Typical Circuit Design and Analysis of Oscilloscope-EEWORLD

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Typical circuit design and analysis of oscilloscope

The conversion circuit is composed of NE555 time base oscillator, 74LS169 counter and MAX309 multiplexer. It can display multiple trace signals simultaneously through one Y channel. The circuit is simple, stable and reliable, with good waveform display effect, which is convenient for signal analysis and research. The circuit is shown in Figure 1. Design of Single/Multi-Trace Oscilloscope Conversion Circuit

Design of Single/Multi-Trace Oscilloscope Conversion Circuit

This circuit uses 555 chip as oscillator, and the square wave output from its 3rd pin is used as the control signal of the switching circuit. The control signal is directly connected to the hexadecimal counter as the selection signal of the multiplexer.

MAX309 is a dual-circuit switch chip, one for DC channel and the other for signal channel. After the two signals pass through the adder, four different signals are displayed simultaneously on the horizontal position of the oscilloscope. Since the input signal is an AC signal, dual power supply is used. In addition to ensuring the normal transmission of the AC signal, it also expands the dynamic range of the signal output.

The external power supply only needs a +5V power supply voltage, a +5V voltage input to the 8th pin of IC1 (ICL7660), and a -5V voltage output at its 5th pin to power the multi-way switch chip and the op amp. IC2 (NE555) is connected in the form of a multivibrator to generate a 35kHz square wave as the time base signal of the hexadecimal counter IC3 (74LS169). Take the lower two bits Qa and Qb of 74LS169 and connect them to the A0 and A1 terminals of IC4 (MAX309) as the selection signal of the multi-way switch. MAX309 is a dual-circuit analog switch chip. One of the circuits is a resistor voltage divider network composed of 2K, 1K, 1K, and 1K, and takes out 3V, 2V, 1V, and 0V DC voltages as the DC components of the waveform to be displayed by the signal, so that it can be displayed at different positions of the oscilloscope. The other is the four input ends of the signal source. Through the control of the four-way switch by Qa and Qb, the loop signal is selected respectively (the two-way selection signal is performed synchronously). After the signal is output by the multi-way switch, it is amplified or attenuated by the operational amplifier circuit IC5 (LF353). The signal is amplified or attenuated by changing the feedback resistors 10K, 10K, 40K, 60K, 60K. Use the four-way switch to select different feedback resistors to achieve different gain control, and finally achieve the purpose of overlapping and adding with the DC component and outputting. Circuit debugging

After the circuit is welded, first perform an appearance inspection, and then perform a power-on test after the inspection is correct. Use a multimeter to measure whether the 5th pin of ICL7660 is -5V. If the output is correct, proceed to the next measurement. Otherwise, immediately turn off the power to check whether there are problems such as short circuit or wrong circuit welding. Use an oscilloscope to observe whether the waveform output by 555 is correct and whether its frequency value is the same as the calculated value. Secondly, use an oscilloscope to observe whether the waveform at the Qa and Qb ends of the counter is the two-frequency and four-frequency division of the 555 time base signal. Finally, observe whether the output of the first switch is a step wave signal, and the step values are 0V, 1V, 2V, and 3V respectively. Then connect 4 different signals respectively, and use an oscilloscope to observe whether the final input waveform is displayed on the horizontal position of the oscilloscope. Changing the xxx switch can change the size of its amplitude.

Analysis of oscillation circuit and analog switch circuit

The accuracy of the output frequency of the NE555 time base oscillator has a great influence on the reliability of the counter composed of 74LS169, so it must be taken seriously. As shown in Figure 1, the oscillation period T = 0.693 (R1 + 2R2) · C generated by the time base oscillator composed of NE555, the oscillation frequency f = 1 / T, that is, f = 1.443 / (R1 + 2R2) · C, the duty cycle of the output oscillation frequency waveform D = t1 / T = (R1 + R2) / (R1 + 2R2)

Note: t1 is the duration of the output pulse

t1 = 0.693 (R1 + R2) · C

When R2 >> R1, D is approximately equal to 50%, that is, the output oscillation waveform is a square wave. The following conclusions are drawn from the derivation of the above formulas.

● The oscillation period has nothing to do with the power supply voltage VDD, but mainly depends on the total time constant of charging and discharging, that is, it is only related to the values of R1, R2 and C.

● The duty cycle of the oscillation signal has nothing to do with the size of the capacitor C, but only with the size ratio of R1 and R2.

The function of analog switches and multiplexers is mainly used for signal switching. At present, integrated analog electronic switches have become the leading products in the field of small signals. Compared with the previous mechanical contact electronic switches, integrated electronic switches have many advantages, such as fast switching speed, no jitter, low power consumption, small size, reliable operation and easy control. But it also has several disadvantages, such as large on-resistance, limited input current capacity, small dynamic range, etc. Therefore, integrated analog switches are mainly used in high-speed switching and small system volume. In the lower frequency band (f<10Hz), bipolar transistor technology is widely used.

When selecting a switch, you need to pay attention to the following indicators:

● Number of channels. Integrated analog switches usually include multiple channels. The number of channels has a direct impact on the accuracy of the transmitted signal and the switching rate of the switch. The more channels there are, the greater the parasitic capacitance and leakage current.

● Leakage current. An ideal switch requires zero resistance when turned on, infinite resistance when turned off, and zero leakage current. The conventional CMOS leakage current is about 1nA. If the internal resistance of the signal source is very high and the transmitted signal is a current, it is particularly necessary to consider the leakage current of the analog switch. Generally, the leakage current is expected to be as small as possible.

● On-resistance. The on-resistance will cause signal loss and reduce accuracy, especially when the load connected in series with the switch is low impedance. Therefore, the better the consistency of the on-resistance, the smaller the error caused by the switch when the system collects various signals.

● Switching speed. Refers to the speed at which the switch is turned on or off. For occasions where fast-changing signals need to be transmitted, the analog switch is required to have a fast switching speed. At the same time, it should also be considered to be compatible with the speed of the A/D converter of the subsequent sampling and holding circuit, so as to select the device with the best performance-price ratio.

In addition to the above indicators, the power supply voltage range of the chip is also an important parameter, which is directly related to the on-resistance and switching speed of the switch. The higher the power supply voltage, the faster the switching speed and the smaller the on-resistance. Conversely, the larger the on-resistance.

Conclusion

During the circuit design and debugging process, it was found that the factor that has the greatest impact on the waveform display effect is the stability of the NE555 oscillator's oscillation frequency and amplitude. In order to obtain a complete and stable waveform on the oscilloscope, the frequency cannot be too low and the amplitude should not be too small.

Keywords：Oscilloscope Reference address：Typical Circuit Design and Analysis of Oscilloscope

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