Radio broadcast .. (1922-30)

Rear rieu> of audio-frequency voltmeter. WHILE ENGAGED in activities concerned with the development of audio-frequency amplifiers it is necessary to have at hand apparatus by which the values of input and output voltages of the amplifier can be determined. There is very little diffi- culty associated with input voltage de- terminations. It may be accomplished, for instance, by measuring the audio- frequency current flowing through a re- sistor of known value across which are connected the input terminals of the am- plifier, the voltage being the product of the current and resistance. But determina- tions of output voltage are not accom- plished as easily. To the output terminals of an amplifier is connected a load which in measurement practice usually is a re- sistor, and in service some sort of loud speaker. Now any device placed across this load for the purpose of measuring the voltage across it must satisfy three con- ditions: the current passed by the device must be very small compared with that flowing through the load; the indications of the device must be independent of frequency over the audio-frequency range (30 to 10,000 cycles); and the indications of the device must be controlled by the effective rather than by the peak or aver- age value of voltage across the load. A voltmeter has been de- veloped to satisfy these three conditions and also to cover a sufficiently large range of voltages to adapt it to meas- urements identified with phonograph and broadcast amplifiers. The two pictures accom- panying this article show the external appearance and the arrangement of the vari- ous parts of the voltmeter. An additional feature of this apparatus is a unit called the load device. This occu- pies the bottom panel of the voltmeter. The useful- ness of this device may be judged from a consideration of the conditions under Developments in HIGH-FREQUENCY MEASURING EQUIPMENT By H. D. OAKLEY General Electric Company which measurements on a.f. amplifiers are made. Audio-frequency amplifiers make use of vacuum tubes and, when making meas- urements to determine their characteris- tics and power output, it is usual practice to terminate the amplifier (i.e., load the output tube) with a resistor the value of which is specified by the tube manufac- turer. It is convenient, therefore, to have a variable resistor with which any value of resistance likely to be used in the meas- urements can be easily and quickly ob- tained. Also when using a resistance load it is wise to supervise proceedings with a loud speaker or a pair of phones connected across the load resistor—being certain, rmummlmmillmnliltil Voltmeter Errors ERRORS DUE TO MULTIPLIER RESISTOR Multiplier Kalios Nominal 1 2 5 10 20 SO 100 200 True 1 2.001 4.98 10.01 19.90 49.7 99.8 199.0 Per Cent. Error 0 0.05 0.4 0.1 0.5 0.6 0.2 0.5 FREQUENCY ERRORS (Multiplier Tap 1. Input held at 0.9 volt) Frequency in Cycle* 30 60 80 100 200 500 1000 2000 5000 7000 10000 Meter Heading 0.875 0.892 0.897 0.900 0.901 0.902 0.905 0.905 0.905 0.905 0.900 Per Cent. Error 2.8 0.9 0.33 0.00 0.11 0.22 0.44 0.44 0.44 0.44 0.00 Fig. I — Circuit of A.F. voltmeter and loading device. however, that the impedance of the loud speaker or phone circuit is many times that of the load resistor so that no appreciable errors will be introduced in the measure- ments. There is still another condition en- countered in this class of measurement. It has to do with the type of output cir- cuit used in the amplifier. For example, in some amplifiers the d.c. supply for the last tube passes through the load resistor but in others it does not. When this second con- dition prevails, the supply must reach the tube through a high-impedance reactor and the load resistor must be connected to the output tube through a low-reactance cap- acitor. The circuit for this second condit- ion is called the shunt-fed type, the first being the series type. The loading device attends to all these little details. It consists of a decade resis- tor box with which any desired value of load resistance can be obtained, a large reactor and large condenser for shunt fed circuits, pin jacks for a loud speaker, a 100,000-ohm resistor which can be in- cluded or excluded from the loud speaker circuit, and two key switches. Each switch has three positions, those for one being "Choke," "Neutral," and "Resistance," and for the other "Monitor," "Neutral," and "Loud speaker." On the front of the loading device are two binding posts marked "High" and "Low." The output terminals of the amplifier upon which measurements are to be made are con- nected to these. Fig. 2 shows five circuits which may be obtained with this device and it also indicates the positions of the two key switches for each circuit. Circuit No. 1 (Fig. 2) is the shunt-fed type with the loud speaker monitoring connection and is obtained by throwing one switch to the "Choke" position and the other to the "Monitor" position. Notice that there are 100,000 ohms in series with the loud speaker, thus preventing the loud speaker circuit from producing an appreciable effect on the load of the amplifier under measure- ment, since the load resist- ance never exceeds the value of 10,000 ohms and is usually around 4000 ohms. It is ob- vious from Fig. 2 how the other circuits are obtained and it is only necessary to name the circuits. Circuit No. 2 is the series circuit with monitoring connection; Nos. 3 and 4 are, respectively, 324 • RADIO BROADCAST FOR APRIL