Abstract
Numerous instrumentation engineers and re- searchers frequently manage analog electronic issues when moving toward sensitive estimations. Regardless of whether off-the-shelf measuring solutions exist, per- ception of the analog electronic of the measuring so- lutions is frequently a need. The function generator is also known as the signal generator is become the most used testing device. The function generator delivers an accurate calibrated range. This report talks about the overview of a signal generator, how to generate function using analog electronics, and changing its characteris- tics like waveform, modulation, frequency, and ampli- tude voltage. Contents
1 INTRODUCTION 2
2 Electronic Design 2 2.1 Design Specifications.......... 2
3 Wave Genaration 2 3.1 Square Wave.............. 2 3.2 Trangular Wave............. 3 3.3 PWM Wave............... 3 3.4 Sawtooth Wave............. 3 3.5 Sine Wave................ 4
4 Output Circuit 5 4.1 Summing Amplifier........... 5 4.2 Push-Pull amplifier........... 5
5 Power Supply 5
6 Component Selections 5
7 Simulations 7
8 PCB Designs 10
9 Enclosure Design 11
10 Discussion 12
- INTRODUCTION
A function generator is an electronic device that is used for generating various types of electrical wave- forms such as sine waves, square waves, triangular waves, Sawtooth waves over a frequency range. These waveforms can be either single-shot signals or repeti- tive signals. In addition to changing the type of wave- form, the function generator is also capable of varying the waveform characteristics such as frequency, ampli- tude. Uses of the function generator,
Sweep generation
AM/FM generation
Phase-Locked Loops
In addition to the above applications function gen-
erators are also used for repair, development, and test
of electronic devices. Function generators are primar-
ily used for working with analog circuits, related pulse
generators are primarily used for working with digital
circuits.
Most of the modern function generators are made
using Digital components. The objective of this project
is to build and implement a fully functional analog func-
tion generator which generates various waveforms with
varying amplitude and frequencies.
In this report, we describe the different circuits
used for the generation of noise-free waveforms and
how potentiometers and transistors are used to change
the frequencies and the amplitudes of the waveforms.
- Electronic Design 2.1. Design Specifications
In this project, we will fulfill thesespecifications.
The function generator should be able to generate
sine, triangular, saw-tooth, square and pulse width
modulation(PWM) waves.
The frequencies of the waves should be able to
vary from 20 Hz to 20000 Hz.
The amplitudes of the waves should be able to vary
up to 20V peak-to-peak.
In the PWM wave, the pulse width should be able
to vary from 1% up to 99%.
The function generator should be able to supply
the waves to a 50W load.
The function generator should be able to give a DC
shift to the waves.
- Wave Genaration 3.1. Square Wave
A square wave is a wave which is a non-sinusoidal periodic waveform. A square waveform can be repre- sented as an infinite summation of sinusoidal waves. Square wave generator can be constructed using the Schmitt trigger circuit. A Schmitt inverter is the in- verse to that of its input. It uses a Schmitt trigger action that changes state between an upper and lower threshold level as the input voltage signal increases and decreases about the input terminal.
Figure 1. Schmitt Trigger
This simple square wave generator circuit consists of a single Schmitt inverter logic gate with a capacitor connected between its input terminal and ground and the positive feedback required for the circuit to oscillate being provided by the feedback resistor. Let us take,
V 2 =Voltageat invertingterminal V 1 =Voltageat thenon−invertingterminal Vid=di f ferential voltage
When the charge across the capacitor is below Schmitt’s lower threshold level it makes the input to the inverter at a logic “0” level resulting in a logic “1” out- put level (inverter principals). At this stage,
V 2 = 0 V There fore,Vid=V 1
As Vid is positive, it will drive the Vout to the pos- itive saturation voltage. (Since the capacitor does not have any charge at the initial stage, the gain is maxi- mum)
At this time capacitor gets started charging and it will increase the voltage of the capacitor. Therefore, V2 gets an increase. After V2 getting a little higher than V1, it will give negative output and Vout will be switched from positive saturation voltage to nega- tive saturation voltage.At this negative saturation volt- age,V1 can be given as, At this negative saturation voltage,V1can be given as
Vid=−V 1 +V 2
−V 1 =(R 4/R 4 +R 3)*(−Vsaturation)
Since V1 is negative, the capacitor starts to dis- charge through the resistor towards negative saturation voltage. After reaching V2 slightly less than -V1, the output will again be switched to a positive saturation voltage. This process will happen again and again, and a square wave is generated. Overall, |V 1 |=(R 1/R 1 +R 2)*|Vsaturation|
Let
R 1 +R 2 =R
We can write the product as,
T= 2 RC×ln
2 R 4 +R 3 R 4
since f=1/T=1/2 RC×ln^2 R^4 R+ 4 R^3 Hz
Since R4 and R3 are constants,
f α 1/RC
Therefore, by changing R and C values, we can change the frequency of the square waveform.The R2 is used to prevent the resulting resistance from becoming zero. 3.2. Trangular Wave
Method I This method is based on passing the square waves through an integrator. This method has two main parts. Which are,
Square wave generator – which generates square
waves.
Integrator – which converts square waves to trian-
gular waves.
Method IIWhen the capacitor charges and dis-
Figure 2. Triangular Wave Genaration
charges,theoretically the shape of the voltage across the capacitor is an exponential waveform. But practically,it will be triangular. So, we can use a capacitor as a trian- gular waveform generator. we are using second method because easy to inte- grates with other circuits.
3.3. PWM Wave
PWM signal is used to get low-frequency signal by high-frequency source by switching the voltage be- tween upper and lower levels, and output can be taken as the average of voltage over the period of switching between upper and lower level. The PWM waves are generated by passing the tri- angular waves through a comparator.
3.4. Sawtooth Wave
A triangular waveform is a waveform that is linear,non-sinusoidal, triangular shape waveform. When the rise time and the fall time are equal, the wave- form is named as a pure triangular waveform. When the rising time and falling time are differ- ent the waveform named Sawtooth Waveform.The saw-
Figure 3. With Schmitt Trigger
Figure 4. Comparator PWM Genaration
tooth waveform can be generated using a cascade com- bination of a Schmitt trigger circuit and an integrator. The Schmitt trigger gives the output as a square wave. By feeding this output by an integrator we can gener- ate triangle and sawtooth waveforms.In our circuit we use PWM as the input, by doing that we can adjust the rising/falling time by using PWM duty adjuster. In this circuit, the non-inverting terminal of the in- tegrating op-amp is given a dc voltage. The second op- amp integrates the Schmitt trigger output of the first op- amp and gives a saw-tooth waveform. By varying the dc voltage given to the non-inverting input of the inte- grator, we can change the rising and falling time of the saw-tooth waveform. When this dc voltage becomes zero, the output will be a triangular waveform. The fre- quency of the output voltage can be varied by varying R1 resistor and capacitor.
Figure 5. Intergrating PWM Wave
3.5. Sine Wave
There are many ways to create a sine wave. Such as Wien bridge oscillator, Phase-shift oscillator, Colpitts crystal oscillator,Square waveform and filter, Pulse- based sine wave generator.
We have chosen the square waveform and filter. From the Fourier series theory, a square wave can be thought of as sinusoidal waves. We can similarly think of triangular waves. So, we can obtain the sinu- soidal waves by sending the amplified triangular waves through a low pass filter.
Figure 6. Sine Wave Genaration
Figure 7. Output Circuit
- Output Circuit 4.1. Summing Amplifier
Summing amplifier is used to give DC offsets and Changing Amplitude of the final output signal.
Gain= Vout/Vin =R f/ Rin
Rf is constant.So
when Rin= 10 k
Gain=10 k/10 k = 1 Also Rin= 1 k
Gain=10 k/1 k= 10
By changing Voltage of non-inverting terminal, We can add DC offsets to the output circuit. 4.2. Push-Pull amplifier
A push-pull amplifier is an amplifier that has an output stage that coulddrive a current in both directions through the load. The output stage of a typical push-pull amplifier consists of two identical BJTs or MOSFETs one sourcing current through the load while the other one sinking the current from the load. 5. Power Supply
Figure 8. Power Supply Circuit
In this project power supply wants to give sev- eral outputs. They are DC outputs because electronic components need DC power. To complete our require- ment, we must create +12V and -12V DC power sup- ply. There were two options for this. They are, Using a rechargeable battery. Using the AC power supply. Con- sidering these two options we are mostly thinking about the second option. Our product is laboratory equipment. And it must be work for a long time. If we choose a rechargeable battery it cannot withstand against those conditions. We use a step-down transformer and plug its inputs to the main power supply through the fuse for the pro- tection of the circuit. Then there is a diode bridge for the full-wave rectifier. Then use a smoothing capac- itors and then use a regulation done by LM78712 and LM7912 regulators. Finally,there is another capacitors for further improving output voltage quality.
- Component Selections
In this project we have use RC oscillator for mak- ing and changing frequiency of the wave. First we de- cided to usevariable resistor. Then we realised that we need more resolution so we usedMulti turn variable resistorsfor fine tuning. Also we usedDescrete set of capacitorsfor make it more fine. Op-amp selectionThe waveform generation and amplification is done by op-amp circuits. To meet the given specifications a suitable op-amp must be se- lected.The selected op-amp for this project is TL084. Which has higher GBP 1MHz and This low noise op- amp has a high slew rate of 13Vus.This is an important
Figure 9. Capacitor set
factor in square wave generation.
maximumgain=1 × 106/2 × 104 = 50
Transistor selectionThe push-pull amplifier output stage is to drive a 50Ωload through the function gener- ator. The maximum power and current which should be given across a 50W load by a 20V peak-to-peak wave can be calculated as follows.
maximum power = 102/50 = 2 W
maximumcurrent=10/50= 200 mA
So We selected TIP 31 and TIP 31 transistors. In power supply circuit we choose LM7812 and LM7912 which has maximum output cuurent of 3A. In that case, we have used GBU6M/DGBU6A bridge rec- tifier. We have used DPDT switches which has locking mechanisms on it for selecting options. 7. Simulations
Power Supply
Figure 10. Power Supply Circuit
Figure 11. Output Results by Power Supply
Output Circuit Main Circuit
Figure 12. Output Circuit
Figure 13. Input Signal for Output cct. Simula- tions
Figure 14. Amplitude Changing
Figure 15. DC Offsets
Figure 16. Main Circuit
Figure 17. Square Wave
Figure 18. 20 Hz
Figure 19. 20000 Hz
Figure 20. Triangular Wave
Figure 21. PWM Wave
Figure 22. Sawtooth Wave
Figure 23. Sine Wave
- PCB Designs
- Enclosure Design
(exploded view, isometric view of front and back)
- Discussion
By using analog electronic components we can make waves. We can determine characteristics such as frequiency, Amplitude, DC offset,Wave shape and Duty Cycle.Using Op amp as the main block of the design, We could implement comparators, Inverters,amplifiers and Schmitt Trigger... etc.By combine them together and using several filters for joints we could make work- ing function genarator at the end.