Radio Broadcast (May 1929-Apr 1930)

Record Details:

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lished on this point, it seems well to discuss it rather thoroughly in connection with this receiver. As compared with the grid-leakcondenser detector, the bias-type detector is often referred to as a "power" detector. This meaning has come from the fact that by operating with the proper values of grid and plate voltages, a much larger power output can be obtained from the {120 ii so v. y ? BIAS rYPE d UY 2r. ETECTO 7 K The operating requirements with and without the first stage of audio-frequency amplification are approximately at 4-volt bias and 18volts bias, respectively. It may be observed that the detector sensitivity is just about twice as great at the low value of grid bias. This two to one dilference, plus the gain of the a.f. stage, makes the resulting overall sensitivity of the set so greatly superior to the "power" detector without the first stage of a.f. amplification, that there is no question regarding the use of the conventional two stage a.f. amplifier. Justification of Power Detection GRID BIAS IN VOLTS Fig. 2 — Sensitivity curves of a power detector. bias-type detector than from the grid-leakcondenser type of detector. The advantages claimed for this use of the "power" detector he principally in the dispensation of the first a.f. stage. No a.f. amplifying stage is perfectly linear in frequency characteristics, and the use or two or more stages usually increases the "infidelity." However, this is not all gained without some loss of other characteristics, as will be apparent immediately. The mathematics of the characteristics of vacuum tubes indicates that when voltage is applied to the grid of the tube, plate currents of various frequencies are produced. These are all harmonics of the fundamental applied frequency and represent distortion. The magnitude of these harmonics depends upon the characteristics of the tube used and the value of the voltage corresponding to the order of the harmonic. That is, the second harmonic will vary as a constant times E2; the third harmonic will vary as another constant times E3; the fourth harmonic will vary as still another constant times E4, etc. Thus, in doubling the applied voltage to the grid of the detector tube, the second harmonic is increased 4 times, the third harmonic is increased 8 times, and the fourth harmonic is increased 16 times! In order to dispense with the first a.f. stage it is necessary to apply a voltage to the detector of approximately five times the value which would be needed if this a.f. stage were present. Under these conditions the fourth harmonic would be increased 625 times! Unless extreme care is taken in design or an unusually large tube (a power transmitter tube?) is used, the resulting distortion would make the gain in fidelity a very dubious advantage! A view of the curve in Fig. 2 will show another disadvantage toward the use of the "power" detector to eliminate the first a.f. amplifier. In order to obtain the high a.f. voltages from the detector necessary to drive the output power amplifier, it is necessary to use high plate and grid voltages on the detector tube. In Fig. 2 are shown a series of curves of sensitivity of an early experimental model against varying grid and plate voltage. It may be seen that for each plate voltage, there is an optimum grid-bias voltage. The dashed curve is the envelope of these curves, and shows the maximum sensitivity which can be obtained (with this particular model) for any value of grid voltage. The demand for better fidelity is a justifiable one, however, and some means should be taken to improve this performance characteristic. A view of the reasons why the customary grid-leak-condenser detector contributes to the "infidelity" and distortion of the average set is desirable. The first point in error is in the use of the grid leak and condenser. Due to the conductance of the grid-coupling condenser at high audio frequencies (2000 and over), the high audio-frequency response is decreased greatly. This point has been discussed in various periodicals and need not be stressed here. The reduction in response due to this cause may be to a value as low as 40 per cent, at 5000 cycles. The use of a bias-type detector automatically cures this fault as the leak and condenser are eliminated. A second point lies in connection with the distortion previously mentioned, the introduction of harmonic currents not present in the applied voltage. It is possible to get more undistorted power from a biastype detector than from the grid-leak Fig. 3 — Schematic tliagram of the power detector circuit. type. This means that in order to drive properly the output tube (or tubes, in push pull) the audio amplifier does not have to amplify so much, and the ratio of the a.f. transformer can be lowered, with a corresponding increase in fidelity. As an a.f. amplifier, with the customary transformer, the usual detector leaves much to be desired. A flat audio-frequency amplification characteristic is obtained when the load impedance is high with respect to the tube impedance. For an average case, the plate resistance of an a.f. amplifier tube may be 8000 ohms. While the primary impedance of the transformer varies continuously with frequency, the output voltage is approximately constant whenever the primary impedance is above 50,000 ohms. Thus, the primary impedance might increase from 50,000 to 500.000 ohms without changing the output. This is due to the way the voltage divides across the two impedances. In the case of the detector, the plate impedance is usually much higher, say 30,000 to 50,000 ohms. Here the change in transformer primary impedance is of much greater importance in determining the amplification characteristic with respect to frequency. Either the transformer must be correspondingly increased in impedance (size and cost) or some means taken to prevent the usual great change in load impedance. Fortunately this means is simple. A resistance shunted across the primary of the transformer, prevents a great change in impedance, and consequently good fidelity. This is at a slight expense to sensitivity, but, as sufficient sensitivity is gained in the tuned antenna stage, the overall result is ahead both ways. Increasing the resistance increases the sensitivity, but decreases the fidelity. A good compromise gives almost the fidelity of pure resistance and the amplification of the transformer. The transformer is fed through a coupling condenser instead of being connected directly. This enables a saving in cost (no primary, but equivalent to a tapped autotransformer) and slightly increases the response to low audio frequencies due to resonance with the transformer inductance. The circuit is shown in Fig. 3. Overall Characteristics With the discussion of the major improvements in this set, in mind, a study of the overall characteristics is apropos. Fig. 4 shows a type of curve which will be new to almost all readers of this publication. It represents the percentage of the power of the harmonics in the output of the radio receiver, plotted against the output voltage, These are true distortion curves and show how the percentage of harmonics increases as the output voltage increases. The curve marked a in Fig. 4 is a most peculiar looking affair but is relatively simple. This represents the distortion present in the Jewelbox at any output voltage. It may be observed that there is a certain minimum distortion, about f per cent. This is an inherent property of any receiver, and is properly blamed, not on the receiver, but on the characteristics of broadcasting methods. This minimum power distortion (in any receiver) is equal K2 to r-7 , where K is the percentage of modulo lation. As these curves were taken at 30 per cent, modulation, the minimum dis(0 30V2 tortionis ' ■ =0.0053 = 0.53 per cent. Id From a physical viewpoint, this distortion is all second harmonic, and represents that power obtained at audio frequency due to the "beating" of the sidebands. Follow along the curve a from left to right. This means an increasing radio frequency applied, (or an increase in the volume control) resulting in an increased audio voltage output. With an output of 65 volts, 1 /b Oh * -1° ■ A V v — <(1.7Watts s fTo Walt 30 40 50 60 70 AUDIO VOLTS OUTPUT 80 90 100 Fig. 4 — Curve A shoics how distortion in Jeivelbox decreases; Curve B shows distortion occuring sooner in grid leak and condenser detector. 156 • • JULY 1929 •