US2313906A - Electrical delay circuit - Google Patents

Description

March 16, 1943. K. R. WENDT 2,313,906

ELECTRICAL DELAY C IRCUIT Filed May 25, 1940 I 2 Sheets-Sheet 2 HG. 4. I i ORIGIN/7L PULSE E6; 4 b. DELHYED PuL-sE Zhwentor Karl E. Wendzi Gttorneg Patented Mar. 16,1943

ELECTRICAL DELAY Karl n. Wendt, Audubon, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application May 25, 1940, Serial No. 337,158

6 Claims.

My invention relates to a method and apparatus for delaying and/or shaping electrical waves, and particularly to a method and apparatus for delaying an electrical pulse-or a series of recurring electrical pulses.

In the television art, especially, it is frequently found desirable to delay recurring pulses which are rectangular in wave form. An example of this is described in Patent No. 2,132,655, issued October 11, 1938, to J. P. Smith and assigned to the Radio Corporation of America, this patent describing an electrical generator in which electrical pulses are properly delayed, shaped and combined to produce the desired synchronizing signal. Another example is the case where it is desired to key a circuit at the end of an impulse, the keying or switching to be done by an impulse which starts as soon as the original impulse ends. Such a keying pulse may, of course,be looked upon as the original pulse which has been delayed by an amount equal to the du ration of the original pulse.

An object of my invention is to provide an improved means for and method of delayin electrical pulses.

Another object of my invention is to provide an improved and simplified circuit for delaying and shaping electrical pulses.

Still another object of my invention is to provide an improved means for and method of de laying an electrical pulse by an amount equal to the duration of the said pulse.

In practicing my invention, I employ what may be described as a non-linear differentiating circuit, the non-linear feature residing in the fact that a vacuum tube which is associated with the difierentiating circuit presents a difference impedance thereto during one part of the differentiating action than during a diilerent part.

One of the preferred embodiments of my invention comprises a capacitor-resistor circuit located in the input circuit of a vacuum tube which has a positive bias voltage applied to its grid. When a positive rectangular pulse is applied to the differentiating circuit, the vacuum tube input impedance is so low that there is very little voltage on the grid of the tube for the duration of the pulse. At the termination of the pulse, however, the voltage which results from the difierentlatlon causes the grid to become negative whereby the tube input impedance becomes high, and there is produced a voltage pulse of substantial amplitude which is delayed. If the amplitude of this delayed pulse is made large enough, it drives the tube beyond cut-oi! and there appears in the output circuit of the tube a substantially rectangular pulse with the desired delay.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which Figure l is a circuit diagram of one preferred embodiment of my invention,

Figures 2, 2a and 2b are voltage curves which are referred to in explaining the operation of the circuit shown in Fig. l, v

Figure 3 is a circuit diagram of another embodiment of my invention,

Figures 4, 4a and 4b are voltage curves which are referred to in explaining the operation of the circuit of Fig. 3, and

Figure 5 is a modification of the circuit shown in Fig. 3.

Referring to Fig.v 1, the differentiating circuit comprises a condenser I0 of comparatively small capacity connected in series with a resistor II which, in the example illustrated, functions also as the grid leak resistor for a vacuum tube I2.

The tube I2 may be a triode having a cathode I3, a control grid I4 and a plate I6. In accord ance with one feature of my invention, the grid I4 has a positive bias applied thereto by means of a suitable biasing source. In the example illustrated, this bias is obtained from the B battery I5 to which the grid leak resistor II is connected- The electrical pulse which is to be delayed is applied to the diiierentiating circuit through an amplifier tube I! having the usual plate resistor I8.

Considering now the operation of the abovedescribed circuit, it will be assumed that it is de sired to delay a rectangular voltage pulse a: which appears across the plate resistor I8 with positive polarity as shown in Fig. 2. The voltage appearing on the grid I4 will then be as showri in Fig. 2a for the following reasons:

Before the start of pulse at, the tube I2 is operating at a slightly positive bias and the grid is drawing current due to the positive voltage applied to resistor I I. The leading edge of pulse :1: tends to drive grid I4 more positive which, of course, tends to decrease the already low gridcathode impedance of tube I2. The leading edge of pulse a: is therefore so sharply and rapidly differentiated that condenser I0 is charged almost immediately, and very little of the voltage due to the leading edge of the pulse :0 appears on the grid of tube I2, as shown by Fig. 2a.

At the end of the pulse :r, however, the grid I4 is driven negative by the edge of the pulse.

condenser l discharges comparatively slowly toward +300 in the example shown, and in the interval up to zero, or cathode voltage, the discharge is essentially linear as shown in Fig. 20. Thus the back edge of the pulse a: is diilerentiated at a comparatively slow rate. At zero voltage the grid it again shorts out the high resistance II, and the condenser discharge ceases. Due to clipping action, this grid voltage wave is converted to a substantially rectangular pulse of plate current which gives the delayed output pulse voltage as shown in Fig. 2b. It is thus seen that the back edge of pulse a: has been diflerentiated very differently from the manner in which the front edge was differentiated. It will be noted that this delay circuit does not reverse the polarity of the voltage pulse. In some apparatus this feature saves the cost of a tube for reversing polarity.

Referring now to Fig. 3, there is illustrated a non-linear diiierentiating circuit in which an inductance coil 26 and a resistor 27 are connected in series to differentiate the applied voltage pulse. The voltage pulse which is to be delayed may be applied to the clifierentiating circuit through an amplifier tube 23. This pulse appears as a positive pulse, such as the rectangular pulse 1 shown. in Fig. 4, across the plate resistor 29 of the tube 28.

The input electrodes of a vacuum tube 3| are connected across the coil 26. This tube may be a triode having a cathode 32, a grid 33 and an anode 34. In the specific circuit illustrated in Fig. 3, the desired positive bias on the grid 33 is obtained from the plate voltage for the tube 28 by utilizing a. direct current coupling between the tubes 28 and 3|.

The voltage shown in Fig. 40. appears on the grid 33 as a result of the following action: Be-

fore the voltage pulse 11 is applied to the circuit,

the grid 33 is drawing a small amount of grid current Just as in the case of the circuit shown in .Fig. 1. Thus the input impedanceof the tube 3| is low and it remains low while the positive pulse 1/ is being applied. For the duration of the pulse :11, there isa current flow through the coil 26 whereby lines of flux build up in the coil. Thus, the grid voltage increases gradually during this period as shown in Fig. 4a, but the corresponding output voltage is low as shown in Fig. 4b.

At the end of the pulse y, the flux in the coil 26 collapses to produce a negative voltage which drives the grid 33 negative whereby the input impedance of the tube 3| is high. As a result, ther is a voltage pulse of substantial amplitude appearing in the output circuit of the tube 3|, and, since the tube 3| is driven beyond cut-off, as indicated in Fig. 4a. the said voltage pulse is substantially rectangular in wave shape as shown in Fig. 4b. It will be apparent that this pulse, like the pulse shown in Fig. 2b, has been delayed in time by an amount equal to the duration of the original pulse. 4

In Fig. 5 there is shown a circuit which operates substantially the same asthe circuit shown in Fig. 3, the difference in the two circuits beinz that an alternating current connection rather than a direct current connection is employed in Fig. 5 between the vacuum tubes. In the two figures, like parts are indicated by the same reference numerals.

Because ofthe coupling condenser 38 employed in Fig. 5, in this embodiment of the invention, the positive bias is applied to the grid 33 through a grid resistor 31 and the resistor 21 by means oi a suitable biasing source such as a battery 38. A delayed pulse is obtained in the output circuit of the tube 3| as previously described.

In certain figures, merely by way of example. the values 01 certain circuit elements have been indicated in megohms, micromicrofarads, and millihenries. Also, a suitable tube type and suitable voltages have been indicated by way of example.

It will be apparent from the foregoing that various modifications may be made in my invention without departing from the spirit and scope thereof.

I claim as my invention:

1. The method of utilizing a difierentiating circuit for delaying an electrical pulse which comprises diilerentiating said pulse in said circuit and simultaneously presenting a low impedance to said circuit the entire time that the differentiated signal is of one polarity, and presenting a high impedance to said circuit while the difierentiated signal is of the opposite polarity.

2. The method of utilizing a differentiating circuit which is connected to impress the difleren tiated signal upon the input electrodes of an electric discharge tube, said method comprising the step of impressing a voltage pulse of such large amplitude upon said differentiating circuit that the input impedance of said tube is low the entire time the differentiatedsignal .is of one polarity and is high while the differentiated signal is of the opposite polarity.

3. The method of delaying a positive voltage pulse which comprises diiferentiating the front edge of said pulse to produce a narrow differentiated voltage pulse of small amplitude and differentiating the back edge of said positive pulse differently to produce a pulse of comparatively large amplitude.

4. The method of delaying a positive voltage pulse which comprises diilerentiating the front edge of said pulse to produce a narrow differentiated voltage pulse of small amplitude and differentiating the back edge of said positive pulse diilerently to produce a comparatively wide voltage pulse of comparatively large amplitude.

5. The method of delaying a positive voltage pulse which comprises differentiating the front edge of said pulse at a rapid rate and cliiferentiating the back edge of said pulse at a comparatively slow rate.

6. The method of utilizing a difierentiating circuit and a vacuum tube having a cathode and a positively biased control grid connected to said dlfierentiating circuit which method comprises impressing a voltage pulse of such large amplitude upon said differentiating circuit that said grid is driven further positive while the difierentiated signal is of one polarity and is driven so negative while the differentiated signal is of the opposite polarity that said tube is driven beyond plate current cut-ofi'.

KARL R. WENUI'.

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