Electronic Measurements
Purpose: Common laboratory electrical measurement devices are introduced in this experiment, and all devices are checked for calibration. These test instruments and/or the measurement functions that they perform are widely employed in chemical instrumentation.
Reference: Malmstadt, Enke, and Crouch, Electronics for Scientists Chapter 1.
Equipment: Vacuum tube voltmeter, or high input impedence (FET) voltmeter, regular voltmeter, oscilloscope, signal generator, voltage reference source, a voltage calibration source (standard cell), and various electronic components including a 1.0 µF capacitor.
Procedure:
1. Equipment Check
a. Vacuum-Tube Voltmeter (VTVM) Check: Put the VTVM in operating position. Before turning on, check to see if the meter reads zero. If not, carefully turn the set screw on the meter face using a screwdriver. This adjustment is only made with the meter turned off and is required infrequently.
Turn the VTVM on. Set the function selector switch on the +DC position. Wait a minute until the meter needle comes to a steady position. Adjust the meter zero with the zero adjust control. This adjustment is required frequently.
b. The Voltage Reference Source (VRS): The VRS is basically a voltage divider across a regulated dc voltage. It has four basic voltage ranges which are selected by the range switch. The actual output voltage, in millivolts, is the sum of the voltages indicated by the coarse and fine voltage controls times the factor on the range switch. The coarse and fine dials are labeled so that in the x1 position they are direct reading in millivolts and in the x1000 position they are direct reading in volts. The polarity of the voltages at the output terminals may be reversed by a polarity switch. The position of the output switch determines the function of the unit. The voltage reference source will be used to provide a standard DC voltage for calibration in STD VOLT position, to compare the reference voltage with a DC signal voltage in the SUM-DIFF position, provide a low-voltage bias supply in the STD VOLT position, and to provide a calibrated bucking voltage in the SUM-DIFF position.
In the SIGNAL position the signal voltage is connected directly to the output terminals; by depressing the spring-return ZERO switch, the output is shorted in all function positions. This ZERO (shorting) switch is convenient in many applications.
The ac off switch of the VRS is one position of the OUTPUT selector switch. For maximum stability, the unit should be left on during the entire working period (left in one of the SIGNAL, STD VOLT, or SUM-DIFF positions).
Adjust the voltage of the VRS to +12 volts. (RANGE x1000, COARSE voltage control at 10, FINE voltage control at 2.0, POLARITY "normal," OUTPUT at STD VOLT.) Switch the VTVM to the 15-volt range, and readjust the zero. Measure the output voltage of the VRS with the VTVM. Connect the common lead to the black output terminal and the probe to the red output terminal. Be sure that you have the proper leads, or that your probe switch is in the proper position for a dc voltage measurement. Don't be concerned about small errors, because the calibration of neither unit has been checked yet.
c. Oscilloscope Check: Obtain a sharp horizontal line on the screen by following the oscilloscope operating directions. Connect the output of the VRS to the vertical input of the oscilloscope using a probe. The input switch should be set in the dc position and the vertical sensitivity control should be set at 2 volts/cm. Adjust the VRS output voltage to 0 volts (Range x1000, OUTPUT=STD. VOLT). Now vary the Fine control of the VRS until the sweep moves from the center to the top ruled line on the screen. Press the Zero switch on the VRS and note the vertical deflection of the line back to the center. Reverse the polarity of the VRS output and note the vertical displacement of the line from the top to the bottom ruled line.
Note: most oscilloscope test probes are "x10" which means they divide the real voltage by a factor of 10. In this case, a reading of 0.1 V is expected for a 1 V input.
d. Function Generator Check: The function generator contains a sine-wave oscillator whose frequency is determined by the position of the FREQ MULTIPLIER and FREQUENCY controls. The sine-wave output is divided, and the fraction of voltage available at the output is determined by the DC offset and the AMPLITUDE control.
Turn on the function generator. Turn the FREQ MULTIPLIER switch to 100, the FREQUENCY control to 10, and the AMPLITUDE control to about one-half of full amplitude. Turn the scope TIME/DIV control to the 0.2 msec position. Adjust the HORIZONTAL and VERTICAL controls so that the sweep line is confined to the screen and roughly centered. Connect the sine-wave output terminals to the scope INPUT using the probe. Count the number of cycles observed. Rotate the FREQUENCY control of the function generator and note what happens. Also note the triangle and square wave output of the function generator.
2. DC Calibration
To take full advantage of measurement equipment, it is necessary to put it and keep it in calibration. Even the calibration of new equipment should be checked, and the aging of components causes the equipment to drift out of calibration. The following calibration procedure is recommended for the measurement equipment used in these experiments.
a. Calibration of the Voltage Reference Supply: The voltage of a standard cell is very reproducible. As such it is a good standard for calibration test equipment to within 1%. The standard cell will be used to calibrate the voltage reference source. The voltage regulator in this source is very stable and will require only infrequent checks once calibrated. The voltage reference source then becomes a suitable secondary standard for the calibration of the other test equipment. Calibrating the equipment with an exhausted cell is avoided. Because precision resistors are used in the reference supply, calibration at one value calibrates all other values indirectly. Standard voltages from zero to 100 volts are available for calibrating the various ranges of the test equipment.
Connect the standard cell to the signal terminal of the VRS. (Observed the polarity markings.) Adjust the VRS to the voltage of the standard cell. Switch the OUTPUT function to the SUM-DIFF position, and the POLARITY to DIFF position. Connect the VTVM to the output terminals. Put the VTVM on the 1.5-volt dc range; depress the ZERO switch on the VRS and move the ZERO adjust knob on the VTVM so that its zero is at an arbitrary reference position near midscale. If there is any difference voltage between the standard cell and the voltage from the VRS, adjust the VRS CALIBRATE control (screwdriver adjust on back of unit) so that the VTVM returns to the arbitrary present zero position. Alternately depress and release the ZERO switch to detect any small difference voltage. When properly calibrated, the VTVM reading should remain constant when the ZERO is alternately depressed and released. Return the VRS voltage to zero and remove the standard cell.
b. Calibration of the VTVM: Select a range on the VTVM between 1 and 3 volts, and set the VTVM ZERO adjust at the zero dial reading. Switch the POLARITY of the VRS to the NORMAL position, turn the OUTPUT function to STD VOLT position, and adjust the RANGE, COARSE, and FINE controls to the nominal full-scale voltage of the VTVM. If the VTVM does not read the applied full-scale voltage, record the reading so that corrections can be made in the subsequent calculations.
c. Calibration of the Oscilloscope Deflection: The position of zero signal deflection, as set by the VERTICAL position control, should not change when the VERTICAL attenuator and gain controls are varied. If the zero does shift, an adjustment of the vertical dc balance control is required. If this adjustment is not on the front panel, its position ca be determined by reference to the instruction manual. To adjust the dc balance, turn the VOLTS/DIV control completely counter-clockwise and adjust the zero position with the VERTICAL position control to the center horizontal line. Turn the VERTICAL gain control full clockwise and adjust the dc balance control to return the beam to the zero position.
d. Calibrated Voltage Scale for the Voltmeter: Use the low-resistance voltmeter to measure 1, 2, 5, and 10 volts from the voltage reference source, on as many of the 3 scales as you can. The readings will be in error due to the current drain through the internal resistance of the VRS. Connect the VTVM at the output of the VRS, with the low-resistance voltmeter still connected, and compare voltage readings of the two meters for various outputs from the VRS. Now alternately connect and disconnect the low-resistance voltmeter while keeping the VTVM connected to the output of the VRS and observing its reading. Determine the percentage error at each of the above voltages caused by the loading of the low-resistance voltmeter. Note that the observed voltage values on the VTVM should correspond closely with the dial readings on the VRS when the low-resistance voltmeter is disconnected.
It is important to keep in mind during subsequent experiments two facts demonstrated by the above experiment:
i) The resistance of a voltmeter at a given range must be sufficiently high compared to the output resistance of the measured source. For sources of high output resistance, the voltmeter can significantly change the output voltage and give erroneous results.
ii) For the voltage at the terminals of the VRS to be accurate to within 1% of the dial values, the load should not draw more than about 50 µA on the low ranges.
3. The Oscilloscope as an x-y Plotter
In this experiment the frequency dial of the sine-square generator is calibrated using the line frequency as a standard and the oscilloscope as a comparator. Connect the voltage from the VRS SIGNAL terminals to the scope vertical (Y) input terminals. Set the VRS Output Selector Switch to SIGNAL. Connect the sine-wave output of the signal generator to the horizontal (X) input terminals. Turn the TIM/DIV switch to XY to connect the generator signal to the horizontal deflection system. Adjust the signal generator FREQUENCY to about 60 Hz. A straight diagonal line, circular, or elliptical pattern will be observed when the signal generator frequency is equal to that of the power line. For the calibration adjustment of the signal generator, refer to the instruction manual. Vary the frequency of the signal generator and observe the patterns. The stable patterns are called Lissajous figures. The ratio of the frequencies of the X and Y channels may be calculated from the ratio of the number of nodes along the outside of the figures. Check whether the observed frequency reading on the signal generator corresponds to the theoretical multiples or sub-multiples of the 60 Hz line frequency.
Figure 1 Impedance measurement with capacitor discharge method
4. Determination of the input impedance of the oscilloscope, voltmeter, and VTVM Measure the input resistance of the oscilloscope by observing the discharge time of a 1.0 µF capacitor as shown in Figure 1. (Note: the 1 µF capacitor is probably an electrolytic. Electrolytic capacitors have polar leads. The positive potential used to charge this capacitor must be connected to the lead marked +). Adjust the reference supply used to charge the capacitor to about 8 V. Adjust the oscilloscope to locate the 0 V and 8 V reading positions on the screen. Also, take note of where the 3 V deflection would be on the oscilloscope screen.
Momentarily connect the positive potential to the capacitor to charge. Disconnect the contact and measure the time required for the capacitor voltage to decay to 3 volts, or 37% of the initial value. This time is very nearly equal to the 1/e time of the RC circuit and is therefore tRC given by the equation
E = E0 exp(-t/tRC)
where tRC=RC. Therefore, R=tRC/C. Repeat the measurement at least 5 time to be sure there was no measurement error. Be sure to give the average value and standard deviation in your report.
Using the above procedure, measure the impedance of the voltmeter and the VTVM. To do this, place the voltmeter measurement leads across the capacitor and repeat the measurements with the oscilloscope. The decay time constant will be very short in this case so look sharp! For VTVM impedance measurement, remove the oscilloscope probe and make measurements with the VTVM alone. Again, 5 measurements should be taken of each decay time.
Report: