Introduction

This chapter is not required reading for a general knowledge of VT fuzes sufficient for in-telligent handling and use. It is primarily for those who have some knownledge of radio circuits and operation and who seek a more detailed explanation of VT circuits and wave behavior. It is a more detailed explanation of the theory of operation of the transmitter, receiver, amplifier, wave-suppression feature, and firing circuits.

Transmitter

A schematic diagram of a transmitter-receiver circuit is shown in fiugre 16. It is a modi-fication of a "grounded-grid" Hartley oscillator circuit. (Navy VT fuzes also use a modifi-cation of the Colpitts oscillator circuit.) The tuned circuit is comprised of the coil, the distributed capacity (largely between the antenna and the projectile body), and the in-ter-electrode capacities of the tube. This "distributed capacity" is represented in the diagram by a condenser connected across the coil by dotted lines. The plate is held at projectile body voltage, so far as radio-frequency currents are concerned, by the bypass condenser.

Figure 16. Schematic diagram of transmitter-receiver (T-R) circuit.

As with all radio-frequency oscillators, the oscillation is started by some stray electrical voltage in the circuit impressed on the grid of the 32 tube. This is amplified by the tube, and enough of this amplified signal is fed back from the plate to the grid in the proper phase and frequency, through the resonant circuit, to sustain the process. The excess over that necessary to sustain the oscillation is emitted from the antenna as the radio signal.

The radiation pattern is that of a standard dipole with it greatest strength prerpendicular to the axis of the projectile body. This modified by the amplifier-response characteristics to produce an effective sensitivity pattern that more closely matches the fragmentation pattern. The standard dipole pattern in shown in figure 17, and the effective sensitivity pattern is compared with the fragmentation pattern of a projectile in figure 18. The am-plitude of the radiated wave also varies with the distance from the projectile, as shown by the curve in figure 19.

Figure 17. Standard dipole radiation pattern.

Figure 18. Effective sensitivity pattern of a VT fuze compared with the fragmantation pattern of a five-inch projectile

Figure 19. Variation of transmitted wave amplitude with distance from projectile.

Receiver

In the presence of a reflecting target a portion of the radiated wave is returned to the oscillator. As the amplitude of the radiated wave varies with direction and distance from the projectile, so will the amplitude of the reflected wave depend upon the direction and distance from the target, as well as on its size, shape, aspect, and material.

Figure 20 (right) picture the relationship between the transmitted and reflected wave when the total distance the wave has to travel to the target and back to the oscillator is an exact number of wave lengths of the radio wave. When this occurs, the signal re-turns to the oscillator an excat number of cycles later. As shown in figure 20 (right), the reflected signal is then of the same polarity as the transmitted signal. The two voltages therefore reinforces each other at the grid of the oscillator.

Figure 20. Wave relationship between transmitted and reflected waves.

This results in a larger amplitude of oscillation than would occur if the oscillator were far from any reflecting target. This causes a greater average plate current, as shown by figure 21 (right).

Figure 21. Relationships within transmitter-receiver of transmitted and reflected waves.

If the oscillator is moved to a position where the total distance the radio wave has to travel is an odd number of half wave lengths, as shown in figure 20 (left), the conditions are reversed. The reflected signal is opposite in polarity to the signal being emitted; the two signals therefore subtract from each other; the amplitude of oscillation is less; and the resultant plate current, as shown in figure 21 (left) is smaller than that which occurs when the oscillator is far from a target.

As the projectile approaches a target, it passes alternately, every quarter wave length (equivalent to one-half wave length of total wave path), through regions in which the oscillations are reinforced, and the plate current is higher than normal, and through regi-ons in which the oscillations are reduced, and the plate current is lower than normal. As the distance lessens these variations from the steady state become greater and greater. The frequency with which these variations occur is many times less than the frequency of the radio wave, because the projectile travels much slower than the radio wave; but the reversals still occur several hundred times a second.

The plate current drawn by the oscillator must flow through a load resistor, as shown in the circuit diagram, figure 16. The voltage drop across this resistor varies at a slow rate with the average plate current drawn by the tube. The radio-frequency current compo-nent does not pass through the resistor, but returns through the bypass condenser.

This is an idealized picture; actually the strength of the reflected wave from a target, such as an airplane, varies in a complicated way as the direction from the target chan-ges. In spite of the complexity of this signal, it is characterized by regular fluctuation about the indisturbed value, increasing sharply in amplitude as the target is approached.

Amplifier

A schematic diagram of the amplifier circuit is shown in fiugre 22. The amplifier is con-nected, in effect, across the load resistor in the transmitter-receiver circuit. The output of the amplifier is connected to the input of the firing circuit and the wave-suppression circuit. Direct current from the wave-suppression circuit is fed back to the grid of the first amplifier tube through the grid resistor. The amplifier is connected electrically through the fuze body to the projectile body and by leads to the "A" and "B" batteries.

Figure 22. Schematic diagram of amplifier circuit.

This is accomplished by a conventional resistance-coupled pentode amplifier of two sta-ges. The input blocking condenser keeps the positive plate voltage of the transmitter-re-ceiver tube from the gird of the first pentode, but charges and discharges as the voltage across the oscillator load resistor varies. This charging current flows through the grid re-sistor of the first pentode, and thus impresses an alternating potential on the first pen-tode grid.

The varying potential on the first pentode grid causes a variation in the plate current of the first pentode. This in turn causes a proportional variation in the voltage drop across the first pentode plate resistor. The varying voltage drop, which is greater than that in the output of the transmitter-receiver, is in turn applied through a blocking condenser and grid-return resistor to the grid of the second pentode, and a similar amplified signal appears across its plate resistor. It is this signal which appears at the output of the amplifier and which operates the firing circuit and the wave suppression circuit.

In the pentodes, there are two grids in addition to the one to which the signal is applied. One of these shields the plate from the filament and therefore makes the plate current independent of the plate voltage. It is called the "screen" and is held at a positive volt-age with respect to the filament. The other, called a "suppressor", is held at the same potential as the filament by a connection inside the tube. Its purpose is to prevent cur-rent flow from the plate to the screen in those instances in which the screen is more po-sitive than the plate. The screen is connected to the positive side of the "B" battery through a screen resistor and is bypassed to ground by a condenser.

By a proper selection of values for screen and plate bypass condensers, the amplifier is made sensitive to a band of frequencies and insensitive to others. The frequency of a target signal lies between two extremes: that frequency generated when a projectile traveling at its maximum speed approaches a plane coming toward it at its maximum speed, and that frequency generated when a projectile traveling at its minimum speed approaches a plane going and insensitive to others. Figure 23 shows how amplification varies with frequency. The limitation upon the frequencies that are amplified modifies, in effect, the standard radiation pattern of figure 17 to produce the effective sensitivity pattern of figure 18.

Figure 23. Amplifier frequency-response characteristics.

The amplifier is made less sensitive to relatively low frequencies by the screen bypass condenser, while a similar function is performed by the plate bypass condenser for relati-vely high frequencies.

When a realtively low frequency is impressed on the grid of the pentode, as the grid vol-tage increases the bypass condenser does not prevent the screen current (and hence, also, the screen voltage) from increasing. This is at the expense of the plate current and decreases the amplification of the applied signal. However, when a higher frequency is impressed on the grid, screen is held at a substantially constant voltage because the by-pass condenser cannot charge and discharge quickly enough to allow a corresponding variation in screen voltage. Thus the amplification of the signal is not affected.

In performing the complementary function, the plate bypass condenser decreases the amount by which relatively high frequency signals are amplified. Lower frequency signals are amplified. Lower frequency changes are unaffected, and the amplified signal can pro-duce its equivalent output voltage.

Wave-Suppression-Feature Circuit

As is pointed out above, the amplitude of a target signal increases rapidly as the projec-tile approaches. In contrast, other signals, such as those resulting when a projectile passes over oceans waves, have a nearly constant average amplitude. They are some-times large enough so that the fuze would be operated by them, except for a wave-sup-pression circuit which decreases the sensitivity of the amplifier in the presence of steady signals. The curve in figure 24 shows how amplifier sensitivity is decreased as strenght of signals increases.

Figure 24. Wave-suppression-feature sensitivity curve.

A schematic diagram of the wave-suppression feature circuit is shown in figure 25. The output from the amplifier is applied through a blocking condenser and a load resistor to the plate of the diode tube in the wave-suppression-feature circuit. When the signal is positive, the diode draws current, and the resultant drop in the load resisitor prevents the diode plate from becoming very positive; but when the signal is negative, no current flows in the diode, and the plate goes as far negative as does the amplifier output signal. The average rectified d-c voltage on the plate is therefore negative by an amount pro-portional to the amplifier output.

Figure 25. Schematic diagram of wave-suppression-feature (WSF) circuit.

This average d-c voltage is filtered by a filter resistor and bypass condenser and applied to the grid of the first amplifier pentode. This pentode, is so designed that its amplificat-ion decreases with increasing negative voltage on the grid.

The filter resistor and bypass condenser cnanot follow the rapid increase in signal ampli-tude as the target signal gets trough the amplifier and operates circuit before the wave-suppression volatge builds up. This assumes that the fuze is armed while over waves or that it approaches the waves gradually, as at the end of flight. In a sudden appearance of waves after the fuze is armed, the signal from the waves may get through and ope-rate the firing circuit. A signal of constant amplitude, however, does knot give sufficient output to operate the firing circuit, no matter how great its value.

Firing Circuit

Figure 26 is a schematic diagram of the firing cicuit. When the round is fired, current flows from the "B" battery, through the charging resistor, into the firing condenser. This condenser provides a means of storing electrical energy which can be rapidly expended, when called upon, to fire the squib.

Figure 26. Schematic diagram of firing circuit, with safety switches.

The rate at which energy accumulates in the condenser is limited by the charging resis-tor. Before the electrical arming time has passed, there is insufficient energy in the con-denser to operate the squib; but after arming there is an excess of energy to insure re-liable operation. This is shown by the curve in figure 27.

Figure 27. Average arming characteristics of firing condenser.

The thyratron is an electronic switch which discharges the condenser through the squib when the projectile is near a target. It is a triode radio tube with a small amount of ar-gon gas in the hub. The grid of this tube is normally maintained at a negative voltage with respect to the filament by A.C. or grid battery, shown in the ciruit diagram of figure 26. This negative voltage prevents any current flow in the tube.

When a target signal is present in the output of the amplifier, however, the blocking con-denser shown in the grid circuit of the thyratron charges and discharges with the signal through the grid resistor. The charging current, flowing through this resistor, causes the grid end of it to be alternately positive and negatibe with respect to the negative "C" voltage. A signal of sufficient amplitude will eventually swing sufficiently amplitude will eventually swing sufficiently positive to overcome the control supplied by the negative voltage of the "C" battery, and current can flow in the thyratron.

As soon as current starts to flow, the argon gas in the thyratron is ionized, and a heavy current flow, carried by the argon ions, can take place between the thyratron plate and its filament. This current flow, which is now independent of the voltage on the grid, is so heavy that the thyratron plate is effectively shorted to the filament, and the firing con-denser rapidly discharges through the thyratron and squib. The squib is exploded by the surge of current.