Radio Broadcast (May 1928-Apr 1929)

A Simple Unit for Measuring Impedances By F. J. FOX and R. F. SHEA MR. FOX and Mr. Shea have described in this article a very excellent use for vacuum tube voltmeters in the measurement of coils, condensers, and resistors that are ordinarily used in the audio-frequency and power circuits of modern radio receiving equipment. The circuit arrangement used is such that the unknown impedance is compared to a known resistance and at the balance point the unknown impedance is equal to the known resistance, whence we may easily calculate the inductance or capacity by the formulas given in the text. The comparative simplicity of the device is commendable, and since the device may be readily constructed for use in the home laboratory we feel that the article will appeal to our many friends who have shown such great interest in the descriptions of home laboratory equipment that have appeared in Radio Broadcast. — The Editor. IN THE design of radio equipment there are many instances when it is desirable or necessary to know the impedance of large inductances or capacities which are used in the building of a piece of apparatus — for example, a Bpower unit. Large inductances and capacities may, of course, be measured on a bridge (an expensive piece of apparatus) but in many cases, especially where the necessary standards of inductance and capacity are not at hand, the method to be described will be found useful, accurate, and inexpensive. Most methods of measuring large impedances require the use of expensive apparatus and delicate instruments. The authors have aimed, in the method described here, to provide a simple, accurate and inexpensive method of measurement. The apparatus utilizes vacuum tube voltmeters and provides a quick and accurate means of measuring impedance. The accuracy of the method is limited only by the precision of the meters used and the care used in making the instruments. With inexpensive instruments this device will give results to three figures and therefore is accurate enough for practical radio design work. HOW THE UNIT WORKS IT IS [possible to measure high impedances by 1 the use of a vacuum tube voltmeter in a circuit as indicated in Fig. i. With this arrangement the vacuum tube voltmeter is used to read the voltage across the unknown impedance, and the current through the impedance is read by the milliammeter. The impedance is then equal to the voltage divided by the current. From the standpoint of simplicity this method has two disadvantages: it is necessary that the vacuum tube voltmeter be calibrated to read voltages, Impedance to be — > measured THE COMPLETED UNIT All the instruments used in making the measurement are housed in this box, with the exception of the decade resistance box, which is connected in the circuit by means of terminals behind the rear tube. The C batteries are also housed in the box. . Vacuum Tube / \y Voltmeter FIG. I and an a.c. milliammeter — an expensive and delicate instrument — is necessary. To eliminate these disadvantages the circuit shown in Fig. 2 has been devised. In this case two vacuum tube voltmeters are used, one across a variable resistance, R, and another across the unknown impedance, Z, connected in the circuit at X. To calibrate this set-up a known value of resistance is first connected at X. The resistance box, R, is then set exactly equal to the known resistance so that the voltages across the two voltmeter circuits will be exactly equal to each other. A.C. voltage from the transformer, T, is then applied and the readings of the vacuum tube voltmeters as indicated by the meters, Mi and M2, are carefully taken. The tubes for the set-up should preferably be very nearly matched so that when calibrating, the meter readings are very nearly alike for voltages of the order of those being used. That is, with the two equal resistances in place, the input voltage should be varied by means of the variable resistance in the primary of the supply transformer, T, and the meter readings noted. Tubes should be selected which give meter readings that correspond very closely over the entire range of voltages. From this calibration we know the corresponding readings of the two meters at any points on their scales under the condition that the voltage impressed across the input of each vacuum tube voltmeter is the same. The cali bration is complete and we can now use the circuit to measure unknown impedances. The unknown impedance to be measured is connected at X in place of the known resistance and the resistance of R is varied until the readings of the two meters correspond. We know from our calibration that when the readings correspond the voltages across the two vacuum tube circuits are the same. It therefore follows that under the condition that the readings of the two meters correspond, the voltages across resistance R and the unknown impedance are the same. Since the current through R and through the unknown impedance at X is the same it follows that [RJ= IZ where I is the current R is the value of resistance at which the readings of the meters corresponded Z is the value of the unknown impedance Therefore the impedance Z is equal to the resistance R. FIG. 2 279