Radio Broadcast (May 1929-Apr 1930)

RADIO BROADCAST .X" tion. From the point of view of quality there is no difference, as with small signals both types have a square-law characteristic (i.e. an audio-frequency output voltage proportional to the square of the signal voltage.) The lack of ability to operate satisfactorily (vith large signal voltages usually attributed to the grid leak detector is due to improper use if this method of detection, and not to any defect in the method itself. In fact, in every case ,vhich the writer has examined, power grideak detection properly used was found to be Vom two to five times as sensitive as plate letection with the same tube. That is to say, ;he grid-leak power detector will give the same output voltage as the plate detector yhen the signal voltage is only 20 to 50 per ;ent. as great. Not only is the grid rectifier nore sensitive, but it can also put out from ;wo to four times as much undistorted audiofrequency power as the plate rectifier, when 30th are operating at the same plate voltage, [n addition to this, the grid-leak power desector, when properly adjusted, will give less iistorlion than the plate power detector. Fig. 4 gives the rectified plate current of a ilO-type tube when operated as a power grideak detector and as a power plate detector with !47 volts on the plate in both cases. It is apjarent that grid rectification is superior in sensitivity (because it gives more rectified jurrent with the same signal), in power capacty (because it can give more rectified curent), and in distortion (because its characeristic is closer to a straight line). The answer to the question "Plate or grid letection?" is clear in the light of the points ,hat have been considered. Grid-leak detection s superior to plate detection in respect to lensitivity, power capacity, and distortion, ind should be used for both weak and strong ignals. In view of its all-around inferiority, )late detection need not be given further consideration. The usual receiver when not using power letection has one stage of audio-frequency implification between detector and power ube. This stage generally gives a voltage implification of about twenty-five times. If this tube is removed, and the detector made ,o supply directly the input to the power ,ube, an increase in the radio-frequency implification of fifteen times will about make lp for the loss in the audio end. Again, f the power tube is removed and the detector tself is made to supply the power, it will be lecessary to increase the r.f. amplification an ideational fifteen times, or to a total increnent of 200-300 times, in order to keep the volume from the loud speaker the same. The advantages in putting all possible implification in the radio end are that the jower detector has a straight-line characterisic, and that with each added radio stage one ;ains additional selectivity that in many ;ases is badly needed. On the other hand, mdio-frequency amplification, while giving 10 selectivity, also requires no tuning ad 201-A Ef = TUBE = 5.0 » i f <&/ ♦ / * / <§> * Region i plate de ised for section S i /' «** justments, needs no shielding, and is relatively stable. In the final analysis the decision as to whether or not to use power detection is determined by how easy it will be to get the extra radio-frequency amplification. If this can be obtained by making the tuning coils a little more efficient and using a little higher plate voltage, as will frequently be the case, then power detection should unquestionably be used. This is also the case when the r.f. amplifier is sufficiently stable to allow an added stage without introducing oscillation 35 30 \~ e25 3_ a-20 Oh czuj15 a^io 210 TUBE r,p= Ef= &itr2 VOIIS = 7.5 Volts A? -16 -12 -8 -4 GRID VOLTAGE + 4 Fig. 3 — Relation between grid voltage and plate current in a typical vacuum tube. 0 5 10 15 20 25 30 SIGNAL VOLTAGE (CREST VALUE) Fig. 4 — Rectified d.c. plate current produced by an unmodulated signal applied to the grid of a 210-type tube acting as a plate and as a grid power detector. troubles, or when the amplifier already has more amplification than is necessary with the usual amount of audio-frequency amplification. A power detector is particularly desirable in the case of screen-grid tubes, where the stable amplification is so great that it is easy to get sufficient voltage to operate the power detector, and at the same time it is necessary to use at least three stages to obtain satisfactory selectivity, even though two stages often give all the amplification required for the weak-signal detector. Detection of Weak Signals WHEN the signals to be rectified are weak, which means about 0.1 volt or less, the detector sensitivity and quality are the factors to be considered in adjusting the circuit. The things that have a bearing on these matters are type of tube, grid-leak resistance, grid-condenser size, and plate voltage. With small signals the rectification will only be partially complete, and will depend upon the operating point on the grid-current curve. With all types of tubes the rectification is best at small grid currents, but the completeness of rectification is the same for all grid currents below a certain critical value. Adjustment of the detector so as to operate at grid currents greater than this critical value results in loss of sensitiveness. The dividing point is indicated on Fig. 2. The actual point on the grid-current characteristic used for detection is determined by the grid-leak resistance, as explained in an earlier article. The greater the leak resistance, the smaller will be the grid current at the operating point. Thus, a low-resistance leak causes operation with large currents, and results in poor rectification. On the other hand, all leaks having a resistance sufficient to place the operating point below the critical value of grid current will give the same completeness of rectification of the small signal no matter how small the current is and this degree of rectification will be the maximum obtainable with that particular tube. Values of grid-leak resistance that will put the operating point at the critical grid current have been determined for various standard tube types, and are tabulated in the fourth column of Table I. Resistances lower than those tabulated give poor sensitivity (i.e., poor rectification), while values much higher than those indicated will tend to cause dis. tor tion. The grid-leak type of detector will reproduce the low notes better than the high ones unless properly adjusted. The loss in sensitivity on the higher notes is due to the fact that when the average value of rectified grid current (see Fig. 1b) is varying at a high audio frequency, this average current will be more or less short-circuited by the grid condenser, and so will not produce much audiofrequency voltage across the leak-condenser combination. The reproduction of the higher notes is improved by using the smallest possible grid condenser, and by operating with the lowest possible grid-leak resistance (in order to put the operating point where the grid current is high and the grid resistance correspondingly low) . The best value of grid-leak resistance to use is approximately that tabulated in Table 1, which is correct to give maximum sensitivity and a minimum of distortion. If the leak resistance is as given in the table, and the grid condenser capacity is that in the last column of Table 1, then the detector will have the greatest sensitivity possible with that tube, and at the same time will reproduce 5000-cycle notes 70 per cent, as well as the lower pitches. The detector characteristics with different leak and condenser sizes are shown in Fig. 6. The effect of using large grid condensers, or large grid leaks is shown and it is evident that such changes increase the distortion. It must be remembered that grid condensers much smaller than 0.0002 mfd. use up signal voltage and reduce sensitivity. Slightly improved quality with lower leak resistance is gained at a very great loss in sensitivity, while the effect of too high a leak resistance is the same as a big grid condenser, namely, poor reproduction of the high notes. In general, whenever the grid-leak resistance recommended in Table I is exceeded, the grid condenser capacity should be correspondingly less than the tabulated figure if the high notes are to be preserved. On the other hand, if the grid capacity used is smaller than that given in the table the gridleak resistance can be proportionately higher without loss of quality or sensitivity. The completeness of rectification obtainable with a grid-leak detector when small signals are applied is inversely proportional to a tube constant called the " gridvoltage constant," values of which are given in Table I. The audio frequency that is applied across the grid and filament of a grid detector by the rectification process is inversely proportional to the "grid-voltage constant." Thus, reference to Table I shows that a 227 heatertype tube will have 47/23 as much audio voltage on the grid as a 20lA-type tube for the same signal. The best rectifiers are tubes with oxide-coated filaments having high electron emission. The 112a tube is about twice as good a rectifier as the 201 \ tube, and so is preferable for detector use in storage-battery sets. The mu of the detector tube, and the plate voltage, determine the amount of amplification given the audio-frequency voltage produced on the grid, and have no effect on the rectification itself. With transformer coupling, a moderate-mu tube and a rather high plate potential (45 to 90 volts) are best, while with resistance coupling it is desirable to use a high-mu detector, and the highest possible plate voltage. With impedance coupling it is best to use a high-mu detector with a moderate plate voltage. When the detector is adjusted according to the recommendations of Table I. it is a simple matter to compute the approximate detector performance. The effect of applying to the detector grid a radio signal having the carrier wave peak value of Es and modulated to a degree m, is to produce • may, 1929 . . . page 38 •