Department: Department of Electronic Engineering Major: Applied Electronic Technology Session: 07 Name: Li Xianchun
abstract
The system uses ICL8038 integrated block as the core device to make function signal generator, which has low manufacturing cost. Suitable for students to learn electronic technology measurement. ICL8038 is a precision oscillating integrated circuit with various waveform outputs. Low distortion sine wave, triangular wave, rectangular wave and other pulse signals of 0.001Hz ~ 30kHz can be generated only by individual external components. The frequency and duty ratio of the output waveform can also be controlled by current or resistance. In addition, because the chip has a modulation signal input, it can be used for frequency modulation of low-frequency signals.
ICL8038, waveform, schematic diagram, common connection.
I. Overview
Various signal waveform generators are often used in electronic engineering, communication engineering, automatic control, telemetry control, measuring instruments, meters and computers. With the rapid development of integrated circuits, various signal waveform generators can be easily constructed with integrated circuits. Compared with other signal waveform generators, the signal waveform generator realized by integrated circuit has greatly improved the waveform quality, amplitude and frequency stability.
Second, the scheme demonstration and comparison
2. 1, system function analysis
The core problem of this design is signal control, including signal frequency, signal type and signal strength control. In the design process, we comprehensively considered the following three implementation schemes:
2.2. Scheme demonstration
Scheme 1: The traditional direct frequency synthesizer is adopted. This method can realize fast frequency conversion, low phase noise and the highest working frequency among all methods. However, due to the use of a large number of frequency multiplication, frequency division, mixing and filtering links, the direct frequency synthesizer has complex structure, huge volume, high cost, and is easy to produce too many stray components, so it is difficult to achieve high spectral purity.
Scheme 2: PLL frequency synthesizer is adopted. The output frequency of the voltage controlled oscillator is locked at the required frequency by using the phase locked loop. This frequency synthesizer has good narrow-band tracking characteristics, can select the required frequency signal well, suppress stray components, avoid quantization filters, and is conducive to integration and miniaturization. But because PLL itself is an inert link, the locking time is long, so the frequency conversion time is long. Moreover, the parameters such as amplitude and frequency of sine wave synthesized by analog method are believed to be difficult to control.
Scheme 3: Using 8038 single-chip voltage control function generator, 8038 can generate sine wave, square wave and triangular wave at the same time. Changing the modulation voltage of 8038 can realize numerical control adjustment, and its oscillation range is 0.00 1 Hz ~ 300 kHz.
Third, the working principle and analysis of the system
3. Application of1and ICL8038
ICL8038 is a precise waveform generator and voltage-controlled oscillator. Its basic characteristics are: it can generate and output sine wave, triangle wave, sawtooth wave, square wave and pulse wave at the same time; The external resistance and capacitance can be changed, and the frequency range of the output signal can be 0.001Hz ~ 300 khz; The output distortion of sine signal is 65438 0%; The linearity of triangular wave output is less than 0.1%; Duty cycle varies from 2% to 98%; The external voltage can modulate or control the frequency and duty ratio of the output signal (asymmetry); The temperature stability (typical value) of the frequency is120 *10-6 (ICL8038acjd) ~ 250 *10-6 (ICL8038cpd); As for power supply, single power supply (V+):+ 10 ~+30V, and dual power supply (+V) (V-): 5V ~ 15V. Figure 1-2 is the pin arrangement diagram, and figure 1-2 is the functional block diagram. 8038 is packaged in DIP- 14PIN, and the pin functions are shown in table 1- 1.
3.2, ICL8038 internal block diagram introduction
The circuit structure of the function generator ICL8038 is shown in the dotted box (Figure 1- 1), and * * * has five components. The currents of the two current sources are IS 1 and IS2 respectively, IS 1=I and IS2 = 2I；; The threshold voltages of the two voltage comparators I and II are sum respectively, the input voltage of which is equal to the voltage uC at both ends of the capacitor, and the output voltage of which controls the S end and the rs end of the RS flip-flop respectively; The state output Q of RS flip-flop is used to control the switch S to realize the charging and discharging of the capacitor C; The charging currents Is 1 and Is2 are determined by external resistance. When Is 1=Is2, a triangular wave is output, otherwise it is a rectangular wave. Two buffer amplifiers are used to isolate the waveform generation circuit from the load, so that the output resistance of the triangular wave and rectangular wave output terminals is low enough to enhance the load capacity; The triangle wave to sine wave circuit is used to obtain sine wave voltage.
3.3, internal block diagram working principle
★ When the function generator ICL8038 is turned on, the voltage of the capacitor C is 0 V. According to the voltage transmission characteristics of the voltage comparators, the output voltages of the voltage comparators I and II are low. Therefore, the output q of RS flip-flop is 0;
★ Turn off the switch S, the current source is 1 to charge the capacitor, and the charging current is
IS 1=I
Because the charging current is constant, the voltage uC on the capacitor rises linearly with time.
★ When it rises to VCC/3, the output of voltage comparator II is high. At this time, when S=0 of the RS flip-flop, Q sum remains unchanged.
★ When it rises to 2VCC/3, the output voltage of the voltage comparator I jumps to a high level. At this time, when the RS trigger is set and Q= 1, the switch S is closed and the capacitor C begins to discharge. The discharge current is is2-is1= i. Because the discharge current is constant, the voltage uC on the capacitor decreases linearly with time.
At first, although the decline of uC makes the S terminal triggered by RS jump from high level to low level, its output remains unchanged.
★ Until uC drops to VCC/3, the output voltage of voltage comparator II jumps to low level. At this time, Q=0, so that the switch S is turned off and the capacitor C starts charging again. Repeating the above process, the circuit produces self-excited oscillation.
Because the charging current is equal to the discharging current, the voltage on the capacitor is a triangular wave, and the Q sum is a square wave, which is output by the buffer amplifier. Triangle wave voltage outputs sine wave voltage through triangle wave to sine wave circuit.
Conclusion: By changing the charging and discharging current of the capacitor, rectangular wave and sawtooth wave with adjustable duty ratio can be output. However, when the output is not a square wave, the output will not get a sine wave.
3.4, the scheme circuit working principle (see figure 1-7)
When the external capacitor C can be charged and discharged by two constant current sources, the thresholds of the voltage comparators I and II are 2/3 of the total power supply voltage (+Vcc and -VEE) and 1/3, respectively. The constant current sources I2 and I 1 can be adjusted by external resistors, but I2 must be greater than I 1. When the output of the flip-flop is low, the constant current source I2 is turned off, and the constant current source I 1 charges C, and the voltage UC across it rises linearly with time. When it reaches 2/3 of the power supply voltage, the output voltage of the voltage comparator I jumps, so that the output of the trigger changes from low level to high level, and the constant current source I2 is turned on, because I2 > I 1 (let I2=2I 1). When the voltage drops to 1/3 of the power supply voltage, the output voltage of the voltage comparator II jumps, so that the trigger outputs a square wave, and the square wave signal is output from the 9 pins through the inverting buffer. The voltage UC on C has the same rising and falling time (triangular wave), and the triangular wave signal is output from pin 3 through the voltage follower. Converting triangular wave into sine wave is realized by a nonlinear network (sine wave converter). In this nonlinear network, when the two ends of the triangular wave become smooth sine waves, they are output from two pins.
Where K 1 is the output frequency band selection switch, K2 is the output signal selection switch, and the potentiometer W 1 is the output frequency fine-tuning potentiometer, the potentiometer W2 adjusts the square wave duty ratio, and the potentiometers W3 and W4 adjust the nonlinear distortion of the sine wave.
Figure 1- 1
3.5. The voltage transmission characteristics of the two voltage comparators are shown in Figure 1-4.
Figure 1-4
3.6, ordinary connection
Figure (1-2) shows the pin diagram of ICL8038, in which 8 pins are frequency modulation (FM) voltage input terminals, and the oscillation frequency of the circuit is proportional to the FM voltage. The FM bias voltage output by pin 7 is the difference between pin 7 and power supply +VCC, which can be used as the input voltage of pin 8.
As shown in the figure (1-5), ICL8038 has two common basic connections. The rectangular wave output is in the form of open collector, which requires an external resistor R 1 to +VCC. In the circuit shown in Figure (a), RA and RB can be adjusted independently. In the circuit shown in fig. (b), the values of RA and RB are adjusted by changing the sliding position of the potentiometer RW.
Figure 1-5
When RA=RB, the waveform of each output terminal is as shown in the following figure (a), and the duty ratio of the rectangular wave is 50%, so it is a square wave. When RA≠RB, the rectangular wave is no longer a square wave, and the output of pin 2 is no longer a sine wave. Figure (b) shows the waveform of each output terminal when the duty cycle of rectangular wave is 15%. According to the internal circuit and external resistance of ICL8038, the expression of duty cycle can be deduced as follows.
Therefore, ra < 2RB.
In order to further reduce the distortion of sine wave, a circuit as shown in figure (1-6) can be adopted. Resistor 20K and potentiometer RW2 are used to determine the DC voltage V8 of 8 pins, usually V8≥2/3Vcc. The higher V8, the smaller Ia and Ib, and the lower output frequency, and vice versa. The adjustable frequency range of RW2 is 20 Hz and 20 kHz. V8 can also be provided with a fixed potential by seven pins, in which case the output frequency f0 is only determined by the capacitances of the pins Ra, Rb and 10. When Vcc is powered by two pairs of power supplies, the DC level of the output waveform is zero, and when VCC is powered by a pair of power supplies, the DC level of the output waveform is Vcc/2. A circuit consisting of two potentiometers of100kΩ and two resistors of10kΩ can reduce the sine wave distortion to 0.5% by adjusting them. When RA and RB are unchanged, adjusting RW2 can make the ratio of the maximum and minimum oscillation frequency of the circuit reach 100: 1. The input voltage is directly applied between pin 8 and pin 6 to adjust the oscillation frequency, and the difference between the highest frequency and the lowest frequency can reach 1000: 1.
3.7, the actual line analysis
An LF35 dual-channel operational amplifier can be added at the output for waveform amplification and impedance conversion. According to the selected circuit components, the output frequency range of this circuit is about 10 Hz ~ 20 Hz. Amplitude modulation range: sine wave 0 ~ 12V, triangle wave 0 ~ 20V and square wave 0 ~ 24V. In order to obtain a higher frequency, the value of the third capacitor can also be changed.
Figure 1-6
Table 1- 1 ISL8038 Pin Functions
Working energy of pin symbol
1, 12 SINADJ 1, SINADJ2 sine wave adjustment is over. SINADJ 1 is usually open or connected to DC voltage.
SINADJ2 is connected with resistors REXT to V- to improve the sine wave shape and reduce the distortion.
2 SINOUT sine wave output
Three-channel output triangular wave output
4,5 dfadj1,the adjustment of repetition frequency and duty ratio (or waveform asymmetry) of DFADJ2 output signal is finished. Generally speaking, DFADJ 1 terminates resistors ra to V+, and DFADJ2 terminates resistors RB to V+. Changing the resistance can adjust the frequency and duty cycle.
6 V+ positive power supply
7 DC bias voltage for FM bias frequency modulation operation.
8 FMIN FM voltage input terminal
9 SQOUT square wave output
10 C external capacitor is connected to V terminal to adjust the frequency and duty ratio of output signal.
1 1 V- negative power terminal or ground.
13, 14 CNC empty feet
Fourthly, the printed circuit board is manufactured.
First, make the printed circuit board according to the drawing, and be careful not to have broken lines and short circuits. Then, solder the components according to the schematic diagram and the components of the printed circuit board. You can arrange the components on the panel according to your own habits and follow reasonable principles, minimize the length of the connecting wires and keep the transformer away from the output terminals. Then turn on the power supply for debugging, adjust the working point of the amplifier of the discrete element oscillation circuit, and make it in the amplification state to meet the conditions of amplitude oscillation. Check the feedback condition carefully to make it meet the positive feedback condition, so as to meet the phase start condition.
After the production is completed, the whole machine should be debugged. First, measure the voltage of the power supply branch to make sure it is correct, then plug in integration express and install the connecting wire. The corresponding change of waveform can be observed by oscilloscope, and the magnitude and frequency of amplitude can be read by oscilloscope.
Verb (abbreviation of verb) System Test and Error Analysis
5. 1, test instrument
Double trace oscilloscope YB4325(20MHz), multimeter.
5.2, test data
Basic waveform frequency measurement results
Frequency/kHz
Sine wave preset 0.0 1.022 20 50 100.
Measured 0.0095 0.0196 2.0003 20.0038 50.00096100.193.
Square wave preset 0.0 1 0.02 2 20 50
The measured value is 0.095 0.01971.0002 2.0004 20.0038.
The preset value of triangle wave is 0.01.021.220100.
The measured value is 0.0095 0.01961.0002 2.0004 20.0038100.05438+0.
5.3, error analysis and improvement measures
Sine wave distortion. Adjusting R 100K potentiometer RW4 can reduce the distortion of sine wave to 1%. If a sine wave with distortion close to 0.5% is needed, just connect two 1 1 potentiometers between pin 6 and pin1.
The output square wave is asymmetrical. By changing the resistance of RW3 to adjust the frequency and duty cycle, a square wave with a duty cycle of 50% can be obtained. Potentiometer RW3 and external capacitor C jointly determine the frequency of the output waveform, and adjusting RW3 can make the waveform symmetrical.
There is no oscillation. The 10 pin is short-circuited with the 1 1 pin, and then it can be disconnected.
Waveform distortion may be caused by signal interference caused by long capacitor pins, and the problem can be solved by cutting the pins short. It is also possible that the power of 2030 is too high and the waveform is distorted, so it is enough to install a heat sink.
5.4, debugging results analysis
The output sine wave will not distort the frequency. Due to the limitation of the rising rate of the post-stage operational amplifier, the high-frequency sine wave (F >；; 70KHz) produces distortion. The output can be stepped by 0.2V, and the peak-to-peak value can be extended to 0 ~ 26 V. ..
Figure 1-2
Figure 1? seven
Conclusion of intransitive verbs
Through the design of this paper, we have an essential understanding of the working principle of ICL8038, and have mastered the internal structure and working principle of ICL8038, such as pin function and working waveform. The function generator made of ICL8038 has simple circuit, convenient debugging and complete functions. Sine wave, square wave and triangle wave can all be output, and the output waveform is stable and clear, with good signal quality and high precision. The system has a wide output frequency range and is economical and practical.
Seven. refer to
1 Xie Electronic Circuit Design, Experiment and Test (3rd Edition) Wuhan: Huazhong University of Science and Technology Press. July 2000
2 Yang Bangwen's Household Circuits of New Integrated Devices. Beijing: Electronic Industry Press, 2002.8.
Organizing Committee of the 2nd National Electronic Design Competition for College Students. Selected works of the national electronic design competition for college students. Beijing: beijing institute of technology press, 1997.
Li Yanqing's Graduation Thesis Writing and Examples. Xiamen: Xiamen University Press. 2006. 10
5 Tan Boxue, Miao, Principles and Applications of Integrated Circuits, Beijing: Electronic Industry Press. 2003.9
Chen Zicheng, Design and Debugging of Household Electronic Circuits, Beijing: China Electric Power Press. 2006