Radio Broadcast (May 1929-Apr 1930)

Record Details:

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same wavelength and for a transformer of mutual inductance equal to 200 X 10-6 henries is 51,000 ohms. Simplified Design Thus, the amplification problem has been considered without resort to the more complicated amplification formulas that must be used for other types of tubes. The very high plate resistance of the screen-grid tube makes for simplification ' in design as'well as simplification in mathematical circuit considerations. In the original experimental work on this receiver, it was necessary to measure the amplification under actual operating conditions in order to check the theoretical results. At that time there was no calibrated oscillator with a calibrated signal output of a wide range of values available and a method of making these measurements had to be developed. Because of the high amplification of the screen-grid tube the required input voltage had to be so small as to offer considerable diiliculty in measurement. Several methods of measuring these small voltages were tried with only partial satisfaction. Finally it was decided to use a method that did not require the measurement of the input voltage. A relatively large signal current that can be measured with a sensitive thermo-couple was made to flow through a definite length of straight copper rod of negligible resistance. The inductance of this short, straight rod of copper could be calculated and thus the reactive voltage drop was known. The circuit used in making of these measurements is illustrated in Fig. 1. The copper rod is 29.2 cm. in length and 0.14 cm. in diameter with a self inductance of 0.341 microhenries at 550 kc. and 0.339 microhenries at 1500 kc. 100 SPI.PCTIVITV CAI IRRATK IN A T 200 MI :ters Microvolts required in a 0.00025 ^ mfd. Antenna to produce a Stanc • \ u oi^nai Its impedance at 545 meters is, therefore, 1.179 ohms and at 200 meters it is 3.197 ohms. With a current of 2 milhamperes flowing through it, the reactive voltage drop is 2.358 miUivolts at 545 meters and 6.394 millivolts at 200 meters. This method is not absolutely accurate because of end effects and the mutual reactance between the rod and the rest of the circuit and ground. But with a little care in arrangement of the rod itself, and the lead wires to the rod, errors arising from this source can be reduced to a very small value. Actual amplification measurements as obtained with this apparatus for a few of the more interesting conditions are shown in the graphs (Figs. 2 and 3) on the preceding page. Amplification per Stage Nothing has been said up to the present regarding the conditions that affect the actual value of amplification per stage. Begardless of the kind of coupling used, the amplification is inversely proportional to the radio-frequency resistance of the resonance circuit. Low-loss coils of large size wire and good shape factor are very important, and this implies further that the coil shielding itself must be as 30, 30 20 10 KILOCYLES BELOW RESONANCE 10 20 KILOCYLES ABOVE RESONANCE 30 Fig. 7 Fig. 5 large as it is practical to make it. As a second condition for good amplification, the ratio of L to C should be large. Making this ratio large also improves the selectivity of the circuit. There is, however, a very practical upper limit to the value of the ratio of L to C. The larger L is made, the smaller will be the maximum capacity of C necessary to reach the upper limit of the broadcast range. There are, however, fixed capacities in the circuit such as the plate-filament capacity of the tube, and capacity of the compensating condenser, and the minimum capacity of the tuning condenser itself, the total value of which fixes the upper limit of L. Attention may be called to the amplification curves of Figs. 2 and 3 which were made by using the circuit of Fig. 4. It will be seen at once there is a discrepancy between the calculated amplification for impedance coupling and the measured values. It is obvious that the results that can be obtained with impedance coupling are largely dependent on the characteristic of the r.f. choke coil supplying the tube plate voltage. The measurements on impedance coupling already referred to were made before the r.f. choke coil used in this receiver was designed. This choke coil has three sections. It has an impedance at 500 meters of 70,000 ohms which increases gradually as the frequency increases to a value of about 500,000 ohms at 200 meters. The resonance point of this choke coil falls at approximately 200 meters, or a little higher. Thus far we have considered only the amplification per stage and find that the amplification of two stages on the basis of the results already discussed, is sufficient and all that is desirable in a good receiver. There will, of course, be some regeneration though none has been purposely introduced, and the actual shape of the amplification curve may be modified to some 10 SENSITIVITY CALIBRATION Microvolts required in a J 0.00025 mfd.Antenna to . produce a Standard _ Signal of 50 Milliwatts _ Modulation 30% 700 900 1100 1300 1500 FREQUENCY Fig. 6 extent in the completed receiver by the antenna coupling method used and other factors. Actual overall amplification measurements which will be discussed later indicate, however, that these effects are small and need not be considered further. Detection System Having obtained an overall radiofrequency amplification of approximately 5000, we are at liberty to use plate rectification (C bias) in the detector. It is less efficient than the grid rectification so commonly used up to the present time, but this efficiency is not necessary with the amplification available. It is capable of more faithful reproduction, or, to put it more accurately, the tendency to frequency distortion found in grid circuit rectification with the values of grid leak and condenser generally employed, does not exist in anode detection. A second advantage of plate rectification is the greater audio output power of the anode detector. Detector overloading has been one of the most serious limitations of radio receivers in the past. With the use of plate detection, and the high radiofrequency amplification that makes it possible, a serious defect of past receivers has been eliminated. As already stated, there is a signal selector of band-pass characteristics pre^ ceding the first radio-frequency amplifier tube. There are at the present time many types of band-pass filters, and such filters have been used for a long time in tele \ 1 ! ! \ SELECTIVITY CALIBRATION / AT 550 METERS # Microvolts required in / a 0.00025 mfd. Antenna 1 to, / Sig roduce a S nal of 50 M tandard illiwatts "20 10 0 10 20 30 Kilocycles Delow Resonanct Kilocycles above Resonance Fig. 8 172 • • JULY 1929 •