Oscilloscope calibrator. Encyclopedia of radio electronics and electrical engineering

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Oscilloscope vertical and horizontal amplifier calibration device

Most oscilloscopes do not include a built-in reference signal generator. Of course, some older models have a 1V full-amp calibration output, but this output is limited to 50Hz and is not accurate enough to make adjustments. Somewhat more customization options are provided by the special oscilloscope calibrator described in this article. This block produces a 1 Vp-p, 1 kHz square-wave signal that can be used to set up the oscilloscope's vertical and horizontal amplifiers.

This device can also be used to trim the compensation elements of an oscilloscope probe or as a signal source for measuring transients in audio amplifiers. This device is battery operated for portability. The device circuit is insensitive to changes in the supply voltage: the output frequency remains constant when the battery voltage changes from 7.7 to 9.8 V. In addition, the low current consumption - about 2 mA - can significantly extend the battery life.

Description of the circuit In fig. 1 shows a schematic diagram of the calibrator. The oscillatory part contains two of the six sections of the 4049 CMOS inverter (DD2.1 and DD2.2), as well as timing components C2, R7, R8, and R9. The elements of this part of the circuit determine the output frequency. The exact frequency value can be calculated using the formula:

Rice. 1 Schematic diagram of the calibrator (click to enlarge)

f=2,2(C2)(R7R8).

Let's assume that the input DD2.2 (pin 5) is initially in a low state, then the output DD2.2 (pin 4) will be high. Since the input DD2.1 (pin 3) will also be in a high state, a low level signal will appear at the output DD2.1 (pin 2). The high voltage from the output of DD2.2 will charge the capacitor C2 through R7 and R8. When the voltage across the capacitor C2 reaches the threshold value, the output of the element DD2.2 and the input of the inverter DD2.1 will be in a low state. For this reason, the DD2.1 output will switch to a high level state. Since the voltage across capacitor C2 cannot change instantly, the voltage at the input of DD2.2 will increase significantly and reach approximately 150% of the supply voltage. This positive feedback loop switches logic levels at the highest frequency that can be achieved on a CMOS element.

When the logic level is inverted on DD2.1 and DD2.2, C2 recharges in the other direction and the voltage at pin 5 starts to drop. When the threshold level is reached at pin 5, the output DD2.2 and the input DD2.1 will switch to a high level state, and the output DD2.1, respectively, will go to a low level state. Again in this case, the voltage at C2 cannot change instantly, and the voltage at the input of DD2.2 will drop to about 50% below the supply voltage. This, in turn, inverts the logic levels at the outputs of the specified elements. Resistor R9 limits the current at the input of DD2.2 when the voltage at C2 exceeds the supply voltage, thus protecting the input diodes from destruction. This resistor prevents the timing RC circuit from being discharged through the internal protection diodes. Otherwise, there is a tendency to tighten the signal edges. As a result, the shape of a square wave with 50% filling is relatively little dependent on the voltage of the power supply.

The rectangular signal from the DD2.1 output is fed to the parallel-connected inputs of the four remaining inverters from the 4049 case, the outputs of which are also connected in parallel. At the moment when the voltage at these outputs goes low, the 2.5V LM336Z (DD1) voltage reference is turned on through resistor R1 and diode D1. At this point, the output voltage of the calibrator becomes high.

The combined load capacity of the four inverters DD2.3 to DD2.6 exceeds 14mA. The circuit uses only 2 mA of this current, providing a steep edge to the square wave output. In order to ensure the amplitude of the output calibration voltage of 1 V, a resistor assembly R2-R6 is used with 2% accuracy. The resistors in this assembly are 470 ohms and sectioned to provide 40% of the 2,5V square wave amplitude, which corresponds to 1V on the L (calibrator output) pin. Contact J2 is used as "Common". When an output voltage pulse appears at the output of the inverters, the voltage across the diode D1 does not exceed 0,5 V. At the same time, it is closed, and the output current does not flow through R1 and DD1. At this point, the output calibration signal is zero. Bilateral limiting of the output signal is provided, on the one hand, by a dynamic resistance of the order of 0.2 Ohm LM336Z in the open state and, on the other hand, by a completely turned off current at the moment when a high level voltage is present at the output of inverters DD2.3-DD2.6.

The accuracy of the amplitude of the calibration signal is maintained by DD1 in the range up to 1%. Despite the fact that the resistive assembly has a claimed accuracy of 2%, the resistance deviations between individual resistors in it are much less. The output impedance of this circuit is approximately 1000 ohms.

The output square wave depends mainly on the current through R2-R6, so a large filter capacitor is not required for the 9V battery B1. Capacitor C1 is needed only to smooth out peak current surges at the moment of switching inverter DD1.

Design

The author's prototype was assembled on a special breadboard. The layout of the components in this device is not critical, so you can use any options that are convenient for you. For those who want to build this device on a printed circuit board, Fig. 2 shows the wiring drawing, and the circuit in fig. 3 shows the placement of the components.

Rice. 2 Wiring drawing

According to the correct mounting sequence, the least sensitive components should be installed first. Solder the wires of the battery box, the DD2 block, the switch, then the potentiometer and the output connector. Then install the rest of the passive elements: first the resistors, then the capacitors. To achieve a minimum frequency drift of the output signal, the capacitor C2 must be a film, R7-Me-tallium oxide resistor with an error of 2%, and it is desirable to use a wire-wound multi-turn potentiometer as R8. Lastly, you need to install D1, DD1 and DD2.

Rice. 3 Placement of components

Check carefully the orientation of the polarized components, and if you haven't used a PCB, then check the wiring. Depending on the sensitivity of your oscilloscope, you may need a different output amplitude. If this is the case, then you can remake the output stage of the circuit as follows: connect two LM336Zs in series and reduce the resistance of R1 to keep the divider and LM1Z at about 336 mA. This will provide twice the output voltage.

Setup and Calibration

The output voltage of the calibrator can be checked with any good digital multimeter. Temporarily short the connection point of R1 and D1 to ground. This will set the output of the device to 1V DC. Check and verify that this is the case.

You can use a digital frequency counter to check the output frequency. However, there is another exact method that can be used if you have a test CD. Turn on the test disk to reproduce a sinusoidal frequency of 1 kHz and connect it to one channel of a stereo amplifier. Connect your oscilloscope calibrator to the other channel. Turn potentiometer R8 to adjust the output frequency of the calibrator so as to obtain zero beats of the audio frequency. This sonic balancing process is similar to how a piano or guitar is usually tuned.

Using the Calibrator

An oscilloscope's vertical deflection amplifier can be tested by connecting a calibrator and comparing the peak-to-peak square wave on the oscilloscope screen with the markings on the cathode ray tube. The sweep generator is checked by setting the sweep knob to the 1 ms position and comparing the rectangular signal edges with the vertical markings of the tube. In addition, using this calibrator, you can check the input probe-divider of the oscilloscope (x10, x100). Since the edges of the square wave generated by the calibrator are quite steep, any distortion in its shape becomes very noticeable. If the remote probe incorporates tuning elements, then by adjusting them, you can restore the original rectangular shape of the calibration signal passing through the divider.

Solid state components: DD1 - LM336Z precision voltage reference (Jameco 23771 or equivalent) DD2 - 4049 six CMOS inverters D1 - 1 N4148 silicon diode

Passive Components:
Resistors (all fixed resistors 0,25 W, 5%, except where noted)
R1 - 2,2 kOhm
R7 - 39 kOhm
R8 - 10 kOhm, trimmer (see text)
R9 - 1 MΩ
R2-R6 - 470Ohm5, 2% resistor assembly
Additional details and materials:
C1 - 0,1uF ceramic disc capacitor
C2 - 0,01uF precision film capacitor
S1 - miniature switch
L, U2-tips (red and black)
B1 - 9 V battery

Author: Charles Hansen. Translation and editing Vladimir Volkov; Publication: radioradar.net

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