Radio Broadcast (May 1928-Apr 1929)

-z. c Fig. 2 — Circuit for a grid-leak gridcondenser detector Features of Practical Detection IT WILL be observed that the grid-leak detector combines two distinct functions. First is the rectification of the signal voltage, and the utilization of the rectified signal current to produce a change of grid potential, and second is the amplification of this change of grid potential in the plate circuit. This latter problem is purely a matter of audiofrequency amplification, and is quite generally understood. The real problem of grid-leak detection is therefore centered around the determination of the change of grid potential by the rectifying process, and the rest of this article will be devoted to a discussion of -the factors controlling the rectified grid current, and the voltage drop it produces. In the analysis of practical detection it is necessary to consider only the audio-frequency components of the rectified grid current. The direct-current component can produce no sound in the loud speaker arid so is unimportant. The sensitiveness of the detector (i.e. the change of grid potential produced by a given input signal voltage) is obviously determined by the effectiveness with which the signal voltage is rectified, and by the amount of opposition which the grid leak-condenser combination offers to the flow of the audiofrequency components of the rectified current. The impedance which the grid leakcondenser combination offers to the rectified grid current depends greatly upon the frequency of this current. At low frequencies, such as 50 cycles, this impedance is very high because the low-frequency current has difficulty in getting through the grid condenser and is accordingly forced through the high resistance of the grid leak. On the other hand, at high audio frequencies, such as 5000 cycles, the grid condenser offers an easy path to the current, practically short-circuiting the grid leak. The result is that the rectified grid current tends to produce less change of grid potential on the high audio frequencies than on the low notes. This reduction of sensitiveness at the higher audio frequencies can be quite serious, and unless the detector is adjusted properly will lead to very bad quality. Satisfactory reproduction of the high notes requires that the smallest possible grid condenser capacity be used in order to minimize the short-circuiting effect of the grid condenser on the grid leak. If the grid condenser is made too small, however, an appreciable part of the radio-frequency voltage developed by the tuned circuit supplying the grid will be used up in the grid condenser, and the signal voltage, Es, actually applied to the grid will be seriously reduced (see Fig. 2). The best value of grid condenser for all standard type tubes is from 0.0001 to 0.00025 mfds. Larger capacities should never be used. The Equivalent Circuit FROM the discussion that has been given it is seen that the most important features of the grid-leak detector are the amount of rectified grid current produced by a given signal, and the amount of voltage drop which this rectified current produces in flowing through RADIO BROADCAST the grid leak-condenser combination. Recent investigations, both theoretical and experimental, have shown that the rectified grid current produced by the application of a small radio signal voltage to the detector grid acts exactly as though if were produced by a suitable series of low-frequency generators acting between the grid and filament in series with the grid-filament resistance of the tube. There is one such generator for each component of the rectified grid current. The most important of these equivalent generators is the one of modulation frequency. The action that takes place in the grid circuit of a grid-leak detector can be conveniently described in terms of the equivalent grid circuit shown in Fig. 4. Here the rectifying effect of the grid circuit on the radio signal is replaced by the equivalent generators, Er, which are considered as producing the rectified grid current in place of the radio signal that actually does. These equivalent generators act in a circuit consisting of the grid leakcondenser combination, RC, in series with the grid-filament tube resistance, Rg. This grid resistance, Rff, is analogous in all respects to the plate resistance, RP, and is the reciprocal of the slope of the grid voltage-current curve shown in Fig. 1. While the grid resistance in amplifiers is commonly considered as infinite, this is not the case with grid-leak detectors because the detector works with a small but o Unmodulated Simple Modulation i Complex Modulation Time Fig. 3 — Rectified current for various kinds of modulation definite grid current. The grid resistance, Rg, depends upon the grid, plate, and filament voltages of the tube, and becomes higher as the grid is made more negative. High grid-leak resistances accordingly give a high grid resistance because such leaks give a more negative operating grid potential, while low resistance leaks will fix the operating grid potential where the grid resistance is low. Tube Capacity THE capacity Cg1 indicated in Fig. 4 is the input grid-filament tube capacity to audio frequencies. This capacity is larger than the interelectrode capacity by an amount depending upon the plate circuit impedance, and will be in the order of 70 mmfd. with 226, 227, 112a, and 201a tubes when there is a transformer in the plate circuit. The capacity Cgl is in parallel with the actual grid condenser, C, so that the effective grid condenser capacity is the actual capacity plus Cg1. The generators that can be assumed acting between the filament and grid in series with the dynamic grid resistance to produce the rectified grid current have no actual existence. It is merely that the effect of applying a signal voltage to the grid is the same as though these fictitious generators actually were present, and as though they and not the signal voltage were the forces really producing the rectified grid current. The voltage developed by this series of fictitious generators can conveniently be called the rectified grid voltage, and will be represented by the symbol Er. One of the fundamental features of the law of detectors is that the size of the equivalent rectified grid voltage, Er, which can be considered as acting to produce the rectified grid current, depends only upon the strength and type of signal and upon the tube characteristics at the operating grid potential. The size of grid condenser and grid leak has no effect on the amplitude of the rectified grid voltage, Er, except in so far as the grid-leak resistance affects the operating grid potential. Part of the rectified voltage, Er.' in the equiv • march, 1929 . . . page 304 • S alent grid circuit of Fig. 4 is used up as voltage drop in the grid leak-condenser combination and part is used up across Rg. The change of grid potential which the rectified grid current produces in flowing through the grid leak and condenser is the grid voltage change which is amplified by the tube in the usual audio-frequency manner. Under ordinary circumstances only the modulationfrequency component of rectified grid voltage, Er, need be considered in this process as this component represents the useful output of the detector. Detector Voltage Constant THE size of rectified grid voltage, Er, used in the equivalent detector circuit of Fig. 4 depends upon the tube characteristics at the operating grid potential and upon the signal voltage. The action of the tube in rectifying the radio-frequency signal voltage can be completely taken into account by a single tube constant called the voltage constant of the grid and represented by the symbol Vg. The voltage constant, Vg, is measured in terms of volts and varies between — 0.2 and — 0.5 volts for nearly all properly adjusted detectors. The voltage constant of the grid depends upon the slope of the "grid voltagecurrent characteristic at the operating point, and upon the way in which this slope varies with grid voltage. Mathematically it is defined by Vg=2*? dRg It can be measured by determining the grid resistance at the operating grid voltage, and at grid voltages slightly more and less than the operating voltage. As has been pointed out, the modulationfrequency component of the rectified grid voltage is the important part. The crest or peak value of this component of Er is equal to mEs2/Vg where m is the degree of modulation, which must lie between 1.00 and zero, and will only reach 1.00 when the music or speech is very loud, and Es is the peak amplitude of the signal carrier wave. The crest amplitude is 1.414 times the effective (or r.m.s.) amplitude. It is to be remembered that field strengths, etc., are ordinarily expressed in effective values, while amplifier inputs must be expressed in crest amplitudes because it is the crest amplitude of the sine wave that overloads the amplifier. Practical Example AS A simple example, consider the case of a signal modulated 20 per cent, or 0.20 at 1000 cycles, with a carrier crest amplitude of 0.05 volts being applied to a detector grid operated where the voltage constant is — 0.25 volts. The crest value of the 1000-cycle component of rectified grid voltage is then 0.20 x (0.05)2 / (—0.25) = —0.002 volts crest value. The amount of 1000-cycle rectified grid current existing in the grid circuit of the actual detector is the same as the current which this — 0.002 volts of 1000 cycles will produce acting in the equivalent grid circuit of Fig. 4. The 1000-cycle voltage drop produced ■a/vwvnaat1 R | 1 J Fig. 4 — Equivalent circuit of gridleak grid-condenser detector