US2444338A

US2444338A - Oscilloscope circuit

Info

Publication number: US2444338A
Application number: US597458A
Authority: US
Inventors: Thomas L Dimond
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1945-06-04
Filing date: 1945-06-04
Publication date: 1948-06-29
Anticipated expiration: 1965-06-29

Description

.Pune 29, 1948. r. DlMoND OSCILLOSCOPE CIRCUIT Filed 'June 4, 1945 INVENTOR T. L. D/Mo/vo BV www ATTORNEY Patented June 29, 1948 UNITED STATES PATENT OFFICE osoILLosooPE CIRCUIT Application June 4, 1945, Serial No. 597,458

4 Claims. (Cl. 315-26) f This invention relates to cathode ray oscilloscopes of the type in which two signals are viewed substantially simultaneously in displaced relation on the same screen.

't`The object of the invention is an oscilloscope of this type which is capable of operating satisfactorily on very high frequency signals.

It is common practice in the oscilloscope art t'o use a single cathode ray tube for Viewing two different signals simultaneously by connecting the vertical deecting plates of the tube to the two signal sources alternately in cyclic succession, as for example by means of an electron switch. In these prior systems the patterns are separated in Various ways some of which involve `the use of extra tubes which increase the cost and complexity of the circuit. In other cases the operation of the beam shifting means produces transient effects which retard the motion of the beam and make the system unsuitable for high frequency applications.

According to this invention the beam is shifted` from one position to the other, in synchronism with the switching between the two signal sources, by changing the grid bias on one of the tubes associated with the deection plates of the oscilloscope. This change in bias is made in a portion of the circuit where there are no capaci- #ties large enough to introduce a time constant which would appreciably retard the changes in .static grid potential produced by the switching operation.

It is well understood in the art that the deflecting plates are preferably excited from a `balanced circuit which may be of any suitable type such as the cathode phase inverter described on page 118 of Time Bases by Puckle, wpublished 1943 by Chapman and Hall, Limited, of' London. The present invention is readily applicableto a system of that type since the dif- :ferent biases required may be applied to the grid of the inverter tube which is normally conduc- `tively connected to ground.

' VA; further advantage of shifting the beam in #this particular manner is that when the oscilloscope is energized continuously from either one of the `signal sources, the signal pattern pro- `duced by that source appears and remains in the 'same position as it occupies when both signals are .being observed.

f' In the drawing, Fig. 1 shows an oscilloscope "according to the invention and Fig. 2 is a series of wave forms showing the voltage variations in diierent parts ofthe circuit.

"="In Fig'. 1 the conventional cathode ray tube I projects a beam from the cathode 2 to the screen 3 between the

horizontal deecting plates

4, 4 and the

vertical deilecting plates

5, 5. A sweep voltage wave 6 (Fig. 2) from the multivibrator `I is applied to the horizontal plates through the usual amplifier 8 and the retrace is blanked out by blanking pulses `Si, 9 (Fig. 2) applied from the multivibrator to the grid I0 of the tube at appropriate times over the conductor II, all in the well-known manner.

The signals to be observed are transmitted from the sources A and B through condensers I2 and I3 to the grids I4 and I5 of the tubes I6 and I1, respectively. These tubes function as an electron switch for transmitting signal pulses alternately from the sources A and B in cyclic sequence in the usual manner. For this purpose a conventional square wave generator 44 controlled by the multivibrator I over the conductor 45 generates an output wave I 8 which varies at the sweep frequency as indicated in Fig. 2. This square wave is applied directly to the grid I5 over conductor I9 and through a phase reversing tube 20 and conductor 2I to the grid I4 which is then subjected to potential Variations according to the wave 22. It is then clear that with the waves I8 and 22 of suitable amplitudes the tubes I6 and I'I will conduct on alternate pulses of the wave I8 and will be biased to cut-off by the other alternate negative pulses of the respective waves.

In this system it is assumed that the outputs of the sources A and B are modulated carrier waves of some high frequency such as 455 kilocycles per second and that the plate circuits of tubes I6 and I'I are energized from the power supply through a network 23 which is sharply anti-resonant at the carrier frequency so as to transmit the modulated signals at high efciency and at the same time attenuate other extraneous frequencies.

The output circuits oi the tubes I6 and I'I are coupled through a condenser 24 to a diode detector 25 and associated voltage dividing resistor 26 so that the demodulated signals are applied at suitable level to the grid 21 of an amplifying tube 28. Since the application of the square wave pulses to the tubes I6 and I'I produces transient disturbances which would otherwise obscure the signal patterns to .be observed on the screen 3, the circuit constants must b e such that these transients have effectively disappeared during the retrace of the beam under the control of the blanking pulses 9, `9. The output circuit of the tube 28 is coupled to the input 3 circuit of tube 29 by a condenser 30 and the plate of the tube 23 is connected through the plate resistor 3| to a point of suitable positive potential in the power supply as indicated. The grid 32 of the tube 29 is connected to ground through resistor 33 and the potentiometer 34. Positive potential is applied to the grid 29 through resistor 35 and the arm 3E of the po tentionieter is connected through a resistor 55 to a source of potential negative with respect to ground. With resistors 35 and 55 of the proper values the static potential of grid 32 of the tube 29 can be made either positive or negative with respect to ground to position the beam on the screen 3 of the tube I.

The tubes 29 and 3.1 are connected as a cathode phase inverter in which the signal potentials are applied to grid 32 as stated above and the normally balanced output across the plate resistors 38 and 39 is applied to the deiiection plates 5, 5 'as shown. The cathodes of the tubes 29 and 31 are connected to ground through the common resistor 40 and to the source of negative potential through a variable resistor 4I for adjusting the plate to cathode voltage of the tubes to the proper value.

In order to separate the signal traces from each other on the screen the grid 42 of the inverter tube 31 is connected to an adjustable point on the resistor 43 which forms a conductive path from the plate of tube 20 to ground as shown. Plate voltage for the tube 20 is supplied by connecting the cathode 4B to a Ipoint A41 in the power supply which is at a suitable negative potential with respect to the grounded plate. When the potential from the square wave generator 44 is negative, the grid 48, which is connected through resistor 49 to the negative terminal of the power supply, biases the tube to cut-off and the grid 42 of tube 31 is therefore at ground potential.

In operation, the zero potential pulses of the output wave I8 of the square wave generator 44 will unblock the tubes I1 and 20 thereby connecting signal source B to the cathode ray tube and driving the plate of vtube 20 negative with respect to ground due to the potential drop produced by the plate current in the resistor 43. This drop in plate potential biases the tube I6 Vto cut-off thereby eiectively disconnecting the signal source A from the cathode ray tube and at the same time driving the grid 42 of the inverter tube 31 to a suitable negative potential with respect to ground, as determined by the setting of contact 50.

This change in bias on the follower tube grid reduces the plate current of tube 31 with the result that there is a corresponding increase in the .plate current of tube 29 due to the cathode follower action of the circuit, The potential of the plate of tube 31 is therefore increased and the potential of the plate of tube 29 is decreased with the result that the cathode beam is shifted toward the deflection plate connected to the tube Upon the negative pulses of the wave I3 from the

generator

4,4 the tubes I1 and 20 are biased to cut-oil?, the signal souIce`B is therefore eiectively disconnected from the circuit and the corresponding positive pulses at the grid I4 of tube I6 due to the return of the plate of tube 20 to ground potential, will unblock the tube I6 and connect signal source A to the circuit.

The signals I, 5I from the source B will therefore appear on the screen 3 during the positive pulses of the wave IB and the

signals

52, 52 from the source A will appear during the positive pulses of the wave 22 as shown in Fig. 2. These two sets of pulses will be in vertically displaced relation, as indicated, by a distance proportional to the change in bias produced on the grid 42 by the action of the tube 20. It will be noted that since the changes in bias on the grid 42 are obtained from the circuit of the tube l2l), which is required in any case for phase reversing purposes, no additional circuit elements are required. While the grid by-pass condenser 55 of tube I6 is in shunt to the resistor 43 of the phase reversing tube, this capacity is so small that it introduces no appreciable delay in the change of bias on the grid 42 from one value to the other and these changes occur almost instantaneously.

If it is desired to view only the signals from one of the sources, the switch 53 is operated to disconnect conductor I9 from the square wave generator 44 thereby stopping the cyclic operation of the electron switch. To view only signals from' source A the switch is moved to an intenmediate open position, such as on the contact 54, in which case conductor I9 and the grids I5 and 43 are biased negatively to B potential and the tubes I1 and 2u are cut oi. The grid I4 of the tube I6 and grid ,42 of the tube 31 are then at ground potential and signals from the source A will be displayed in the normal manner with the beam in its down position as indicated by the pulses 52 of Fig. 2. When the switch 53 is moved to the grounded contact 51, the tubes 20 and I1 are then continuously in the `same condition as when under the control of the positive pulses of the wave I8 from generator 44. That is to say, the blocking bias on tube 20 is reduced by the drop in potential in resistor 49 suiiiciently to produce current in resistor 43. This potential negatively biases grid 42 with respect to ground to shift the cathode beam to its upper position and blocks the tube I6 to cut off the signals from source A. At the same time grid I5 of tube I1 is raised to ground potential thereby unblocking this tube and .permitting the signals 5I to appear on the screen in their usual position so that there is no possibility of confusion as tothe source from which the observed signal is derived.

The invention has been described in connection with a cathode phase inverter type of circuit which is preferred because of the simplicity with which the beam may be shifted in such a circuit, but the same fast action can be obtained also in systems using output stages having both the input and output circuits connected in push-pull. If, for example, the push-pull connected tubes are of a type having two control grids, the signals may be applied in push-pull to one control grid of each tube and the static biases on the other control grids may be switched in the same general manner as in the circuit shown. It will be noted, however, that in such a cir-cuit the two biases must be changed in opposite senses but this can be effected very easily by deriving the second bias voltage from a voltage divider connected to ground from the output lead I9 of the square wave generator 44.

What is claimed is:

1. In an oscilloscope the combination with a cathode ray tube, two signal sources, an ampliiier having an input circuit and two vacuum tubes having output circuits connected in push-pull to the cathode ray tube, and switching means for successively connecting the signal sources to the amplifier input circuit in cyclic sequence, of a. grid in each of 'the vacuum tubes, circuits for biasing the grids to position the beam of the cathode ray tube, at least one of the biasing circuits including stray capacities only and means in at least said one circuit for changing the bias on the grid in synchronsrn with the operation of the switching means to separate the patterns of the signals from the two sources.

2. In an oscilloscope the combination with a cathode ray tube, two signal sources, an amplifier including two vacuum tubes having plate, grid and cathode electrodes connected to form a cathode phase inverter with an input circuit and a balanced output circuit connected to one pair of deflecting plates in the cathode ray tube, and switching means for alternately connecting the signal sources to the input circuit, or" means for determining the static bias between the grid and cathode of the inverter tube and means for adjusting the bias between two values in synchronism with the operation of the switching means to separate the reproductions by the cathode ray tube of the signals from the two sources.

3. In an oscilloscope a cathode ray tube, two signal sources, an amplifier having an input circuit, and output circuit terminals connected to the cathode ray tube, electronic switching means for successively connecting the signal sources to the input circuit in cyclic sequence, means for generating sweep voltage for the cathode ray tube, square wave generating means controlled by the sweep voltage generator for operating the switching means, and a circuit including stray capacities only between the square wave generating means and the amplifier for cyclically changing the average relative potentials of the output terminals in synchronism with the operation of the electronic switching means to separate the patterns produced by the signals from the two sources.

4. In an oscilloscope, a cathode ray tube, two signal sources, an amplifier including two tubes each having grid, cathode and plate electrodes cathode-coupled to form a phase inverter with the plate electrodes connected to the cathode ray tube, electronic switching means for connecting the two sources to one of the grids in cyclic sequence, a source oi" control waves for operating the switching means and a path including stray capacities only, extending from the control wave source to the other grid for cyclically changing its potential to separate the signal patterns produced on the cathode ray tube by the signals from the two sources.

THOMAS L. DIMOND.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date Re. 22,672 Shumard Aug. 28, 1945 2,208,254 Geohegan July 16, 1940 2,221,115 Shepard, Jr NOV. 12, 1940 2,300,999 Williams Nov. 3, 1942