Detector and Rectifier Circuits -- GRID-LEAK DETECTOR CIRCUITS

Grid-leak detection combines the functions of detection and amplification into one triode vacuum tube. As we said before, the primary purpose of detection is to demodulate a carrier cur rent, or carrier wave, which has been modulated (i.e., changed) by the audio-frequency current representing the intelligence being conveyed. Like the diode detector discussed in the two previous sections, the grid-leak detector is useful only for amplitude modulated carrier currents.

CIRCUIT DESCRIPTION

Figs 6-1 through 6-4 represent successive half-cycles of operation during a modulation trough and a modulation peak. Since the peak is separated from the trough by many hundreds or even thousands of radio-frequency cycles, the operating conditions depicted by Fig. 6-3 do not immediately follow those of Fig. 6-2.

The circuit components in Figs. 6-1, 6-2, 6-3, and 6-4 and their functional titles are as follows:

R1-Grid biasing resistor.

R2-Load resistor.

C1-RF tank capacitor.

C2-Grid leak capacitor.

C3-RF filter capacitor.

C4-Coupling and DC blocking capacitor to the headphones.

L1-RF choke.

T1-RF transformer.

V1-Triode tube.

The tank (consisting of C1 and the secondary winding of T1) is adjusted, or tuned, to resonance at the basic carrier frequency being transmitted. After being received and amplified by the receiver, the amplified signal will then be delivered to the grid leak detector circuit for demodulation.

A total of four different groups, or families, of currents are at work in this circuit. These currents, and the colors they are shown in, are as follows:

1. Radio-frequency alternating currents (blue).

2. Radio-frequency unidirectional currents (solid red).

3. Audio-frequency unidirectional currents (green).

4. Audio-frequency alternating currents (dotted red).

The order in which these currents are listed follows closely the one in which they are normally considered in a circuit. This conforms with the idea that the "input"--i.e., control-grid portion--of a circuit must normally be understood before the "out put," or plate portion, is discussed.

The four colors chosen do not follow exactly this breakdown of currents into four families. However, this was not done to confuse the reader! One of the most important points to be understood about any current is its complete path through a circuit. This essential information can frequently be made clearer in this series by using different colors than by some other means.

Consequently, the reader. should not try to match colors faith fully with these current families.

RADIO-FREQUENCY ALTERNATING CURRENTS

Fig. 6-1. Operation of the grid-leak detector-negative half-cycle of RF during modulation trough.

Fig. 6-2. Operation of the grid-leak detector-positive half-cycle of RF during modulation trough.

Fig. 6-3. Operation of the grid-leak detector-negative half-cycle of RF during modulation peak.

Fig. 6-4. Operation of the grid-leak detector-positive half-cycle of RF during modulation peak.

The current shown in blue in the grid tank circuit is the final amplified version of the modulated carrier wave. This oscillatory current is sustained by a similar current (also in blue) which flows in the transformer primary. In Fig. 6-1, the tank current is shown moving upward through the transformer secondary.

Since a current is made up of electrons in motion, electrons are withdrawn from the lower plate of tank capacitor C1, making the plate positive as indicated by the blue plus sign. These electrons are then delivered to the upper plate of C1, making the plate negative as indicated by the blue minus sign. This negative voltage in turn delivers electrons to the left plate of grid capacitor C2, but they cannot flow onto the plate unless an equal number flow away from the right plate. The latter component of current, also shown in blue, leaves the grid capacitor and moves toward the grid, making it momentarily negative and thus restricting the flow of electron current through the tube (but not cutting it off entirely). The red line, which follows the conventional plate-current path, indicates that some plate current is going through the tube.

When the operating conditions of a tube are such that the plate current is never completely cut off-even when the grid reaches its most negative voltage--the tube is said to be operating " Class A." That is true of the tube in this example, and it accounts for the flow of current through the tube when the grid reaches its most negative value.

Figs 6-1 and 6-2 depict the approximate conditions during a modulation trough, when the carrier wave is weakest. Fig, 6-2 shows the tank current flowing downward through the secondary winding of the transformer. Electrons are delivered onto the lower plate of tank capacitor C1, making its voltage negative as indicated by the blue minus sign. At the same time, electrons are withdrawn from the upper plate of tank capacitor C1 and from the left plate of grid coupling capacitor C2, making both points positive as indicated by the blue plus signs on the upper plate of C1.

Normal capacitor action will simultaneously draw electrons onto the right plate of grid coupling capacitor C2. This makes the grid voltage momentarily positive and thereby permits more plate current to flow. (The plate current will be discussed more fully under "Radio-Frequency Unidirectional Currents.") Figs. 6-3 and 6-4 represent conditions during the modulation peak. The only essential difference between the oscillating grid current from trough to peak is its size--meaning the quantity of electrons that make up the current. In Fig. 6-3 the tank cur rent flows in the same direction as in Fig. 6-1 and achieves the same results, except for being larger. This is indicated by an additional blue line following the path of the tank current.

A larger tank current results in a larger tank voltage. This is indicated by the additional minus and plus signs on the upper plate of the tank capacitor in Figs. 6-3 and 6-4. Since the amount of conduction by the tube depends on the grid voltage, conduction will be greatest when the grid is most positive (Fig. 6-4). One more radio-frequency alternating current is at work in this circuit-the filter current which flows between ground and the lower plate of filter capacitor C3. This current, shown in blue, will be discussed in the next section, after the tube currents.

RADIO-FREQUENCY UNIDIRECTIONAL CURRENTS

Tube currents-whether plate, screen-grid, or grid-leak-are essentially unidirectional. In other words, they flow in one direction only, and never reverse their direction. This characteristic distinguishes them from alternating currents, which reverse directions at regular intervals. In this section, the plate current is shown in red, and the grid-leak current in green.

Referring to Fig. 6-1, note that the alternating tank voltage has made the control grid negative. However, the tube has not been cut off entirely-a small amount of plate current flows through it, but no grid-leak current does during this half-cycle.

In the second diagram (the next half-cycle), the control grid has been made positive and more plate current is released through the tube. Since these grid-voltage changes are occurring at the basic radio frequency being demodulated, the plate cur rent will be turned "down" and "up" in this fashion once each cycle. Therefore, even though it is a direct current, the plate current pulsates at the applied radio frequency. Hence, the plate current is said to be DC with a radio-frequency alternating component superimposed on it.

Fig. 6-2 reveals a small amount of grid-leak current (in green) coming out of the tube at the control grid. These electrons accumulate on the right plate of grid capacitor C2, until they can leak out through grid resistor R1 and the secondary of T1 to ground, then through ground to the cathode. The grid leak current also flows in one direction only. Since it does not flow from the tube unless the grid is positive, it is obviously an intermittent rather than continuous current. Because of the combined action of the capacitor (which stores the electrons) and the resistor (which passes them), the grid-leak current is smoothed out and becomes a continuous current for the rest of its journey. This resistor-capacitor action is referred to as integration, a term derived from its mathematical background. The RC action is also referred to as a "long time-constant," another term derived from the underlying mathematics. In other words, the electrons come out of the tube on each positive half-cycle and accumulate on the capacitor, but are unable to discharge completely to ground before the next positive half-cycle comes along. A continuous discharge of electrons therefore takes place as the number leaking out during an entire cycle is equaled by the number entering during the brief period when the control grid is positive.

Fig. 6-5. Graphical representation of the grid-voltage, plate-current characteristics of the grid-leak detector.

Fig. 6-5 shows a typical transfer characteristic curve for an amplifier tube. This curve relates an instantaneous grid voltage to the resultant instantaneous plate current, using assumed or average values of power-supply voltage, plate-load resistance, etc.

Graphical representations of two currents and two voltages appear in Fig. 6-5. Each current is shown in the same color as its related voltage or current in Figs. 6-1 through 6-4. Thus, the radio-frequency grid voltage is shown in blue because it is directly related to the radio-frequency tank current.

The green line down the middle of the RF grid-voltage sine wave represents the grid-leak bias voltage. This voltage is indicated in the circuit diagrams by the green minus signs (representing negative electrons) on the right plate of capacitor C2. From the grid-voltage scale in Fig. 6-5, note that the grid-leak voltage goes from -2 volts during a modulation trough to approximately -4 volts during a peak. In other words, the strength of these voltage swings rises and falls in step with the modulation imposed at the transmitter.

The grid-leak voltage in this type of detector circuit marks the first appearance of the desired audio voltage in the receiver system. This voltage will more or less faithfully reproduce the variations in frequency and amplitude of the audio voltage used at the transmitter to modulate the transmitted radio-frequency carrier signal.

As previously discussed, the strength of this grid-leak voltage depends on the amount of grid-leak current drawn from the tube during each half-cycle of radio-frequency voltage. By projecting upward from any point on the grid-voltage sine wave to the characteristic curve and thence horizontally, it is thus possible to construct a curve of instantaneous plate currents. This curve is shown in red because it is most directly related to the plate current, also in red. Reference to the scale in the middle of the diagram reveals that the plate current varies between a low and a high positive value-in the example shown, from 0.6 milli amp to about 1.8 milliamps.

The dotted red line which appears to bisect the plate-current RF sine wave is the average plate current. Reference to the plate-current scale shows that the average current varies roughly between 1.35 and 1.6 milliamperes-or 0.25 milliampere peak-to peak.

In the circuit diagrams, the audio voltage on the left plate of capacitor C4 is shown rising to its highest value (two plus signs) during a modulation peak, and falling to its lowest value (one plus sign) during a trough.

Because the characteristic curve is not a straight line, some distortion of the audio voltage will occur during amplification.

(This is known as "square-law" detection. The derivation of this term is beyond the scope of this book.) On the graphical portion of the diagram, one cycle or pulsation of plate current can be seen occurring for each cycle of radio frequency current. Since the largest pulsations occur during modulation peaks and the smallest during modulation troughs, it is possible to filter out the radio-frequency characteristics of the plate current and retain only the trough-to-peak variations.

Fig. 6-5 shows these variations as "average plate current." They constitute the audio intelligence which the carrier wave has carried from transmitter to receiver.

Capacitor C3 provides the necessary RF filtering in the plate circuit. Its reactance is normally only a few ohms at the carrier frequency. Reactance--in simplest terms, opposition to electron flow-is inversely proportional to both the frequency and the capacitor size, in accordance with the formula: 1 Xi·= 21rfC where, Xe is the reactance in ohms, f is the current frequency in cycles per second, C is the capacitance in farads.

The radio-frequency pulsations of plate current which reach the external plate circuit find three alternate paths available.

They can flow directly into the filter capacitor, or else they can flow through filter choke L1 and into either coupling-blocking capacitor C4 or directly into resistor R2. The strength of these pulsations will divide between the three paths in inverse proportion to the opposition (resistance and, or reactance) offered by each path.

When one or both types of reactances ( capacitive and inductive) are combined with resistance, the common name of impedance represents the sum total of this opposition to the passage of electron current. The filter choke in this circuit is designed to have a high inductive reactance at the radio frequencies in use here. Inductive reactance is directly proportional to both the inductance and the frequency, in accordance with the standard formula: where, Xi, is the inductive reactance in ohms, f is the frequency of the current being passed or impeded in cycles per second, L is the inductance of the coil in henrys.

Since filter choke L1 in effect is in series with load resistor R2 and blocking-coupling capacitor C4 as far as plate-current pulsations are concerned, the impedance of each alternate path can be added to the filter-choke impedance to determine the total impedance of either path. It is hardly necessary to state that most of the plate current (which is pulsating at the basic radio frequency) will choose the low-impedance path to ground offered by filter capacitor C3, rather than the two high-impedance paths through L1 and R2 or C4.

Remember that reactive ohms and resistive ohms cannot be added directly by simple arithmetic; they can only be added vectorially. This is a more complex mathematical process than simple arithmetic and is beyond the scope of this guide.

The filter combination of C3 and L1 could be made more elaborate by adding another capacitor leading from the right terminal of choke L1 to ground. It could also be less elaborate, consisting of capacitor C3 without the filter choke. Even so, the impedance of each alternate path for the RF pulsations in the plate current would be great enough that C3 would bypass most of the energy to ground. The load resistor, for example, will normally be several thousand ohms, as opposed to the few ohms offered by C3 to radio frequencies.

The reason why the other alternate path through C4 will also reject radio-frequency currents requires more explanation. Capacitor C4, which must couple the audio or modulating voltage to the headphones ( or speaker) , must have a low reactance to current flow at audio frequencies. Since audio frequencies are much lower than radio frequencies, C4 must be much larger in value than C3. Therefore, C4 will offer only negligible reactance to the radio-frequency pulsations in the plate-current stream.

At first glance it appears that the energy of these pulsations will flow into C4 and be coupled to the headphones (rather than the desired objective of flowing into capacitor C3 and being filtered back to ground). However, this does not occur because of the resistance of the transducer (headphones) beyond the coupling capacitor. (A transducer is a device for converting energy from one form to another. A headphone or speaker, for example, converts electrical energy to sound energy.) A head phone has an internal resistance of several hundred or even several thousand ohms. The RF pulsations will not flow onto the left plate of C4 unless pulsations of equal size are permitted to flow off the lower plate. Since the headphone resistance is in series with the capacitor, the RF pulsations would also have to flow through the phones. Consequently, the total impedance or opposition to electron flow-at radio frequencies is much greater than the impedance offered by filter capacitor C3. As a result, C3 filters almost the entire radio-frequency component of the plate current to ground. In other words, the RF pulsations in the plate current flow onto the upper plate of C3. Each pulsation drives an equal number of electrons off the lower plate, toward ground. These directions of flow exist only on the alternate half-cycles when the grid is positive (Figs. 6-2 and 6--4). On the alternate half-cycles when the grid voltage is negative and less plate current flows, these directions are reversed, as depicted in Figs. 6-1 and 6-3. During these half-cycles, the electrons which came out of the tube during the preceding pulsation will complete the path followed by all plate current and move downward through load resistor R2, into the power supply.

The current which flows between the lower plate of capacitor C3 and ground is thus one of the radio-frequency alternating currents. Named the "RF filtering current," it will faithfully reflect the frequency of the carrier current. Also, each of its cycles will always equal the quantity of electrons involved in the accompanying RF pulsation of plate current.

AUDIO-FREQUENCY UNIDIRECTIONAL CURRENTS

The first appearance of an audio-frequency voltage and cur rent in this circuit is at the grid, where grid-leak detection has occurred. This is depicted by the increased number of electrons stored on the right plate of grid capacitor C2 during the modulation peak (Figs. 6-3 and 6-4). Another green line is shown flowing through the grid resistor and transformer secondary to ground and back to the cathode. This higher grid-leak voltage and its associated current are due to the increasing strength of the tank voltage as a modulation peak is approached. A close scrutiny of the grid-voltage representation in Fig. 6-5 will assist the reader in understanding the several, intricately related actions occurring with the circuit.

Fig. 6-5 graphically reveals a series of radio-frequency cycles of grid voltage. These begin small during a modulation trough, build up during the modulation peak, and then decrease again as the next modulation trough approaches. They are of course driven by the oscillating voltage in the grid tank circuit. Observe that the grid voltage cannot be raised appreciably above zero.

Thus, as the grid-voltage swing increases, the grid merely be comes more negative during negative half-cycles. A solid green line represents the biasing voltage, which is built up by the grid leak current from the tube. This changing bias voltage is indicated by the collection of electrons, in green, on the right plate of grid capacitor C2. During the first whole cycle (Figs. 6-1 and 6-2) , this collection of electrons consists of one line, and two lines during the second whole cycle (Figs. 6-3 and 6-4). The solid green line in Fig. 6-5 is not actually a smooth curve as shown. Rather, it is a series of short zigzags which are repeated once during each cycle of RF. During positive (second and fourth) half-cycles, grid-leak current will flow and tend to increase the negative bias. The solid bias line "zigs" to the left at the end of each positive half-cycle, since the higher negative voltages are to the left in this diagram.

During any negative half-cycle (represented by Figs. 6-1 and 6-3), the solid line "zigs" back to the right toward the region of lower negative voltages, indicating that the bias voltage is discharging toward zero. This discharge is shown by the green lines going from the grid-storage area (right plate of the grid capacitor), through the grid resistor and transformer secondary, to ground.

In this type of circuit, there can be a few cycles, close to each modulation trough, when the grid voltage is not driven to the zero-voltage line. Consequently, no grid-leak current can begin to flow and no electrons are added to those already stored on the grid capacitor. As a result, the bias voltage discharges continuously toward ground and zero volts during these few cycles. The normal condition is for some grid-leak current to flow on each positive half-cycle, so that the biasing voltage be comes a fair approximation of the carrier wave's modulation envelope and therefore of the audio voltage "carried" by the wave. The difference between an approximation and an exact reproduction of input waveshape is known as distortion, and this circuit is characterized by high distortion.

Fig. 6-5 is a conventional waveform diagram for this type of circuit. It was made using the transfer characteristic curve of the tube. The reader is cautioned that the radio-frequency cycles shown in blue are grid-voltage cycles, whereas those shown in red are plate-current cycles; hence, they are pulsating direct current.

Likewise, the audio cycle shown in green is an audio-voltage cycle representing the accumulation of grid-leak electrons on the right plate of grid capacitor C2, whereas the audio cycle shown in dotted red is a current cycle and represents the approximate average amount of plate current flowing. The latter cycle is shown in dotted red to relate it to the audio current which flows through the headphone, or load, between the right plate of capacitor C4 and the common ground.

Figs. 6-1 and 6-2 are devoted to a modulation trough. In both diagrams the audio current flows downward through the head phones, because of the high average plate current and hence the somewhat lower plate voltage. The amount of plate voltage is represented by the number of plus signs on the left plate of capacitor C4. During a modulation trough, the plate voltage has a low positive value.

The only way this voltage can be altered is by adding or with drawing electrons. The positive voltage stored on the left plate of capacitor C4 can be thought of as a "pool" of positive ions. There is a continuous inflow of electrons into this pool from the plate current, and a continuous drain of electrons out of this pool and through R2 to the higher positive voltage of the power supply.

AUDIO-FREQUENCY ALTERNATING CURRENT

During a period of higher average plate current, electrons are added to this positive-voltage pool faster than they can be drained away through load resistor R2. Consequently, this positive voltage decreases. The normal capacitor action of C4 is such that when additional electrons flow onto the left plate, an equal number are driven off the right plate. This accounts for the downward direction of the audio current through the headphones in Figs. 6-1 and 6-2.

During a period of lower average plate current, the power supply (because of its high positive voltage) withdraws electrons from the positive-voltage pool, making it even more positive.

Again, the normal capacitor action of C4 is such that an equal number of electrons are drawn onto the right plate of C4. This accounts for the upward direction of the audio current through the headphones in Figs. 6-3 and 6-4.