US2975364A - Square wave shaping circuit and attenuator - Google Patents

Description

March 14, 1961 1 T. FLEMING SQUARE WAVE SHAPING CIRCUIT AND ATTENUATOR Filed Sept. 9, 1957 J. I l

SQUARE WAVE SHAPING CIRCUIT AND ATTENUATOR Lawrence T. Fleming, Bellaire, Tex., assignor, by mesne assignments, to Dresser Industries, Inc., Dallas, Tex., a corporation of Delaware Filed Sept. 9, 1957, Ser. No. 682,651

11 Claims. (Cl. 328-31) This invention relates to apparatus for shaping an input voltage, and more particularly to apparatus for forming an input voltage into a square wave voltage.

The prior art includes many different circuits for shaping an A.C. voltage into a square wave output. Perhaps the most effective of these prior circuits utilize one or more cathode-coupled clippers. The present invention represents an improvement over the conventional cathodecoupled clipper circuit to decrease the distortion of the output wave shape and to reduce the expense and complication of the components of the circuit.

lt is also conventional to attenuate a voltage, which may be of square wave configuration, to select a desired amplitude of output voltage. Ordinarilythis is achieved by employing a switch having a movable contact which selectively engages a plurality of fixed contacts connected in a resistor voltage divider circuit. When the movable contact of the switch is changed to different positions, different levels of the voltage input to the attenuator are furnished to the output thereof. This prior type of attenuator is subject to the disadvantage that inherent capacity between the switch contacts is so large as to distoit the wave shape of the output voltage, particularly when the output voltage is of square wace configuration.

The present invention includes an improved attenuator which is designed to minimize the eiiects of the inherent capacitance between the switch sections thereof, in order to decrease the distortion of the wave caused by the attenuator.

The invention will now be more fully described in conjunction with a preferred embodiment thereof shown in the accompanying drawing.

in the drawing, the single figure is a schematic diagram of an apparatus constructed in accordance Withthe invention.

The shaping circuit and attenuator of the invention is designed to accept an input voltage which may be from an A.C. source'l and may also include a positive D.-C. component. In particular, the apparatus is designed to cooperate with a source of sine wave oscillations such as that disclosed in an application of Louis W. Erath, entitled Bridge-Stabilized Oscillator, Serial No. 680,468, filed August 27, 1957, now Patent No. 2,898,553, and assigned to the same assignee as the present application. One of the outputs of the oscillator disclosed in that application is a 9 volt A.C. component associated with a +84 volt DfC. component.

The A.C. input voltage is supplied to a cathode-coupled clipper 2 which includes an input triode 3 and an output triode 4. The cathodes of the two triodes are co-nnected together and connected to ground through a common cathode resistor 5. The plate of the input tube 3 is connected directly Yto a source of positive D.-C. potential 23, which is preferably regulated, and the plate or anode of triode 4 is connected'to the same source through a plate resistor 6. The control grid of the input tube 3 is directly connected to the A.C. source so as to have a ,wat atented Mar. 1d, 19261 a D.-C. component, as well as the A.C. signal, between the control grid and ground.

The control grid of the output triode 4 is bypassed to ground for A.C. through a capacitor 7, and is positively biased through connection to the movable contact of a potentiometer 8 connected in a voltage divider including resistors 9 and 10. The series combination of resistors 9, and 10 is connected between the positive D.C. source 23 and ground.

The anode of the output tube 4 of the first clipper is connected through a coupling capacitor 11 to the second clipper 12, and more particularly to the control grid of the input triode 13 of the second clipper. The input triode 13 and the output triode 14 of the second clipper have their cathodes connected together and through a common cathode resistor 1S to a source of high negative potential 24. The anodes of tubes 13 and 14 are connected to ground through plate resistors 16 and 17, respectively.

The anode of the output tube 14 of the second clipper is directly connected to the control grid of a power amplifier tube 18 which is shown as a tetrode and which is connected as a cathode follower. Power amplifier 18 has its cathode connected to ground through a potentiometer 19 across which the output voltage of the shaping circuit is developed. The screen grid ofthe amplifier 18 is connected to the regulated source of positive potential 23 through a screen dropping resistor 20, and theanode is connected to an unregulated source of positive potential 45 through a plate resistor 21.

The control grid of the power amplifier 18 is positively biased through a resistor 22 connected between the control grid and the regulated positive potential source.

ln explanation of the apparatus so far described, it should be realized that an axiom in circuit design of wave Shapers is that coupling should never be through a lcapacitor, because of the distortion effects of such a coupling. In order to avoid capacitive coupling between the second clipper and the power amplifier, the anode of the output tube 14 of the second clipper is connected directly to the control grid of the power amplifier. In order that this direct connection may be used, the conventional con nection of the anodes of the second clipper to B+ must be avoided, and the anodes are connected to ground, with the cathodes connected to a negative source of potential.

It will be noted that the axiom above referred to is violated by coupling of the output tube `4 of the first clipper through a coupling capacitor 11 to the second clipper. This is done because of the .positive component of the A.C. source 1 which it is convenient to use for the input to the first clipper, and which it is preferable not to place on the control grid of tube 13. Undesirable distortion caused by the coupling capacitor is avoided by selecting with the second clipper only a small portion of the wave supplied by the first clipper. y

With the output of the amplifier 18 taken across itsl cathode resistor and the input of the tube being connected to ground through the plate resistor 17, the output of the amplifier would be limited to the cathode Abias point of the amplifier but for the provision of the positive bias on tube 1S through resistor 22. The magnitude of the resistor is selected to obtain the desired positive-going magnitude of the output voltage. The connection through resistor 22 allows the control grid of the amplifier 1S to be slightly positive when the output tube 14 of the second clipper is cut off, and it also reduces the maximum negative voltage excursion of the control grid of amplifier 18 when the output tube 14 of the` second clipper is fully conducting.

The operation of cathode-coupled clippers is well known and need not be fully explained here, but it will be `appreciated that the output tube 14 of the second clipper must be alternately cut oft and fully conducting in order to achieve the desired clipping action. The plate resistor 17 of that tube must be of sufficient size also so as to cut Off the power amplifier tube 18 when the output tube 14 is fully conducting.

The above described apparatus is designed to operate over a wide range of frequencies between, for example, one cycle per second and 120 kilocycles per second. At the low end of that range, the switch interval in the square wave shaping operation takes enough time that 60 cycle hum from the power supply is capable of distorting the square waves, particularly since the negative supply to which the cathodes of the second clipper are connected is not regulated. A large filtering bypass capacitor 25 is connected between the control grid of clipper tube 14 and ground in order to iilter out this 60 cycle.

hum. The D.C. return for the grid of clipper tube 14 is completed through a resistor 26 connected across capacitor 25, and the D.C. return for the grid of clipper tube 13 is completed with a resistor 27 connected between the grid and the end of resistor 26 remote from ground.

D.C. connection between the cathodes of clipper tubes 13 and 14 and the grids is achieved through a resistor 28 connected between the end of cathode resistor 15 remote from the cathodes of the clipper tubes and the control grid of clipper tube 14. This resistor is shunted by a filter capacitor 29.

At very low frequencies the wave form at the input to the second clipper will be tilted to some extent, that is, will become inclined with respect to the vertical, especially because of the coupling capacitor 11. As indicated above, the undesirable effects of this capacitor may be avoided by furnishing the second clipper a voltage large enough so that it can operate on only a small portion thereof. In order to achieve this effect, the plate resistor 6 of clipper tube 4 must be large enough to develop a relatively large output voltage to drive clipper 12. If the resistor is too small, a signal with a portion thereof tilted will come through at the lowest frequencies of operation of the shaping circuit.

The bypass capacitor 7 for the control grid of clipper tube 4 fulfills an important function in preventing degradation of the positive excursion of the square wave due to grid-following action through the internal capacitances of the clipper tube.

The plate resitsor 16 of the input tube of the second clipper is also important in order to reduce sharp pips" of current normally generated at the switching points of the square wave. Suppressor resistors 20 and 21 of the output tube are important for the same reason.

The potentiomter 8 of the first clipper provides a control of the symmetry of the output wave form. For one setting of the potentiometer the positive-going pulses'and the negative-going pulses may be of the same width, while for other settings the different pulses may be of different widths.A

As indicated above, the negative source connected to the cathodes of the second clipper 12 and the positive source connected to the anode of power amplifier 18 need not be regulated. The reason regulated voltages are lnot necessary is that when the output clipper tube 14 of the second clipper is fully conducting, the power amplifier tube 18 is so biased that it is cut off, with the result that changes in the negative voltage supply have no effect on the output wave during this portion of the cycle. On the other hand, when clipper tube 14 is fully conducting, the power amplifier tube is cutoff, so that changes in the positive supply connected to the anode of amplifier 18 have no effect on this portion of the output wave. The only times during each cycle of the square wave voltages that changes in these two supply voltages could have any deleterious effect are those times occupied by the switch actions of the tubes. Fortunately, switching action takes place very rapidly so that effects of granges in the unregulated voltages are extremely sma It is extremely advantageous to be able to use unregulated voltages for supply of the second clipper and the power amplifier, because then the size and expense of the power supply may be considerably reduced. Since the above-described circuit enables use of the unregulated potential sources referred to above, the circuit design is especially valuable.

The apparatus of the invention also includes an attenuator indicated generally at 30 which is designed to select a desired portion of the output across cathode resistor 19 and supply it to output terminals 31 and 32. Output terminal 32v is connected to a grounding shield indicated schematically at 33, while terminal 31 is connected to movable contact 34 4of a two section ganged switch which is preferably of the rotary type. As indicated in the drawing, the switch has six positions, and, considering the positions to be numbered consecutively from the top to the bottom of the figure, the first four stationary contacts 34a-34d of the switch section corresponding to movable contact 34 are connected together and to the movable contact 35 of the other switch section. The other two stationary contacts 34e and 34f cooperable with movable Contact 34 are connected through a set of resistors to the shield, as will be more fully explained hereinafter.

The rst four stationary contacts cooperable with movable contact 3S ofthe switch are also connected through a set of resistors to the last two stationary contacts of the first switch section, and through those resistors to the shield.

The resistors above referred to include the series combination of resistors 36 through 40 connected consecutively between the movable contact of potentiometer 19 and the grounded shield 3 3. A resistor 41 is connected between the second stationary contact 3Sb of the second switch section and the movable contact of potentiometer 19, and the third stationary contact 35e is connected to the junction between resistors 36 and 37, while the fourth stationary contact 35d is connected through resistor 42 between resistors 37 and 38. As indicated above, the first movable contact 35a of the second switch section is connected to the potentiometer 19, while the fth and sixth stationary contacts 35e and 35j are connected to the grounded shield.

The fifth stationary contact 34e of the rst section of the switch is connected through resistor 43 to the junction between resistors 38 and 39, while the sixth stationary contact 34f is connected through resistor 44 to the junction between resistors 39 and 40.

As indicated in the first part of this specification, a conventional attenuator would include a single switch connectable sequentially to a plurality of contacts between which-voltage dropping resistors are connected. With such an attenuator, the capacity between the output terminals, due to the inherent capacity of the switch, would produce pips at the leading edges of each pulse of the square wave, because of the high frequencies included in the square wave output. For instance, even if the attenuator had an internal capacity of the order of one micro-microfarad, a rather low stray capacity, the impedance therof at one megacycle would be 160,000 ohms. With attenuator resistances of the order of those shown in the drawings, and with the switch in the position corresponding to the lowest voltage, the resistive impedance to ground would be 600 ohms, so that the ratio between the stray capacitive impedance and the resistive impedance would be about 200. However, the resistive attenuation ratio would be 100,000, and the resulting loss of attenuation would produce the overshoots or pips referred to above. On the other hand, the attenuator of the present invention employs a shield between split sections of the attenuator switch, with the last two stationary contacts of the second switch section S connected to the shield. Thereby, the stray capacity is reduced to a level very much lower than that found with the ordinary attenuator, so that the overshoots of pips on the square wave output are decreased or substantially eliminated.

Though the attenuator described could be used with other sources than the wave-Shaper disclosed herein, it is of particular utility in conjunction with a square wave shaping circuit, since the high frequency components in such waves make stray capacity so important in the shape of the attenuator output.

It is not believed that any further description of the operation of the circuit -above described should be necessary, since the general principles of cathode-coupled clippers for square wave shaping, and of switch attenuators are well known. `It will be appreciated that the invention involved in this application resides in particular improvements of these circuits which are described in detail above, and the principles of whose operation is also discussed.

For the purpose of illustration and not as a limitation on the scope of the invention, a set of suitable values of the resistive and capacitive components of the circuit described will now be given:

R C25 R5 15K R26 100K C7 0.01 mfd R27 470K R8 25K R28 100K R9 C29 Rm 68K R35 6K C11 1 R37 Ohms R15 R38 Ohms R13 R39 6 Ohms R17 220K R40 0.667 OhmS 1g R41 Ohms R29 Ohms R42 Ohms R21 220 ohms R43 600 ohms R22 220 Ohms R44 600 OhmS It will be `appreciated that many minor changes could be made in the circuit described without departure from the scope of the invention. Accordingly, the invention is not to be considered limited to the specific embodiment disclosed but rather only by the scope of the Iappended claims.

I claim:

l. A square wave shaping circuit including a pair of cathode-coupled clippers, an output cathode follower, means for delivering an input alternating voltage to the iirst clipper, means for coupling the output wave of the first clipper to the input of the second clipper, the anodes of Said second clipper being connected to ground and the cathodes thereof being connected to a negative potential point, a plate resistor between the anode of the output tube of the second clipper and ground, a direct connection between said last-mentioned anode and the control grid of the cathode follower, and means connected to the cathode resistor of the cathode follower for obtaining an output voltage, said second clipper and the cathode follower being so biased that the cathode follower is cut olf when the output tube of the second clipper is conducting and the output tube of the second clipper is cut off when the cathode follower is conducting` 2. The apparatus of claim l including a resistor connected between the control grid of said cathode follower and a point of positive potential to supply a positive bias for said control grid and thereby permit an increased swing of the cathode follower.

3. The apparatus of claim l including an attenuator connected across at least a portion of the cathode resistor of the cathode follower and including a two-section ganged multi-position switch, a shield separating the two switch sections from each other, the movable contact of one section of the switch being connected to one of the output terminals and the other output terminal being connected to the shield, some only of the stationary switch contacts corresponding to the movable contact 'of said one switch section being connected to the movable contact of the other switch section and the other stationary contacts of said one switch section being connected together by resistors and connected through said resistors to the shield, some only of the stationary contacts of the other switch section being connected together through resistors and through said resistors and the resistors of said one switch section to the shield, said stationary contacts of the other switch section and said first-mentioned stationary contacts of said one switch section being contacted by the respective movable contacts in corresponding positions of the movable contacts, and the rest of the stationary contacts of said other switch section being connected to said shield, whereby the inherent capacity between the output terminals is minimized to decrease distortion of the output signal.

4. The apparatus of claim 3 in which said resistors of said one and said other switch sections include n-l resistors of successively lower resistance connected successively between the side of said cathode resistor remote from ground and the shield, where n is the number of switch positions, and means connecting successive pairs of said some of the stationary contacts of the other switch section and said other of the stationary contacts of said one switch section successively across successively smaller resistors of said n-l resistors.

5. A shaping circuit for forming |an input voltage including an alternating and a positive direct component into ya square wave output, comprising a pair of cathodecoupled clippers each including a pair of vacuum tubes, a cathode follower vacuum tube having \a resistor in its cathode circuit across which the output is developed, means directly connecting the input voltage to the control grid of the input tube of the rst clipper, a capacitive coupling between the anode of the output tube of the first clipper and the `control grid of the input tube of the second clipper, means connecting the anodes of the second clipper to ground and the cathodes to a point of negative potential, said last-named means including a plate resistor fo-r the output tube of the l'second clipper, Aand `a direct connection between the anode of the output tube of the second clipper and the control grid of the cathode follower, the output tube of the second clipper and the cathode follower being so biased that the cathode follower is cut oif when the output tube of the second clipper is conducting and the output tube of the second clipper is cut ofi` when the cathode follower is conducting. i

6. The apparatus of claim 5 including a resistor connected between the control grid of the cathode follower and ia point of positive potential to bias said control grid positively and thereby to increase the possible voltage swing of the cathode follower.

7. The apparatus of claim 6 including a voltage divider connected between said point of positive potential and ground, and means connected to the control grid of the output tube of the irst clipper for adjustably biasing said control grid positively to control the symmetry of the square wave output.

8. A square Wave shaping circuit for forming an input voltage including an alternating and a positive direct cornponent into a square wave output comprising a pair of cathode-coupled clippers each including a pair of vacuum tubes each of which has an anode, cathode and grid and said clippers each having their cathodes connected together, -an output cathode follower having at least an anode, cathode `and control grid, a first and a second source of anode potential, said first source having its negative side grounded and its positive side connected to the anodes of said iirst clipper and the anode of said cathode follower, each of said clippers land said cathode follower having cathode resistors, the cathodes of the first clipper and of said cathode follower being connected to ground through their cathode resistors, said second source having its positive side grounded, the second Y clipper having a pair of anode resistors each connected directly between the respective anodes thereof and ground, the cathodes of the second clipper being connected through its cathode resistor to the negative side of the second source, means directly connecting the input voltage to the grid of the input tube of the first clipper, a capacitive connection between the anode of the output tube of the irst clipper and the grid of the input tube of the second clipper, the output tube of the first clipper having au anode resistor connected between its anode and the positive side of said first source, the anode of the output tube of said second clipper being directly connected to the grid of said cathode follower, the output tube of the second clipper and the cathode follower being so biased that the cathode follower is cut ofIr when the output tube of the second clipper is conducting and the output tube of the second clipper is cut ott when the cathode follower is conducting, said square wave output being available across the cathode resistor of said cathode follower.

9. The apparatus of claim 8 including a voltage divider comprising a potentiometer land at least one other resistor connected across said rst source, with the movable contact of the potentiometer connected to the grid of the output tube of the irst clipper, and a resistor connected between the grid of the cathode follower and the positive side of said iirst source to bias said grid positively with respect tothe cathode thereof.

10. The apparatus of claim 9I including a grid resistor connected between the grid of the output tube of the second clipper and ground, a large iltering capacitor connected across said grid resistor, and a grid resistor connected between the input tube of the second clipper and the grid of the output tube thereof.

1l. The apparatus of claim l0 including a bypass capacitor connected between the control grid of the output tube of the rst clipper and ground.

References Cited in the tile of this patent UNITED STATES PATENTS 2,332,811 Rankin Oct. 26, 1943 2,488,567 Stodola Nov. 22, 1949 2,519,057 Luck Aug. 15, 1950 2,617,023 Weller Nov. 4, 1952 2,821,629 Finkel et al. Jan. 28, 1958 2,863,048 Theall et al Dec. 2, 1958

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