Blocking Oscillators

Blocking oscillators are used to obtain pulses at certain repetition rates. The pulse may be used to drive a pulse amplifier, or it may be used to modulate a UHF oscillator.

A typical blocking oscillator circuit is shown in Fig. 250.

Fig. 250. Blocking oscillator.

The grid is driven hard, and grid current usually is comparable in magnitude to anode current. Grid and anode winding turns are approximately equal. The oscillator operates as follows.

If the grid is only slightly negative, the tube draws plate current and because of the large number of grid turns the transformer drives the grid positive, increases the plate current, and starts a regenerative action. During this period, the grid draws current, charging the bias capacitor to a voltage depending on the grid current flowing into the bias resistor-capacitor circuit. The negative plate voltage swing is determined by grid saturation, so that large positive swings of grid voltage result in virtually constant plate voltage. This continues for a length of time determined by the constants of the transformer, after which the regenerative action is reversed. Because of lowered plate voltage swing, the plate circuit can no longer drive the low impedance reflected from the grid, and the charge accumulated on the bias capacitor becomes great enough to decrease plate current rapidly in a degenerative action. Plate current soon cuts off, and then the plate voltage overshoots to a high positive value and the grid voltage to a high negative value. Grid voltage decays slowly because of the discharge of the bias capacitor into the grid leak. The next pulse occurs when the negative grid voltage decreases sufficiently so that regenerative action starts again. Hence the repetition rate depends on the grid bias R and C.

Either the negative or the positive pulse voltage may be utilized. Instantaneous voltages and currents are shown in Fig. 251 for a load which operates only on the positive pulse. The general shapes of these currents and voltages approximate those in a practical oscillator, except for superposed ripples and oscillations which often occur.

The negative pulse has a much squarer wave shape than the positive pulse, and consequently it is used where good wave shape is required. No matter how hard the grid is driven, plate resistance cannot be lowered below a certain value; so a limit to the negative amplitude is formed. There is no such limit to the positive pulse, and this characteristic may be used for a voltage multiplier.

If the transformer has low OCL, the leakage inductance may be high enough to perform like an air-core transformer. That is, there are optimum values of coupling for maximum power transfer, grid drive, and negative pulse shape, but they are not critical. Comparison of peak voltages in Fig. 251 shows that the usual 180° phase relationship between grid and plate swings do not hold for such a blocking oscillator, if the term "phase" has any meaning in this case.

Fig. 251. Blocking oscillator voltages and currents.

The front-edge slope of the negative pulse is determined by leakage inductance and capacitance as in Fig. 230, with two exceptions: the pulse is negative and the load is non-linear; hence there are no oscillations on the inverted top. The slope of this portion can be computed from Fig. 234, provided tube and load resistances are accurately known. The positive pulse can be found from Fig. 235 if these curves are inverted.

Pulse width, shape, and amplitude also are affected by the ratio of grid turns to plate turns in the transformer. Voltage rise is steeper as this ratio is greater, with the qualification that grid capacitance increases as the square of the grid turns; the ratio is seldom greater than unity. The exact ratio for close control depends on tube data which may not be available and must be determined experimentally. The situation parallels that of the class C low-Q oscillators mentioned in Amplifier Circuits - Introduction.

The circuit of Fig. 250 is called a free-running blocking oscillator. When it is desired to synchronize or otherwise control the pulse repetition rate, an external "trigger" pulse is applied to the blocking oscillator grid or cathode.