Radio Broadcast (May 1928-Apr 1929)

382 RADIO BROADCAST ADVERTISER Do You Read the TECHNICAL JOURNAL of the RADIO INDUSTRY Editor — M . L. Muhleman Associate Editors — John F. Rider and Austin C. Lescarboura Managing Etlitoi — G. C. B. Rowe Radio Engineering has been published for eight years for the engineers, technicians, manufacturers and dealers of the radio field. It covers, in some forty text pages each month, the latest developments in manufacturing and engineering, design, production, etc. It includes valuable material on construction, testing and servicing. In the "Commercial Developments" section, Radio Engineering deals with television, moving picture transmission, aeroplane and train communication, speech amplifying systems, etc. Contributors and authors include the leading engineers in the electrical and radio field. Radio Engineering is NOT sold on newsstands If you are not already a subscriber, use coupon below. RADIO ENGINEERING, Inc. 52 Vanderbilt Ave., New York, N. Y. it i j c j ) $2.00 for 1 year > , Enclosed find J $3.00 for 2 years J 8ub" scription to Radio Engineering. Enclosed find twenty cents (20c) for which send sample copy of Radio Engineering. (please print name and address) Name Address City State Please check classification. □ Manufacturer □ Dealer □ Engineer □ Technician Anything else No. 230 Radio Broadcast Laboratory Information Sheet Filters October, 1928 HOW THE VARIOUS TYPES DIFFER TN TELEPHONE and radio circuits various types of filters are used and in this Laboratory Sheet we will indicate how the several types differ. First let us define a filter. We might say that a filter is a circuit arrangement that will separate direct current from alternating current or vice versa or a circuit that will separate alternating currents of one or a group of frequencies from alternating currents of a different frequency or group of frequencies. Filters can be divided into three general classes: (A) low pass filters; (B) high pass filters; (C) band pass filters. Low pass filters. A low pass filter is designed to pass all the low frequencies below a certain cut-off frequency and to oppose the passage of frequencies above the cut-off frequency. The frequency characteristic curve of an ideal low pass filter is given in sketch A. The r.f. choke coil used in the plate circuit of a detector tube functions as a low pass filter, since it permits audio frequencies to pass into the audio amplifier but excludes from the amplifier the high carrier frequencies. 100 100 0 High FREQUENCY (A) High pass filters. Sketch B gives a frequency characteristic of an ideal high pass filter and it will be noted that it has the opposite effect to a low pass filter in that it permits the passage of high frequencies and obstructs the flow of low frequencies. The r.f. chokes and condensers used in the plate circuits of an r.f. amplifier are an example of a high pass filter, functiong to pass the high frequencies directly to the filament, thereby keeping them out of the plate supply, but obstructing the passage to the filament of the d.c. plate current (which can be considered a current of o frequency). Band pass filters. This type of filter permits the passage of a band of frequencies and excludes all those frequencies below or above this band. A very common type of band pass filter is used in radio receivers — the tuned circuit. When a coil -condenser combination is tuned to a given broadcasting station it permits the passage of that band of frequencies associated with that broadcasting station and excludes to a more or less greater degree frequencies either lower or greater than that of the station we are trying to receive. The ideal curve of a band pass filter is indicated in sketch C. 100 0 High FREQUENCY (B) 0. 0 High FREQUENCY (C) No. 231 Radio Broadcast Laboratory Information Sheet October. 1928 Impedance-Coupled Amplifiers THE EFFECT OF THE SIZE OF THE INDUCTANCE TN CONNECTION with impedance-coupled -*■ audio amplifiers, the statement is frequently made that the coupling inductances should have as large an inductance as possible, creating the impression thereby that the larger the inductance the better the results. Such is not the case. First, let us examine the effect of the inductance at low frequencies. The curve on this sheet indicates how the amplification from a stage of impedance-coupled audio 120 160 LIN HENRIES varies with the inductance in henries of the coupling coil, L. This curve is calculated for a frequency of 60 cycles, assuming that this is the lowest frequency which we desire to amplify uniformly. It is assumed that the tube has a plate impedance of 30,000 ohms and a mu of 20, and that the coupling condenser, C, and the grid resistance, R, are of such values as not to affect the amplification. If 100 per cent, amplification were obtained, the gain would be 20, and with an infinitely high inductance this gain might be realized at low frequencies. With practical values of inductance, however, the gain is less than this and varies with the inductance as indicated by this curve. The value of the coupling inductance should be the smallest value that will give satisfactory gain at the lowest frequency to be amplified, which we have assumed in this case to be 60 cycles. At medium frequencies the amplification obtained from a circuit of this sort is approximately equal to the amplification constant of the tube and we might assume as a reasonable figure that the amplification at 60 cycles shall not be less than 75 per cent, of the amplification obtained at medium audio frequencies. 75 per cent, of 20 is 15, the value therefore of the gain at 60 cycles. This corresponds to an inductance of 100 henries. If a value of inductance much greater than this is used to obtain more amplification at low frequencies, it will be found that the high frequencies begin to fall off due to the shunting effects of the tube and coupling coil capacities. Amplifier curves with various values of coupling impedance will be given and explained in a future Laboratory Sheet. No. 232 Radio Broadcast Laboratory Information Sheet October, 1928 The Voltmeter HOW IT WORKS TN PRECEDING Laboratory Sheets, Nos. 205, ■L 214 and 222 we explained the construction of the galvanometer and the ammeter and indicated how they differed. The voltmeter is quite similar to these two instruments, differing in only one important respect to be explained below. A voltmeter is used obviously to measure voltage. We desire to measure this voltage using as little power as possible, for if the instrument itself requires any great amount of power it is liable to affect the voltage reading of units such as batteries or B-power units which are designed to deliver only a small amount of power. To measure the voltage of some source of potential we might take a very low reading ammeter, one having a maximum scale reading of perhaps 0.01 amperes, place it in series with a known high resistance and then connect it across the source of potential. The ammeter would read the current that flowed and then by Ohm's law, which states that the voltage is equal to the current times the resistance, we could calculate the value of the voltage. In a voltmeter this high resistance is permanently connected inside of the instrument and the scale is calibrated to read volts instead of amperes. In other words we might say that the instrument solves Ohm's law for us and makes it unnecessary to calculate the IR drop every time we wish to measure a voltage. Ammeters and voltmeters may in general be distinguished in one other way other than the fact that they are marked "volts" or "amperes" on the scale of the instrument. It will generally be found that ammeters have fairly large terminals and they are generally of metal. Voltmeters have small terminals and they are always of the insulated type. Ammeters are equipped with metal terminals because no damage results to the instrument or the circuit in which it is connected if the terminals are accidently short-circuited; ammeters are always connected in series with a circuit and have a very low resistance, so that shorting them affects the circuit very little. Voltmeters, on the other hand, are always connected across the source of potential, and if the voltmeter terminals are accidentally shortcircuited then the source of potential is shortcircuited. A short-circuit may not be a serious thing when measuring a B battery, but may cause damage if it occurs when measuring the voltage at a light socket or when measuring the output voltage of a large generator.