Finer Details of Radio

Radio Record, Volume II, Issue 44, 17 May 1929, Page 30

Finer Details of Radio

Three Electrode Valves as Detectors

B y

PENTODE

ANOTHER correspondent, "R.T.C." (Dunedin), asks the following questions: 1. What guage of tinned or enamelled wire should I use in both the antenna coil in the transformer? A.: Within a size or two, guage does not matter. Usually 22 is considered as satisfactory as any. For the transformer try about 85 turns on a 23-inch former. Space the wire so that the primary and the secondary, although close-wound, do not touch one another. IZ wound too tightly, oscillation will be difficult to control. 2. What diameter of former should be used for the tickler coil? A.: If a 24-inch secondary former is used, a 2-inch will be necessary. Wind on about 25 to 30 turns D.S. ©. 30-guage wire. 8. Should a .0001 m.f.d. condenser be connected in series between the antenna and the aerial coil?-Yes. It will sharpen tuning. 4, What should be the value of fixed condenser between "B" plus 135 and "A 5 m.f.d.? 5. What voltage should. be applied to the plate of the .201A as detector? A.; From 224 to 45 volts.

FoLLowine our discussion on the theory of the modern valve when used as an amplifier, let us consider for a while just how and why a valve will detect signals when called upon to do so. The valve is the same as all gre others in the receiver, but why does e first, if that be a R.F. valve, not detect? How is it that the mere fact of the inclusion of a grid leak and condenser specifies which valve is destined to convert the ‘inconceivably high frequency impulses into audible low frequency electrical vibrations. To thoroughly understand the various prin--eiples involved needs a certain amount of imagination, not because there is any doubt as to the accepted explanation, but rather from the numerous factors working together upon which the final result of rectification takes place. A little concentrated thought will be needed, and the writer will en-

deavour, in this short article, to make it as clear as. possible. ; The word "detection" means the rectification of the high frequency alternating currents as they are received from the aerial or amplified by the

radio frequency valves in a receiver. Several methods have been used, but the three of most importance are those using a crystal detector, a grid leak and condenser in conjunction with a valve, and the one known as the "anode bend" detector. As it is only with the valve that we are concerned, the last two methods only will be discussed. The Leaky Grid. "THE most common and universally ' used system is the leaky grid condenser, and as this is the more difficult to fully understand it is dealt: with first. Last week a simple graph was explained, showing the relation between the grid. voltage. and anode current. Reference to the accompanying diagram shows the relation between grid current and grid voltage, irrespective of anode voltage, which is constant at 22 volts. Whereas the anode current amounted to several milliamps, the grid current as shown here amounts to one or two microamps. A milliamp is a one-thousandth of an ampre, a microamp is a one-thousandth of a milliamp. The vertical lines indicating grid voltages show the grid potential relative to the filament. These are expressed as. fractions of a volt, the reason being explained later. It will be noted that even when the grid is made slightly negative a small amount of grid current flows, and it is not until over 4 volt negative bias has been applied to the grid, that grid current is completely prevented from flowing. This is accounted for by the fact that the velocity of the electrons, as

~ they rush toward the grid, is so great that it needs a small retarding potential to completely prevent them from settling in the grid and liberating their. small charge of electricity. It is upon this small bend in the bottom of the grid-current, grid voltage curve that the action of detection depends. The eurve is based upon measurements taken using a 201A type valve with 22 volts B supply and a 6 megohm grid leak. The valve of the condenser is not critical. In the average receiver the grid return, in the case of a detector (the grid leak) is connected to.the filament positive terminal and it ‘would be assumed that the grid would be at a positive potential, the valve being half the filament supply voltage. This is incorrect. The operating point of the valve is that which is finally evolved after ons, or two governing factors have been } satisfied. Let us at first assume the grid to be 8 volts positive with respect to the filament. The graph does not accommodate this supposition, but it would be seen that a large grid current would flow. This, however, has to flow through the grid-leak. Whenever 2 current flows through a resistance, 2 voltage drop occurs across the said resistance. The grid-leak is merely a simple high resistance and the voltage drop across this woulc be comparatively great. In actual practice, the drop is such that the average detector valve operates with the grid at a fraction of a volt from the zero, usually, as shown, slightly on the negative side, So that although the grid return is connectec to the positive filament terminal. the grid does not follow suit, but settles down to an operating point which depends upon the valve of the. grid leak and the current flowing through this resistance, causing a voltage drop across the same. This point has to be remembered. Having arrived at the working point we are ready to apply a high-frequency wave to the grid to understand the various steps towards rectification. The series of waves on the left-hand top corner show the applied R.F. wave. The marked operating point shown, is

correct only when no signals are being applied to the grid. Directly a wave is impressed upon the grid, the operating point moves in sympathy. When at rest, this point is: at 3 volts negative, but when the incoming wave is applied, this point moves from .1 to .5 volts up and down the curve. oo,

Now turn the attention to the effect his has upon the grid current. According to the graph, this will also vary, corresponding to. the movement of the operating point. Owing to the shape of this curve more grid current will flow when the point goes over to the positive side of the cycle, than will be the case when the working point moves to the negative side of the completing half-cycle or wave. Let us run over it once more. The incoming high-frequency wave shown on the left of the graph is directly impressed on the grid. Hach half of this wave is equal and the voltage mounts up to a maximum on one side and repeats itself on the other aide of the zero-line. These variations of potential, however nroduce variations of grid currae shown as the wave form on the tom right side of the graph, But, ng to the peculiar shape of ‘the characteristic grid cure, these current Waves are not equal on each side of the z.-o-point shown by the dotted line. More current flows during one-half eycle than does in the other, and this current is said to be partly rectified. Having obtained these minute impulses of direct current it would be thought that the pro :ss of detection was complete. One or two more steps are necessary before the audio-fre-quency signals find their way into the plate circuit. The small amount of direct current produced on the grid has only one escape, and that is through the grid leak. It was mentioned before that when a current passes through a resistance a voltage drop occurs across loon resistance. One of the factors governing this drop is the amount of "eurrent flowing. The more current the greater the voltage drop. The direct current flowing through the grid leak varies according to the amount of rectified current produced so it can now be seen that these direct current impulses, flowing through the resi.‘ance of the grid leak, produce variations in the voltage drop across the grid leak. As one end of the leak is connected directly to the grid these voltage

variations produce an effect upon the anode current, and the action of the valve as-a detector is complete. Summarised, the process is as follows: High-frequency voltages are first rectified by making use of the grid volt-grid current characteristic. The rectified current’s vibrations flow through the grid leak causing varying potentials in voltage to be again applied to the grid. The valve now works as an ordinary amplifier, and current variations appear in the plate circuit. . we fw. ° ate , Anode Bend Detection. "THE explan:tion that has been given of grid-leak detection differs considerably from the familiar one in which the function of the grid leak is to let the grid condenser. charge. leak off and return to the filament. The explanation of the type known as the "anode bend" detector will be almost _ self-evident. The curve given (Fig. 2) represents a characteristic curve of a typical high impedance valve. This class of valve :s the most efficient when used as detector. No grid leak and condenser are used, and rectification is obtained by making use of the sudden curve at the lower end of the straight portion. The operating point in this case is obtained by using a negative grid bias of four volts, shown by point marked X. The high-frequency wave is impressed upon the grid, but owing to the shape of the curve the anode current wave form appears as represented by the wave on the bottom right-hand side of the graph. More current flows during one-half of the cycle than during the opposite half, and this extra current is the rectified current used to operate the ’phones or speaker in the anode elreuit of the valve.