Keywords: ICL8038；; Signal generator; Frequency modulation circuit; electronic instrument
China Library Classification. : TP39 document identification code: a article number:1009-2374 (2009) 05-0014-03 signal generator is an electronic instrument that can generate standard signals, and it is one of the electronic instruments frequently used in industrial production, electricians and electronic laboratories. In various fields of modern electronics, signal generators with high precision and convenient frequency adjustment are often needed. Generally speaking, the higher the frequency, the more kinds of waveforms, the better the performance of the generator, but the cost and technical requirements of the device are also greatly improved. Therefore, under the premise of meeting the work requirements, the generator with high cost performance is our first choice. The common signal generator is a simple signal generator made of ICL8038, but this signal generator can only generate sine wave, triangular wave and rectangular wave. What this paper is going to study is how to combine its peripheral circuit and direct frequency modulation circuit to generate other signals on the basis of ICL8038.
First, the overall design
Design block diagram of (1) signal generator
The block diagram of the signal generator is shown in figure 1:
Figure 1 signal generator design block diagram
In figure 1, the waveform generator circuit consists of a monolithic integrated circuit function generator ICL8038 and its peripheral circuits, which are used to display square waves, sine waves and triangular waves. The direct frequency modulation circuit consists of a timing crystal and a varactor diode, and the sine wave generated by the waveform generation circuit will generate a frequency modulation wave through this circuit. The display circuit consists of a single chip frequency meter ICM72 16D, a crystal, a capacitor, a switch and an LED digital tube [1 1], which is used to display the frequency value of the output waveform.
(2) General circuit diagram of signal generator
The general circuit of the signal generator is shown in Figure 2:
Second, the circuit design of each part
(1) Design of Waveform Generation Circuit Based on ICL8038
The function generator composed of ICL8038 is shown in Figure 3. The resistor R 1 and the potentiometer RP 1 are used to determine the DC potential U8 of pin 8, usually U8≥2UCC/3. The higher U8, the smaller IA and IB, and the lower output frequency, and vice versa. Therefore, ICL8038 is also called voltage controlled oscillator (VCO) or frequency modulator (FM). The adjustable frequency range of RP 1 is 20HZ~20KHZ. U8 can also provide a fixed potential through seven pins. At this time, the output frequency f0 is only determined by RA, RB and capacitor Ct. When UCC is powered by dual power supplies, the DC level of the output waveform is zero. When using a single power supply, the DC level of output waveform is UCC/2.
(2) The design of crystal oscillator varactor direct frequency modulation circuit.
Fig. 4 is a varactor direct frequency modulation circuit of a 100MHz crystal oscillator. In fig. 4, the T2 tube is connected to the Pierce crystal oscillator circuit, and the frequency is directly modulated by the varactor diode. The resonant circuit on the collector of T2 tube is tuned to the third harmonic of the crystal oscillation frequency to realize the function of triple frequency. T 1 electron tube is an audio amplifier. After the input signal is amplified, it is applied to the varactor diode through a 2.2μH high frequency choke. At the same time, the power supply voltage of T 1 is also applied to the varactor through the 2.2μH high frequency choke as the bias voltage of the varactor.
When the crystal oscillator is frequency modulated, the frequency stability is lower than that of the crystal oscillator without frequency modulation because the varactor diode is introduced into the oscillation circuit. Generally, its short-term frequency stability reaches the order of 10-6 and its long-term frequency stability reaches the order of 10-5.
Fig. 4 crystal oscillator varactor diode direct frequency modulation circuit
(3) Design of display circuit based on ICM72 16D.
The display circuit consists of a monolithic frequency meter ICM72 16D, a crystal, a resistor and a capacitor. ICM72 16D is a special frequency measurement large-scale integrated chip first developed by Intersil Company of the United States. This is a standard 28-pin dual in-line integrated circuit, which is powered by a +5V regulated power supply. It includes a high-frequency oscillator, a decimal counter, a seven-segment decoder, a bit multiplexer, an eight-segment one-segment code driver that can directly drive the LED display screen, and an eight-bit one-bit code driver. The basic frequency measurement range is from DC to 100MHz, and the upper limit frequency can reach 40MHz or 100 MHz if the preset frequency divider circuit is added. The monolithic frequency meter ICM72 16D can be combined with a few devices such as crystal oscillator, range selection and LED display to form a miniature frequency meter from DC to 40MHz, which can be used for frequency measurement and mechanical speed measurement.
Fig. 5 is a display circuit based on ICM72 16D. Taking the oscillation frequency 10MHz composed of crystal and capacitor C 1 and C2 as the reference frequency, the gate time pulse is generated after internal frequency division by ICM72 16D. Use switch K to select the gear position, and use switch S 1 to control the circuit to reset, and S2 can make the circuit in the holding state. Dps of eight LED digital tubes are all connected to the DP decimal input pin (pin 23), and the correct decimal places are generated by the internal decimal logic unit. When the measured signal is output, ICM72 16D counts its frequency, and 8-bit LED displays the measured frequency bit by bit, thus achieving the purpose of measurement and display.
The specific working process is: using a 2.5MHz crystal oscillator, 22MΩ resistors and capacitors C 1 and C2 to meet the normal operation of the internal oscillator. Because the internal oscillator is a high-gain CMOS inverter, a resistor is connected in parallel with the crystal oscillator to provide sufficient bias. At this time, the fundamental oscillator of the chip is 2.5MHz. If the 2.5MHz crystal oscillator is replaced by 1MHz crystal oscillator, the capacitance between pin 25 and pin 26 of the chip needs to be adjusted. At this time, the fundamental vibration of the chip is 1MHz. In addition, the chip also allows the use of external oscillators. If an external oscillator is used, the fundamental frequency of the chip is equal to the frequency of the external oscillator circuit. At this time, the internal oscillator circuit of the chip is still working, but it will not affect the normal measurement of the chip. If the internal oscillation frequency is less than 1MHz or only the external oscillation circuit is working, pin 25 and pin 26 must be connected together to ensure sufficient floating level. If the output of the external oscillation circuit is TTL level, a resistor of 22 mω needs to be connected between pin 25 and pin 26, and pin 24 and pin 25 should be connected together. If the frequency of the external oscillation circuit is less than 100KHz, the external oscillation circuit has no influence on the chip, and the chip still works at the frequency of the internal oscillation circuit.
The measurement signal is input from pin 28. If the input signal is small, a preamplifier circuit can be used. If the input signal is too large, a limiter circuit can be used. The eight-bit driving lines of D 1 ~ D8 are connected to the male * * terminals of the eight-bit LED, the segment driving output lines A ~ G are connected to the corresponding pins of the LED, and the decimal points of 1 ~ 7 bits of the LED are all connected to the 23 pins, so that the internal decimal logic unit generates the correct decimal point. Eight-bit LED is the bit-by-bit display of indicator light, the frequency is 500Hz, the bit signal time is 244μs, and there is a bit blank time of 6μs between two displays to prevent ghosting. The maximum segment driving current of the chip is 15mA, and the rated segment driving current is 12mA. To increase the display brightness, the power supply voltage can be increased to 6V. When measuring and displaying, the zero to the left of the decimal point is eliminated, and the right bit is displayed as usual. When the frequency of the measured signal exceeds the measuring range of the frequency meter and overflows, the eighth digit after the decimal point can be lit inside the chip, indicating that overflow occurs at this time.
In fig. 5, k is a four-speed switch for selecting different ranges. S 1 is a key switch. When pressed, the pin 12 is at a low level, and the main counter stops counting and displays zero. When S2 is pressed, pin 27 is at high level, the main counter stops counting, and the data is self-locked and displayed; When S2 is disconnected, the main counter starts counting again. Because the signal used in the composite control input is a bit drive signal, in order to avoid the composite control signal from affecting the bit signal, a diode is used for isolation, and the resistor and capacitor connected to the chip pin 1 are used to reduce noise and interference.
Fig. 5 shows the display circuit based on ICM72 16D.
(4) Design of shaping circuit
Because ICM72 16D chip can only count pulse signals, sine wave and triangle wave generated by waveform generation circuit must be shaped first, and then they can be sent to display circuit for frequency display. The shaping circuit can be realized with only one NAND gate. In this paper, 74LS20 chip is selected for shaping. Pin diagram of 74LS20 chip is shown in Figure 6:
Third, the result analysis
In this paper, 10V, 5V DC power supply is used, and the output waveform is displayed by digital oscilloscope.
The signal generator can output sine wave, triangular wave, rectangular wave and frequency modulation wave; The lowest frequency of sine wave, triangular wave and rectangular wave is 55. 10Hz, and the highest frequency is 16. 13 kHz. The peak value of sine wave can reach 4.36 v; The peak value of triangular wave can reach 6.6V, and the duty cycle can be adjusted between 44.4% and 50.4%. The peak-to-peak value of rectangular wave can reach 20.2V, and the duty cycle can be adjusted between 4 1.3%~57.5%. The peak-to-peak carrier frequency in FM circuit is 5.6V, and the frequency is13.3mhz.: ; Due to the addition of a divide-by-4 circuit in the display circuit, the upper limit frequency of frequency measurement can reach 40MHz.
Four. conclusion
In this paper, the function generation chip ICL8038 is combined with peripheral circuits to generate sine wave, triangular wave and rectangular wave, and then the generated sine wave is input to the varactor direct frequency modulation circuit of the crystal oscillator to generate frequency modulation wave output, and the display circuit composed of ICM72 16D shows the frequency of each waveform. The system is completely composed of hardware, which avoids programming problems, has simple circuit, is easy to debug and produces various waveforms. refer to
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