Design of Single/Multi-Trace Oscilloscope Conversion Circuit

　　In order to measure and compare signals, it is necessary to display different signals or different parts of the same signal on the screen at the same time. All of these require that several waveforms can be displayed on the screen at the same time. To achieve this goal, dual-scan oscilloscope display, multi-line display or multi-trace display are usually used. This article introduces a device that converts a single-trace oscilloscope into a multi-trace display. It is simple to make and can be used with the original oscilloscope, which not only saves expenses but also improves the level of experimental technology.

　　Typical circuit design and analysis

　　The conversion circuit is composed of NE555 time base oscillator, 74LS169 counter and MAX309 multiplexer chips. 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.

　　This circuit uses the 555 chip as an 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 multi-way switch.

　　MAX309 is a dual-circuit switch chip, one for the DC channel and the other for the signal channel. After the two signals pass through the adder, four different signals are displayed on the horizontal position of the oscilloscope at the same time. Since the input signal is an AC signal, dual power supplies are used to ensure the normal transmission of the AC signal and also expand 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 band 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 soldered, first perform an appearance inspection. After the inspection is correct, perform a power-on test. 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 soldering of the circuit; 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; then 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, and 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 band 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. Based on the derivation of the above formulas, the following conclusions are drawn.

　　● 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 rate, 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 size. In the lower frequency band (f<10Hz), bipolar transistor technology is widely used.

　　When choosing 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 transmitted signals and the switching rate. The more channels there are, the greater the parasitic capacitance and leakage current.

　　● Leakage current. An ideal switch requires zero resistance when on, infinite resistance when 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, the leakage current of the analog switch needs to be considered. Generally, the leakage current is expected to be as small as possible.

　　● On-resistance. 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 on-resistance, the smaller the error caused by the switch when the system collects various signals.

　　● Switching speed. Refers to the speed at which a switch is turned on or off. For situations where fast-changing signals need to be transmitted, the analog switch must have a fast switching speed. At the same time, the speed of the A/D converter in the subsequent sampling and holding circuit should also be considered, 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.