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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Small-sized two-beam oscilloscope-multimeter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology An oscilloscope is one of the most necessary measuring instruments in the complex at the workplace of a radio amateur, but at the same time one of the most expensive of the equipment. That is why the desire to design such a product among radio amateurs never dries up. In this article, readers are invited to familiarize themselves with the original construction of a small-sized two-beam oscilloscope, which is not difficult to make on your own. Looking through the Radio magazines, I did not find a single device using liquid crystal graphic indicators. Therefore, I propose my development as a basis (base unit) for use in various amateur radio designs. I want to warn you right away that this oscilloscope was not created as a ready-made measuring device, but as a device that allows you to demonstrate the main possibilities of combining microcontrollers and graphic indicators. This can explain the absence of service functions in the microcontroller program, such as indication of the operating mode, the dimension of the measured values and the mode of cursor measurements. I hope that the publication of this development will serve as an impetus for the creation by radio amateurs of a number of original and useful designs. Technical specifications
The main part of the circuit diagram is shown in fig. 1. It contains two identical amplifiers A1 and A2, assembled on a dual operational amplifier DA1, microcontroller DD1, meter R, C (A3). As an indicator, a liquid crystal module with a resolution of 128x64 pixels of the MT12864A-1 type with a built-in controller and a power supply driver (-8 V) of the LCD [1] was used. The resistor 1R6 (2R6) is designed to bias the "beams", the dual switch 1SA1 (2SA1) sets the gain of the operational amplifier DA1.
The input divider is structurally assembled on a small-sized connector 1XS-1XS5 (2XS -2XS5). The signals from the outputs of devices A1, A2 and A3 are fed to the inputs RAO, RA1 and RA3 of the microcontroller DD1, configured as analog inputs of the ADC. Switch SA1 is used to turn on the LCD backlight. Switch SA2 sets the operating mode "oscilloscope - multimeter". Button SB1 - "Start", sweep in oscilloscope mode or measure "R" in multimeter mode. Button SB2 - "CLS", clearing the screen. Button SB3 - "kY", software setting of the gain along the Y axis in oscilloscope mode or measurement "C" in multimeter mode. Button SB4 - "kX", setting the sweep speed. An external signal to start the sweep ("Start") must have a positive polarity with a TTL level, it is fed through the input jacks XS1 and XS2 to the transistor VT1. Since the oscilloscope operates in the single-start sweep mode with further storing of the signal on the display screen, there is no need to use synchronization when examining periodic signals, which greatly simplifies the circuit. Through the resistor R4, power is supplied (about -8 V) to the LCD. By selecting the resistance of this resistor, the contrast of the image on the indicator is set. Port C (outputs RC0-RC7) of the microcontroller is used to transfer data to the indicator. Internal "pull-up" resistors are programmatically connected to the outputs RB0-RB4. When operating in the oscilloscope mode, the DD1 microcontroller digitizes in turn the signal from the outputs of amplifiers A1 and A2 (channels 1 and 2) and turns on the corresponding points on the indicator (128 points along the X axis). To increase the sweep speed in the first three sweep modes, only one first channel is used (for this, the microcontroller operation algorithm has been changed). The digitized values of the signal of the first channel are recorded in the RAM of the microcontroller, and then after recording all 120 (the last 8 did not have enough RAM) points are displayed on the indicator. The microcontroller used uses a 10-bit ADC, and the indicator has a total of 64 points along the Y axis, which corresponds to 6 digits. This is used for software gain control. Eight digits are selected for displaying on the screen: in mode 2 (x1) the highest six digits out of eight are displayed on the screen, in mode 1 (x0,5) the middle six digits are used, which is equivalent to a 2-fold increase in sensitivity, in mode 0 (x0,25, 6) - the lower 4 digits, which is equivalent to a 4,6-fold increase in gain. The reference voltage source of the ADC is programmatically connected to the +1024 V power supply, so the ADC “division price” is equal to Ucc/XNUMX. Information about the modes of software gain control and sweep time is displayed as a single-digit number in the upper left corner of the indicator when the corresponding button is pressed briefly. At the same time, the modes are switched "in a circle". In multimeter mode, the ADC is connected to the output of the first channel of the oscilloscope, it periodically displays a code corresponding to the input signal in the form of a two-digit number in the upper left part of the indicator (from 63 to 1), which corresponds to the position of the point along the Y axis in oscilloscope mode. When you press the SB1 button (Fig. 3) "Start / R" in the central upper part of the indicator, a three-digit number is displayed corresponding to the measured resistance value (taking into account the multiplier set by the 1SA800 switch). The maximum value of the number is limited by a value approximately equal to 3, which is due to the limitation of the voltage at the output of the current source, assembled on the 1VT2 transistor (Fig. XNUMX).
The 3HL1 LED is used as a reference voltage source. Resistors 3R3-3R5 set the currents of the current source in each range. The 3VT3 transistor is used to discharge the measured capacitor. When you press the SB3 "kY / C" button, the 3VT3 transistor closes the measured capacitance. When the button is released, the transistor closes and the voltage across the measured capacitance begins to increase. The microcontroller counts the capacitor charging time to a voltage of 0,287 V. This time, numerically equal to the measured capacitance (taking into account the 3SA1 switch multiplier), is displayed in the middle upper part of the indicator and is stored until the next press of the SB3 button. Since the voltage on the measured capacitor does not exceed 0,287 V, in most cases it is possible to carry out measurements without unsoldering the capacitor from the device. The power supply (Fig. 3) is somewhat complicated due to the desire to use a battery from a cell phone with a nominal voltage of 3,6 V (indicator power 4,5 ... 5,5 V). The voltage converter on transistors VT1, VT2 increases the supply voltage to 5 V. The stabilizer on transistors VT6-VT8 limits the voltage to a level close to the minimum allowable for the indicator to work - 4,6 V. The HL1 LED is used as a source of exemplary voltage and as a power-on indicator . The stabilizer on transistors VT3-VT5 generates a voltage of -0,7 V to shift the "beams" on the indicator screen.
To increase the sweep speed of the oscilloscope, you can use an external high-speed ADC with buffer memory or use the stroboscopic effect [2]. Specifications and programming commands for the MT12864A-1 indicator are given in [1]. The microcontroller can be replaced with a PIC16F876 using the same firmware. Descriptions of these microcontrollers in Russian can be found on the Internet [3]. Microcontroller programming and programmer circuit are described in [4]. The microcontroller firmware in a hex file (Oscil873.hex) and the source code of the program in assembler (Oscil873.asm) with comments in quasi-English (MPLAB IDE 6.0.20 "digests" the Russian language very badly): download. It is highly desirable to use an operational amplifier from the KR1446 series. The T1 transformer is wound on a K16x8x5 mm ring made of M2000NM grade ferrite. Winding I contains 2x65 turns with taps from the 45th turn, counting from the midpoint, of the PELSHO 0,5 wire. Winding II contains 15, and III - 30 turns of PELSHO 0,1 wire. The body of the device is made of foil fiberglass and painted with a gray automotive primer in an aerosol package. The device is mounted on a rectangular plate with dimensions of 130x86 mm made of double-sided foil fiberglass. The mounting elements of the device are fixed by soldering on the reference points of individual mounting plates, combined on a common rectangular plate. For the manufacture of breadboards, you can take strips of foil-clad fiberglass of a suitable width; power rails are cut through them (usually along the edges). From the functional units obtained in this way, as from cubes, a finished device is assembled. Adjustment should begin with power sources, since +4,6 V is used as a reference for the ADC. The power supply circuit can be greatly simplified by using a battery of four or more batteries. In this case, the voltage converter can be excluded from the circuit, and the negative voltage for shifting the beams can be taken from pin 18 HG1 (about -8 V). In other modifications of indicators, this voltage may be absent, and then you will have to make another converter to power the indicator (pin 3). Resistor R4 (see Fig. 1) select the required image contrast on the screen. The oscilloscope calibration is tied to points on the screen in the expectation that in the future the cursor measurement mode will be introduced into the program, without this mode it is better to use the grid on the screen. The easiest way to determine its size is by recording a calibrated signal on the screen, for example, a meander. When adjusting the input amplifier, it should be taken into account that the resistance of the resistor 1R11 (2R11) affects both the gain of the operational amplifier 1DA1 (2DA1) and the beam shift on the screen ("sensitivity" of the bias controller 1R6 and 2R6), and the resistors 1R8-1R10 (2R8 - 2R10) - only for amplification [4]. The sweep speed can be controlled by a software delay between ADC samples. On the first three "high-speed" modes, the sweep line is slightly shortened on the right. This is due to the fact that the signal is recorded through the buffer RAM and the PIC16F873 does not have enough memory. When using P1C16F876, such problems do not arise, but the program must be corrected (transfer part of the buffer memory from bank 0 to bank 2 or 3). In multimeter mode, when measuring voltage, the input signal passes through a divider and an operational amplifier of channel 1 (the bias control must be set to zero). The ADC allows you to increase the voltage measurement accuracy to three digits, but then you will have to take measures to eliminate the influence of the bias regulator and select the input divider resistors with the appropriate accuracy. Then, using exemplary resistors, calibration is carried out in the resistance measurement mode with resistors 3R3-3R5 in the corresponding range, and 3R1 - overall. Calibration of the capacitance meter is performed by software delays (if quartz with a different frequency is used). Literature
Author: A.Kichigin, Podolsk, Moscow Region
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