How to Make a Function Generator?

The generator of functions to be constructed must consist, at least, of the following external controls. Additional input and output connectors can be provided to accommodate, for example, the inputs for AM or FM modulation or to separate the outputs according to the waveform.

Introduction

This project consists of a waveform generator or function generator capable of producing triangular, sinusoidal and square signals, as well as sinusoidal signals modulated in AM (amplitude modulation) and FM (frequency modulation).

This waveform generator is based entirely on a single monolithic IC, the XR-2206, and a limited number of passive circuit components (resistors, potentiometers, capacitors, switches, connectors, etc.). Before constructing the final model of the waveform generator, the students (with the help of the teacher) should experiment with the different modes of operation of the XR2206 by assembling the circuits in a protoboard or prototype matrix.

This function generator or waveform generator is an extremely versatile and useful laboratory instrument for the student, the engineer, or anyone interested in electronics. Its cost is only a fraction of the cost of commercial and professional function generators available today.

General Description

The primary circuit and the external components needed to build a high-quality function generator are shown in figure 2. The circuit is designed to operate with a single 12 V power supply, or with an asymmetric split source of ± 6 V For most applications, symmetric source operation is preferred because it produces an output level cc almost equal to the potential ground level (0 V). During the experiments with the circuit, we will also try to feed it with batteries and with symmetric sources of ± 5 V and ± 8 V.

The circuit of Figure 2 provides three basic waveforms: sine, triangle, and rectangular or square. 4 frequency ranges give a total frequency range of 1 Hz to 100 kHz. In each field, the frequency can be finely tuned using a potentiometer (R13) in a variety of 100: 1.

The sinusoidal or triangular output can be varied approximately from 0 to 6 Vp-p from an impedance of 600W.

The rectangular wave output is available in the Sync Out output of the XR2206 and can serve to synchronize an oscilloscope or serve as an input for logic circuits.

BASIC CIRCUIT OF THE GENERATOR OF 3 WAVES FORMS

Frequency ranges: the function generator is designed to operate over a range of 4 overlapping frequency ranges:

1 Hz to 100 Hz connects C3 = 1 mF

10 Hz to 1 kHz connects C4 = 0.1 mF

100 Hz at 10 kHz connects C5 = 0.01 mF

1 KHz at 100 kHz connects C6 = 0.001 mF

These frequency ranges are selected by connecting capacitors of different capacitive values (C3 to C6) between the inputs TC1 and TC2 of the XR2206, using the rotary switch S1 of 1 pole four positions.

The precision of the Frequency: the accuracy of the frequency generated by the XR2206 is achieved using the timer resistor R and the timer capacitor C, and is given by f = 1 / R

            R = R4 + R13

C = C3 or C4 or C5 or C6, according to the selected frequency range.

The formula above is accurate within 15% in any frequency range.

Sinusoidal and triangular output: the amplitude of the sinusoidal or triangular production is variable from 0 to 6 Vp-p. The potentiometer R12 of FIG adjusts the amplitude. 2. At any amplitude setting, the magnitude of the triangular output is approximate twice the amplitude of the sinusoidal output. The internal output impedance is 600W.

The selection of the waveform is made using the triangle/sine selector switch, S2.

Distortion of the sinusoidal signal: the total harmonic distortion (THD) of the sine wave is less than 1% in the range of 10 Hz to 10 kHz and less than 3% over the entire frequency range.

The total harmonic distortion of a sine wave generator indicates the purity of the sinusoidal waveform. If we achieve a THD = 0%, it would mean that we have made a perfect sinusoidal signal.

Square wave output: the circuit of figure 2 has 2 square wave outputs, with a duty cycle of 50%. The direct production of Sync Out of the XR2206 corresponds to the complete variation (Vp-p) of the power supply. The output through resistor R6 (point L) corresponds to the upper half of the power supply.

Frequency modulation (external sweep): the frequency of the output signal can be modulated or swept by applying an external control voltage to the external sweep terminal (a point I). When not in use, the terminal should be left open. The open-circuit voltage of this terminal is approximately 3V above the negative supply voltage, and its impedance is about 1000W.

Amplitude modulation: the amplitude of the output varies linearly with the modulating signal applied to the input AM (point Q of figure 2). The magnitude of the production reaches its minimum when the control voltage AM approaches half of the total energy of the power supply. The phase of the output signal is inverted when the amplitude crosses its minimum value. The total dynamic range is approximately 55 dB (decibels), with a variety of AM control voltage of 4 V relative to half the total energy of the power supply. When not in use, terminal A should be left open.

Power supply: the specifications for the power supply are as follows:

• Symmetric source: ± 6V, 15 mA load current
• Single source: + 12V, 15 mA load current

For operation with a single power supply, polarization resistors R14 and R15 must be used, ground point GND must be left floating and terminal (-) of the source must be connected to ground (GND).

In figure 3, the recommendations for feeding using asymmetric sources or using batteries are illustrated.

•        a      Transformer T1: Primary 220 V Secondary 12.6 V 0.5 A
•             D1 – D4: 1N4001 or similar
•             D5 – D6: 1N4735 or similar
•             R1 – R2: 51W, ½W, 10%

The realization of the power supply corresponds to the practical work of another group, so the information presented here is purely informative.

Details

1. Frequency range selector switch, S1: if additional frequency ranges are needed, these can be added using a switch with more positions. Also, one of the positions of switch S1 could be used to turn the equipment off or on, as shown in figure 1.
2. Triangular/sinusoidal wave selector switch, S2: selects the triangular or sinusoidal wave output. Optionally, we can make this switch select all the output wave options, namely: triangular, sinusoidal, square maximum amplitude, and square half amplitude. This is the option that we should choose to perform the output, as shown in figure 1. To achieve this, modifications must be made to the circuit of number 2, which must be studied and carried out by the students.

Trimmers and potentiometers

3. Adjustment of the DC offset level, R9: it is used to adjust the dc level of the triangular or sinusoidal wave.

4. Adjustment of the distortion of the sine wave, R10: used to minimize the THD of the sinusoidal output.
5. Adjustment of the symmetry of the sine wave, R11: it is used to optimize the balance of the sinusoidal output.
6. Amplitude control, R12: adjusts the amplitude of the triangular or sinusoidal output.
7. Frequency setting, R13: sets the frequency of the oscillator for any range of the setting of switch S1. Therefore R13 serves as a frequency tuner in a conventional waveform generator and varies the frequency of the oscillator in a variety of approximately 100: 1.

Material’s List

• Capacitors
  • C1, C2, C7 10mF / 10V, electrolytic
  • C3 1mF, non-polar, 10%, of mylar
  • C4 0.1mF, 10%, of mylar
  • C5 0.01mF, 10%, of mylar
  • C6 1000pF, 10%, of mylar
  • Resistances:
    • R1 30KW, 1 / 4W, 10%
    • R2 100KW, 1 / 4W, 10%
    • R3, R7 1KW, 1 / 4W, 10%
    • R4 9KW, 1 / 4W, 10%
    • R5, R6 5KW, 1 / 4W, 10%
    • R8 300KW, 1 / 4W, 10%
    • RX 62KW, 1 / 4W, 10% (RX can be eliminated for maximum output)
    • R14, R15 5.1KW, 1 / 4W, 10% (Used in applications with single cc source)
    • Potentiometers:
      • R9 1MW, 1 / 4W, trumpet
      • R10 1KW, 1 / 4W, trumpet
      • R11 25KW, 1 / 4W, trumpet
      • R12 50 KW, linear amplitude control
      • R13 1MW, frequency control, audio taper
    • Switches or switches:
      • S1 Rotary switch with one pole and five positions (1 for On / Off)
      • S2 SPST switch, sliding
    • Others:
      • Materials necessary for the adequate completion of the project: project box, knobs, conductors, terminals, connectors, LEDs

Instructions

1. Purchase items from the bill of materials
2. Study of the requirements and the theory of operation
3. Mounting the circuit on a breadboard and test different operating modes
4. Final assembly diagrams in a project box
5. Purchase of materials necessary for the final assembly
6. Realization of the printed circuit board (PCB)
7. Assembly of the components on the PCB and completion of the construction of the project
8. Measurements of the characteristic specifications in the different operating modes
9. Realization of the report
10. Exam

Minimum measurements

Measuring instruments to be used:

1. Digital or analog multimeter
2. Oscilloscope

Measurements

For each selected frequency range, measure:

Waveforms

Draw the waveform for the triangular signal, as seen on the oscilloscope. Record the maximum and minimum values of the amplitude of the output voltage, as well as the period measured and the calculated frequency of the signal, for the following cases:

a) Triangular signal

b) Sinusoidal signal

c) Square signal

d) AM signal, also note the frequency and amplitude of the modulating signal

e) FM signal, record the minimum rate, and the maximum frequency of the FM output.