetitive waveforms like sine waves, square waves, sawtooth waves, and pulses.

Sine Wave:

The sine wave is a fundamental waveform characterized by its smooth, oscillating shape. It is commonly used in audio signal generation, frequency response testing, and various analog circuit applications. Sine waves provide a pure tone and are ideal for testing linear systems.

Square Wave:

A square wave alternates between two voltage levels, typically high and low, in a square-shaped waveform. It is extensively used in digital electronics for clock signals, timing, and logic level testing. Square waves have fast rise and fall times, making them suitable for assessing high-speed digital circuits.

Pulse Wave:

Pulse waves are similar to square waves but have variable mark-space ratios, representing different pulse widths. They find application in digital systems for generating timing signals and triggering events. Pulse waves are useful for testing pulse-width modulation (PWM) circuits and digital communication systems.

Triangle Wave:

A triangle wave linearly transitions between high and low voltage levels, forming a triangular waveform. It is often used in audio synthesis, modulation techniques, and testing applications where a linear ramp is required. Triangle waves are beneficial for assessing amplifier linearity and waveform generation in function generator circuits.

Sawtooth Wave:

Sawtooth waves have a linear rise and a sudden fall, resembling the teeth of a saw. They are utilized in applications like waveform synthesis, time-based generators, and display technologies. Sawtooth waves are particularly useful in raster scanning displays and signal generation for music synthesizers.

Function generator controls

In addition to a selection of the basic waveforms that are available, other controls on the function generator may include:

Frequency:   As would be expected, this control alters the basic frequency at which the waveform repeats. It is independent of the waveform type.

Waveform type :   This enables the different basic waveform types to be selected:

• Sine wave
• Square wave 
• Triangular wave

DC offset:   This alters the average voltage of a signal relative to 0V or ground.

Duty cycle:   This control on the function generator changes the ratio of high voltage to low voltage time in a square wave signal, i.e. changing the waveform from a square wave with a 1:1 duty cycle to a pulse waveform, or a triangular waveform with equal rise and fall times to a sawtooth.

Types of function generator

There are a number of ways of designing function generator circuits. However there are two main approaches that may be used:

Analog Function Generator:

An analog function generator operates using analogue circuitry and was the earliest type developed, originating in the 1950s. Advantages include cost-effectiveness, simplicity, and the ability to generate non-sinusoidal waveforms like triangles and ramps without high-frequency limitations. Analogue function generators are straightforward to use and remain popular for basic testing needs.

Analog function generator

Digital Function Generator:

Digital function generators utilize digital technology, with the most common technique being Direct Digital Synthesis (DDS). DDS employs a phase accumulator, a lookup table storing waveform digital representations, and a Digital-to-Analogue Converter (DAC) to generate waveforms. Digital function generators offer high accuracy, stability, spectral purity, and low phase noise due to crystal-controlled clocking. They can sweep over a wider frequency range and perform advanced functions like phase continuous frequency hopping. However, they tend to be more expensive and complex due to the requirement of high-performance DACs and additional digital circuitry.

Digital Function Generators

Sweep Function Generator:

A sweep function generator has the capability to sweep its frequency over a defined range. Sweep function generators can be either analogue or digital, with digital versions offering more versatility. They can sweep over ranges of up to 100:1 or more and may offer linear or logarithmic sweeps. Speed of the sweep and the ability to adjust sweep characteristics are important features to consider.

Arbitrary Waveform Generator

Working 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:

Vout=(-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.

Modulation Techniques in Function Generator

Function generators are versatile instruments capable of not only generating basic waveforms but also applying modulation techniques to these waveforms. Modulation is the process of varying one or more parameters of a carrier signal in accordance with the instantaneous amplitude, frequency, or phase of a modulating signal. Here are some common modulation techniques used in function generators:

Amplitude Modulation (AM):

• In AM, the amplitude of the carrier signal is varied in proportion to the amplitude of the modulating signal.
• Function generators can apply AM by modulating the amplitude of the generated waveform with an external signal.
• AM is used in applications such as radio broadcasting, communication systems, and audio signal processing

Frequency Modulation (FM):

• FM involves varying the frequency of the carrier signal in response to the amplitude of the modulating signal.
• Function generators can modulate the frequency of the output waveform with an external signal to produce FM.
• FM is widely used in radio communications, radar systems, and frequency synthesizers.

Phase Modulation (PM):

• PM modulates the phase of the carrier signal based on the amplitude of the modulating signal.
• Function generators can implement PM by adjusting the phase of the generated waveform in response to an external signal.
• PM finds applications in communication systems, radar, and phase-locked loops.

pulse width modulation (PWM):

• PWM involves varying the width of the pulses in a square wave carrier signal based on the amplitude of the modulating signal.
• Function generators can generate PWM signals by modulating the duty cycle of the square wave output.
• PWM is commonly used in power control applications, motor speed control, and digital-to-analog conversion.

Arbitrary Modulation:

• Some function generators offer arbitrary modulation capabilities, allowing users to define custom modulation schemes.
• Arbitrary modulation enables the synthesis of complex waveforms by combining multiple modulation techniques or creating custom modulation profiles.

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.