Description of a Function Generator Instrument A function generator is usually a piece of electronic test equipment that is used to generate different types of electrical waveforms over a wide range of frequencies and amplitudes. Commonly, it is used to produce Sine, square and triangular waveforms. Different Function generators have different properties and functionalities. However, they mostly have main characteristics that every function generator must have. Function generators are widely used by electrical engineers. It is an essential instrument that every lab has to have. Function generators are used to test and troubleshoot electrical circuits for research or even test a product before releasing it to the market. It is very easy to setup, use and transport one of these instruments, which make it a very useful tool for electrical engineers. Definitions Frequency: Is the number of occurrences of a repeating event per unit time Amplitude: Is the maximum absolute value of the signal Duty Cycle: Is the percentage of one period in which the signal is active (Non- zero value) DC Offset: Is the mean value of a waveform BNC Connecter: Is a special type of connectors mainly used in radio- frequency electronic equipment Integrated Circuit (IC): Is a set of electronics on a single semiconductor chip Appearance Basic function generators have a simple interface. As shown in Figure 1, it has a display to show a sample waveform, knobs to control frequency, amplitude, offset and duty cycle. In addition, it has an On/Off button switch, waveform selector buttons and frequency range selection button switches. These knobs and buttons are mostly made of plastic, but sometimes Figure 1 - Basic interface of a function generator the buttons are made of rubber. Also, the basic interface includes BNC- connector output that is used to output the signal to be used in the test circuit. Most of the function generators nowadays have two different output BNC- connectors that can be used to output two different waveforms simultaneously.
Components Different function generators have different techniques in producing waveforms, hence, different circuit components. Nowadays, these components are manufactured on ICs instead of soldering every component on a huge board. This technique allows manufacturing many devices in a small space, which yields to higher technology and functionality. Figure 2 shows a block diagram of the basic components of a function generator with two outputs. Each block can be built in many different ways. It depends on the specification of the design (the range of frequencies and amplitudes that can be produced). Figure 2 - Block Diagram of a basic network For the generator in Figure 2, it contains the following components: 1. A Frequency Control Network to adjust the current supplies and the frequency of the output 2. Two different Constant Current Supply Sources 3. An integrator, or basically an operational amplifier with a capacitor on the feedback loop 4. A Voltage Comparator that is used to form a square waveform 5. Resistance Diode Shaping Circuit that is used to form a sin waveform 6. Output Amplifiers to adjust the amplitude of the waveform The Frequency Control Network First step in using a function generator is to adjust its frequency. Figure 3 shows a basic frequency control network. The input to this network is a DC current source. When C1 and C2 are charging, the output of this network is going to be zero. Then, when they are both fully charged, the resistors tend to dissipate the charge and conduct a current at the output. How frequent this process happens depends on the value of the resistor R3, which is an adjustable resistor. Furthermore, this process is responsible for adjusting the frequency of the output and the Constant Current Supply Sources of the function generator
Figure 3 - Basic Frequency Control Network The Integrator After adjusting the frequency of the current supplies, the integrator comes into play. Its main function is to produce a triangular waveform that is going to be directly used, or converted to a sin or a square wave. It basically performs a mathematical integration to its input signal, which is a constant DC current. Hence, produces an output voltage that rises linearly with time. In other words, the output is going to be a triangular waveform. The slope of the waveform depends linearly on the magnitude of the Constant Current Supply Sources. If the current supplied is positive, the output is going to be a rising line. In contrast, if it was negative, the output is going to be a falling line. The Frequency Control Network controls the polarity of the current sources. The Voltage Comparator The voltage comparator is used to produce a square waveform. The comparator is a circuit that compares an input voltage to a fixed reference voltage as shown in Figure 4. The comparator is supplied with two voltage constant voltage sources, V+ and V- that have the same magnitude but different signs. If the input voltage (The output of the integrator) is larger than the reference voltage (Usually 0), it outputs the value of the V+ source. However, if the input voltage is smaller than the reference voltage, it outputs the value of the V- source. Since the V+ and V- have constant values, the output will switch between the value of V+ and V-, which will produce a square waveform.
Figure 5 - Voltage Comparator Circuit Figure 4 - Voltage Comparator Circuit Resistance Shaping Diode Circuit The resistance diode network changes the slope of the triangular wave as its amplitude changes and produces a sinusoidal wave with less than 1% distortion. It has a very complicated operation that is hard to explain in few paragraphs. Conclusion: The function generators are widely used instruments that makes electrical engineer s job simpler. A function generator can be built in many different ways, depending on the specification of the design. A basic generator has a Frequency Control Network, Two different Constant Current Supply Sources, an Integrator, a Voltage Comparator and a Resistance Shaping Diode Circuit. The Frequency Control Network is used to adjust the frequency of the output. The current sources are used to supply current to the integrator to form a triangular waveform. The Voltage Comparator and the Resistance Shaping Diode Circuit are used to transform the triangular waveform into a square or sinusoidal waveforms.
References: 1. http://www.ee.usyd.edu.au/tutorials_online/topics/labintro/wrange.html 2. http://www.mycircuits9.com/2012/11/function- generator- working- with- block.html 3. https://www.circuitlab.com/circuit/x66cq6/basic- frequency- control- circuit/ 4. http://forums.mikeholt.com/attachment.php?attachmentid=1636&d=12099 36190 5. http://en.wikipedia.org/wiki/function_generator/