Radio Broadcast (May 1929-Apr 1930)

Part III — Practical Aspects GETTING THE MOST OUT OF A DETECTOR FREDERICK EMMONS TERMAN Stanford University THIS article is devoted to a discussion of the practical aspects of detection which come up in connection with broadcast receivers, in contrast with the first two articles, which went into the theory of detection. Problems which the set designer must answer are: Should plate or grid detection be used? Is a power detector advisable? How should the detector be adjusted for best results? How much audio-frequency output will the detector give? It is to questions of this type that an answer will be attempted. Detection and Rectification IN STARTING off it will be well to devote a little time to considering what detection is, and why it is necessary. A typical radiofrequency signal is shown in Fig. 1a, in which the amplitude of the signal varies in accordance with the sound that is being transmitted. A radio-frequency wave of this character will not produce any effect on a telephone receiver because, in the first place, the receiver will not respond to such a high frequency, and second, even if the telephone receiver diaphragm could vibrate at the signal radio frequency, the vibrations would be too high pitched for the ear to hear. Since the amplitude of the signal varies in accordance with the sound being transmitted, what is desired is a current through the telephone receiver that is proportional to the amplitude of the radio signal. Such a telephone current can be obtained by rectifying the radio frequency, as shown in Fig. 1b, in which the negative half cycles of the signal current have been suppressed. The rectified current of Fig. 1b has the average value indicated by the dotted line. This average current is seen to vary in proportion to the strength of the signal, so in passing through the pbones or loud speaker it will produce a response that is the same as the original sound being transmitted. If the signal is not rectified, the average value is zero, and there is no response in tbe loud speaker. Detection is the name that has been given to the process of rectifying radio-frequency signals in order that the received energy may be converted to a suitable form for operating audio-frequency reproducers. (A) Modulated Signal Voltage Average Value (B) Completely Rectified Signal ( Average Value (C) Incompletely Rectified Signal Fig. 1 — These graphs illustrate the difference between completely and incompletely rectified signals. This final article in a series of three on detection summarizes the advantages of the grid leak-condenser type of power detector. Inasmuch as none of the receivers now using power detection employ this type of detector, the Editors hope that the proponents of C-bias detection will step forward and present their side of the discussion. The other articles in this series concerned themselves with weak-signal detection by the grid leak-condenser detector and power detection, — The Editor. The rectification (i.e., detection) shown in Fig. 1b is complete, which is to say that the negative half cycles are completely eliminated. It is possible for the rectification to be partial, as shown in Fig. lc, in which the negative half cycles are present, but are smaller than the positive loops. The average value in Fig. lc is indicated by the dotted lines, and is seen to be smaller because of incomplete rectification. Grid Detection IN GRID detection the radio-frequency signal is rectified in the grid circuit. The relation between the grid current and grid voltage of a typical vacuum tube is shown in Fig. 2. When adjusted to the point "O," the grid circuit will act as a very good rectifier, because when a radio-frequency signal is applied and the grid voltage alternately becomes more positive and more negative than "O," there will be considerable current flowing when the grid is on the positive half cycle, while on the negative half cycle there will be only a small grid current, or perhaps none at all. If the radio-frequency signal is a few volts in amplitude the grid current will be substantially as given in Fig. 1b, indicating complete rectification, while if the signal is less than a few tenths of a volt the grid current will be as shown in Fie. lc, because the rectification is then incomplete. The grid detector utilizes the rectified grid current to affect the plate current of the tube. Referring to Fig. 2, the rectified grid current must flow through the grid-leak gridcondenser combination. The rapid radiofrequency variations of this rectified grid current that are superimposed on the average get through the grid condenser very easily, but the average of the rectified grid current has great difficulty in getting through the leak-condenser combination, and accordingly produces an appreciable voltage drop across it. This voltage drop exists between the grid and filament; it is thus applied to the grid and is amplified in the plate circuit by the feube acting as an audio-frequency amplifier. Plate Detection IN PLATE, or C-battery, detectors the rectification takes place in the plate circuit of the vacuum tube. The relation that exists between grid voltage and plate current in a vacuum tube is given in Fig. 3. Rectification is possible if the grid and plate voltages are adjusted so as to put the operating point within the shaded region. Take the operating point "O," for example, and consider the result of applying a radio-frequency signal to the grid. On the positive half cycles the plate current will be considerable, while on negative half cycles the plate current stays zero most of the time. If the signal is large the rectification is substantially complete, while if the signal is only moderate in strength the rectification will be only partial. In either case, the rectified plate current that is produced flows through the phones, or whatever audio apparatus is present in the plate circuit of the detector, and the effect desired is produced by the average rectified plate current. The important practical question is not how plate and grid detection take place, but which is best. In this regard grid rectification starts off with the initial advantage in that it rectifies the signal in the grid circuit but obtains the power output in the plate circuit. In this way the grid rectifier can be adjusted to give the most complete rectification possible in the grid circuit without regard to the amount of rectified grid power available, and can then obtain a large output power by suitably adjusting the plate circuit. In the plate rectifier, on the other hand, the power output is obtained directly from the rectified plate current, and as a result it is not possible to obtain simultaneously complete rectification and high power output. The operating conditions giving most complete rectification are those in which the plate current is low, giving a plate resistance from five to twenty times the value under usual conditions. The writer has made approximately 1000 measurements of grid detection constants and about 500 measurements of plate detection constants, and not a single case was found where a tube properly adjusted for gridleak detection would not put out at least four times as much audio-frequency voltage with weak signals than would the same tube operating under optimum conditions as a plate detector. In most cases the grid detector will put out ten to twenty times as much audio-frequency voltage as the plate detector when both have the same signal voltage applied. From the point of view of sensitivity to weak signals it is obvious that plate detection has no justifica -3.0 -2.0 -1.0 0 +1.0 +2.0 GRID V0LTGE Fig. 2 — Relation between grid voltage and grid current. • may, 1929 page 37 •