Radio Broadcast (May 1929-Apr 1930)

Record Details:

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RADIO BROADCAST phone work. But a filter that will meet the requirements for one case may not be at all suitable for another. As a result, it has not been easy to supply a filter of bandpass characteristics for broadcast use. All filters that are simple enough mechanically and electrically for radio receiver use have the unfortunate characteristic of varying band width for different frequencies in the broadcast range. In most filters already used the band of frequencies transmitted is narrow at the high wavelengths and very wide at the short wavelengths. This condition is an unfortunate one, not only because the width of band varies with wavelength but also because it varies in the way it does. The selectivity at the short wavelengths is usually not very good even in the best receivers because of the increased resistance of the radio-frequency circuits at the higher frequencies. It is obvious that if the width of the band transmitted by a band-pass filter increases as the wavelength decreases, the tendency toward broad tuning at the shorter wavelengths will be even more pronounced. The Band-Pass Circuit A simplified circuit of a band-pass filter having more desirable characteristics is shown in Fig. 5. It will be noted first of all that no magnetic coupling exists between the two circuits. There are two principle advantages of coupling these circuits as shown and these advantages will be described as follows: We are interested in the width of the transmission band which depends on the value of the quantity VR1/R2 (M2 — R1R2) where Ri and R2 are the circuit resistances and M2 is the absolute value of square of the coupling impedance. It will be seen that this coupling impedance should vary as the product R1R2 varies with frequency, so that the quantity (M2 — Ri R2) is as nearly constant with frequency as it can be made. When the coupling between the circuits is magnetic the variation of the mutual reactance is numerically equal to L and when the coupling between the circuits is capacitive, the variation of the mutual reactance with frequency is numerically equal to l/o>2C Suppose we decide on 4 per cent, coupling as the value which gives the desired width of band at the longest wavelengths. With 230-microhenry tuning coils the mutual inductance will then have to be 9.6 microhenries and the variation of the mutual reactance with frequency as expressed by the first formula above will be 9.6 X 10 — 6. Now, if the coupling between the circuits is capacitive, and a coupling of 4 per cent, is again chosen, the coupling capacity will be about 10,000 micromicrofarads, and at 550 meters the variation of reactance with frequency as determined by the author's mathematical calculations will be 10 X 10 — 6. For either type of coupling the variation of reactance with frequency at •550 meters when the coupling percentage is adjusted for the same width of band is the same. But in the case of magnetic coupling this variation is constant regardless of frequency, and in the case of the capacity coupling the variation in reactance with change in frequency decreases 6000 300 600 1000 AUDIO FREQUENCY Fig. 9 as the frequency increases. Thus we find at 200 meters the variation of reactance with frequency is equal to only 1 X 10 — 6 as compared with 10 X 10 — 6 at the same frequency for magnetic coupling. So in the broadcast range capacity coupling gives a more nearly uniform width of band than magnetic coupling, provided the width of the band is made the same for both type's of coupling at 550 meters. That is, the actual arithmetic variation in band width is less for capacity coupling than for inductive coupling. The second of the two principle advantages of this type of band-pass filter is that whatever variation in band width there is, is in the most desirable direction as already stated. As the receiver tuning dial is turned to the shorter wavelengths, the coupling percentage is reduced constantly. This reduction in percentage of coupling is slightly more than is required to give constant width of band with the result that there is a slight decrease in band width at the lower wavelengths. The use of a band-pass filter is not without some loss in voltage amplification as compared with other methods of signal selection. But in this receiver with its two stages of screen-grid amplification, the full voltage step-up usually obtained in the antenna circuit is not only unnecessary but actually undesirable. In another part of this paper it was stated that there is a voltage gain in the antenna circuit of this receiver through the band-pass filter of about 2 at 550 kilocycles and about 4 or 5 at 1500 kilocycles. Overall Performance The overall performance of the receiver is adequately described by four sets of measurements made on the receiver with the aid of a calibrated signal generator. These measurements are illustrated in Figs. 6, 7, 8, and 9. In obtaining the data for all of the curves, the input to the dummy antenna of the receiver was adjusted until a standard signal of 50 millivolts in a resistance connected across the secondary of the output transformer was obtained. In all of these measurements an 0.00025-mfd. antenna was used. In Figs. 7 and 8 the data obtained in making a set of selectivity measurements at high and low wavelengths are plotted as a function of kilocycles below and above resonance. In Fig. 9 we have an overall fidelity curve of the receiver. The absence of sideband cutting is apparent, yet the selectivity measurements just described show that the receiver is unusually selective at both high and low wavelengths. In obtaining the fidelity curve the modulation was held constant at 30 per cent, as the audio frequency was varied. As before, the input was adjusted for a standard signal in the output, and the ratio of the input at various frequencies to that required at 800 cycles was taken as the basis for obtaining the variation in transmission units of the overall radio-frequency characteristic of the receiver. A circuit diagram of the radio-frequency circuits of the new Fada-35 receiver which embodies the features that have been described is shown in Fig. 10. Provision is made for connecting the output of a phonograph reproducer or a condenser microphone in the detector grid circuit. This provision makes the receiver suitable for certain kinds of public-address work. The output is designed to match a 3-ohm voice coil of a dynamic loud speaker. The receiver can be used with a short antenna in most localities with good results. In apartment buildings and other localities where an antenna is not available, the link switch can be closed connecting the receiver input to one side of the power line. With this connection no external antenna of any kind is required. UX-245 Fig. 10 — Schematic diagram of the Fada-35. • JULY • 1929't 066 666666 1 23456789 10 10-Wire Cable to Power Supply Unit • 173