Radio Broadcast (May 1929-Apr 1930)

How the Engineers Solved Their Problem DESIGN DETAILS OF THE FADA SET among the most important of the l\ recent contributions to radio are _/~ the screen-grid tube and the bandpass filter. The first of these permits a flexibility in circuit design that has been possible with no other tube. It makes possible the separation of the function of station selection from the function of amplification. Separating the functions of amplification and selection is essential if signal selection is to be accomplished in any other way than by the method of singleresonance circuits in cascade, with its attendant frequency distortion. There is another advantage in this separation of functions. If the desired signal is not entirely separated from the undesired signals before being passed on to the first amplifier tube, the latter may modulate the former, because of the non-linear characteristic of the amplifier tube. This modulation is especially noticeable in the neighborhood of a strong broadcast station, which, as a consequence, may be heard over a large portion of the broadcast range whenever another carrier is tuned-in, the frequency of which may be as much as 40 or 50 kilocycles from the fundamental frequency or harmonic frequency of the interfering station. For the reason described above, if for no other, it is important to precede the amplification in the receiver by adequate signal selection. These functions can be separated by using a band-pass filter to precede the first amplifier tube with the result that very good selectivity can be combined with very slight frequency distortion in the radio-frequency part of the receiver. The receiver described in this article has a band-pass station selector unit of preassigned characteristics followed by two stages of radio-frequency amplification, a detector of the platerectification type, and an audio amplifier having in the output stage two 245 tubes in push pull. Requirements of a Good Set k good receiver should have a J\ radio-frequency voltage amplification of about 5000. If the amplification is considerably less By E. A. UEHLING Engineering Department, F. A. D. Andrea, Inc. than this value the receiver may not be capable of distance reception and a detector of the plate-rectification type is not entirely satisfactory. If the amplification is greater than this value, the receiver will, in general, amplify signals that are below the noise level, and it may be unsuited for general use in localities in which there are powerful broadcasting stations in this immediate vicinity. Before considering the methods by which this value of amplification is to be obtained, let us consider the single stage. The amplification per stage with a screengrid tube is g=Gm Rl where Gm is the mutual conductance of the tube and Rl is the load resistance. This relationship holds for the 224-type tube because all values of Rl that can be attained in practice are small compared with rp. If R is 10 ohms at 535 meters, L is 200 microhenries, and C is 400 micromicrofarads, Rl is equal to approximately 50,000 ohms. Since the mutual conductance of the 224-type tube is about 1000 micromhos, the total amplification that can be obtained with one stage is equal to the product of 1000 X 10-6 and 50,000 or 50. Two stages will then give an amplification of about 2500, and, with a gain in the antenna and filter circuit of only 2 or 3, we have acquired the required amplification of 5000. Impedance Coupling When the amplification problem was first considered, all methods were subjected to theory and experiment. The final choice of tuned impedance coupling is based on the results of actual experi ment, but these results check the theory so well that a description of the advantges of impedance coupling can be illustrated very adequately by considering the actual physical problem involved. Because of the very high plate resistance of the screengrid tube, the coupling between plate and grid circuits must be very close in order to satisfy the conditions of optimum gain. As a matter of fact, this condition can be realized only partially in practice. In ordei to realize this condition with transformer coupling the primary inductance must be very high, that is, equal to or greater than the secondary inductance, and the coefficient of coupling must be as near unity as possible. In general, this design means a transformer of rather high dielectric losses and high mutual capacity. There are other considerations of even greater importance. If the optimum resonance relation is as nearly satisfied with transformer coupling as it would be with impedance coupling, the primary will require a greater number of turns than the secondary even though the coefficient of coupling be very near unity. The effect of the plate resistance on the selectivity of the circuit then begins to assume proportions no longer negligible. The effective resistance due to the internal plate resistance added to the secondary circuit may amount, at 200 meters, to 10 ohms or more. The actual amplification that can be obtained with transformer coupling is approximately equal to that which can be obtained with impedance coupling under the same conditions. The mutual inductance, of course, must be very high, and practical difficulties are encountered that are not met when impedance coupling is used. That the amplification with impedance and transformer coupling is about equal becomes evident when the comparative values of L/R2C and o2M2/R2, the load resistance for tuned impedance and transformer coupling, respectively, are considered. The value of the first of these two quantities in a given case we have found to be equal to 50,000 ohms at 535 meters. The value of the second of these two quantities at the 350 400 WAVE LENGTH Fig. 2 500 300 350 400 WAVE LENGTH Fig. 3 "550 • JULY 1 929 • • 171