US2938140A - Time base circuits - Google Patents

Description

3 Sheets-Sheet 1 Filed Oct. 31, 1958 MTBG COM

INVENTOR FRANCIS WALTER COOK.

BY (\QWMM ATTORNEY May 24, 1960 F. w. COOK 2,938,140

TIME BASE CIRCUITS Filed Oct. 31, 1958 3 Sheets-Sheet 2 SYNC.PULSE TIMEBASE WAVEFORM w PuLsEsHAPrfl Mmfi MAIN AND I TIMEBASE TIMEBASE AMPLITUDE GENERATOR AMPLIFIER I LIMITER I 14 a ADJUSTABLE Y ATTENIJATOR AMPLIFIER STROBE 17- MAGNIFIED PULSE\ TIMEBASE mm TIMEBASE \I/WAVEFORM INVENTOR few/vow M175? 6004 ATTORNEY TIME BASE CIRCUITS Francis Walter Cook, Bridgnorth, England, assignor to A. T. & E. (Bridgnorth) Limited, Bridgnorth, England, a British company Y Filed Oct. 31, 1 9 58, Ser. No. 771,079

Claims priority, application Great Britain Nov.- 9, 1957 2 Claims. c1. 315-9 The present invention relates to indicating arrangements and is more particularly concerned with arrangenients which use a cathode raytube having a linear time base'for observing recurrent phenomena. 7

'Frequently the duration of the phenomena to be observed is short compared withlthe time elapsing between consecutive phenomena. Such, is the case, for instance, in radar systems and also in many vibratory systems which may be of a hydraulic or pneumatic nature.

It is frequently desirable to'examine the phenomena with considerable accuracy and in radar systems using linear time bases it has been proposed to display a selected portion of the time base on a magnified time base on a second cathode ray tube, for instance, to obtain greater accuracy in range determination. In such systems, the triggering of the time base generator for the second cathode ray tube is controlled by the selection of appropriate calibration pulses which are usually available in such systems. I

However, in examining, for instance, transient effects present in a vibratory system, calibration pulses are not normally available and one possibility is to provide a magnified time base by arranging that the time base generator is triggered by the leading edge of, the transient waveform. The transient waveform is also passed through a suitable delay device the output of which is applied to the Y-deflection plates of the cathode ray tube. It is arranged that the time base waveform is short compared with the time between successive transients so that a magnified display of the transientwill be obtained.

One disadvantage of this method is due to the fact that the use of a delay device may lead to distortion of the transient waveform. A further disadvantage is due to the fact that the arrangement is not applicable to low recurrence frequencies since the construction of a suitable delay device then becomes an expensive matter.

An alternative possibility is one in which the time base is synchronised with the transient waveform at a frequency which is half the recurrence frequency of the transient waveform; Alternate transient waveforms are thus displayed and if the amplitude of the time base waveform is increased so that the trace is considerably greater than the diameter of the catho'de ray tube, the transient waveform is expanded in the direction of the time base and can be observed in greater detail. If the expansion ,of the time base waveform isnot symmetrical it may 1 be necessary to employ a D.C. shift control to bring the transient waveform back onto the screen. This method. does not suffer fromthe disadvantages mentioned for the previous arrangement but is not entirely satisfactory due to the fact that it is usually difiicult to maintain steady synchronisation, so that a steady display of the transient waveforms is difiicult to obtain. There is also a further disadvantage that since alternate waveforms; only areobserved, the illumination in the trace is decreased by half. This is of importance since the 1 United States Patent O ice 2 writing speed is very high owing to the amplitude of the time base waveform.

A problem of a similar nature occurs where it is ,dje

sired to examine individual parts of a waveform, the individual parts having a duration which is short confrpared with the duration of the entire waveform. problem may arise, for example, in the examination or telegraph waveforms, television waveforms and the wave forms obtained from some types of vibratory systems.

In examining a small part of a completewaveform there are again two alternative procedures which may be adopted. In the first, the time base generator is trig gered by the commencement of the waveform which is fed through a delay line as previously explained and is displayed on a short time base. Such an arrangement is satisfactory for examining parts of, the waveform which occur shortly after its commencement but if it is desired to examine parts of the waveform which are delayed for a considerable period after its commencement it will then be necessary to use along duration time base such that the trace is very much greaterthan the diameter of the screen and then to apply a D.C. shift control to the X-defiection' plates of the cathode ray tube so that the appropriate part of the waveform is brought back on to the screen of the tube. If the expansion of the time base waveform is symmetrical, it

may not be necessary to use the D.C.shift control de-.

pending on the position along the time base of the ertion of the waveform it is desired to examine. However, this arrangement is also unsuitable for use at-lowjfre= quencies and further the amplification of the time base waveform which is necessary is likely to introduce an at a frequency different from the required recurrence frequency. p a

The object of the present invention s to provide an improved indicating arrangement which is more flexible than the known arrangements and which providesin a simple manner a magnified time base to facilitate the examination of transients or parts of recurrent waveforms; According to the present invention, a magnified time base generator is controlled jointly by the main time base waveform and a DC. potential which is adjustable; according to the position on the main time base at which" the magnified time base generator is to be triggered.

Preferably the D.C. potential is provided by an ac curate potentiometer, the range of potentials supplied by the potentiometer being equal to the potential swingof the main time base waveform. v

In the preferred embodiment of the invention,,the'main' time base waveform and the potentialare applied to a circuit which develops a triggering pulse for application to the magnified time base generator when the instantaneous potential obtained from the main time base waveform is equal to the potential obtained from the potentiometer. I

The invention will be better understood from the. following description of a preferred embodiment taken inconjunction with the accompanying drawings comprising Patented May 24, 1960 increase in the Figs. 3A and 3B show the components of the indicating arrangement also in block schematic form, and

Figs. 4 and 4A show the circuit diagram of the gate circuit or comparator, together with the magnified time base generator.

Referring first to Figs. 3A and 3B, the indicating arrangement includes two cathode ray tubes CRTl and CRT2, the complete waveform being displayed on tube CRTI while the magnified portion of the complete waveform is displayed on tube CRT2. Preferably the screen of tube CRT2 is larger than that of tube CRT1. The arrangements for generating the timebase waveform for application to the X-deflection plates of tube CRTl are conventional and consist of a pulse shaper and amplitude limiter 11 to which a synchronising pulse is applied from terminal 10, a main timebase generator 12 and a timebase amplifier 13 which delivers a push-pull output to the X-deflection plates. Similarly the magnified timebase waveform for application to the X-deflection plates of tube CRTZ is generated by the magnified timebase generator 18 and the magnified timebase amplifier 19 which delivers the push-pull output to the X-defiection plates. The magnified timebase generator is triggered from the gate circuit or comparator 17 in a manner to be described subsequently. The waveform which is to be examined is applied to terminal 14 and after passing through an adjustable attenuator 15 is fed to the Y amplifier 16 which provides push-pull output for application in parallel to the Y-deflection plates of both the tubes CRT 1 and CRT2.

The output from the main timebase generator is also connected directly to one end of a potential divider consisting of resistors R1 and R2 in series. The resistor R2 is also connected to the slider of an accurate linear potentiometer P1 which is connected between earth and the negative H.T. supply. The junction of resistsors R1 and R2 is connected to the gate or comparator circuit 17 which, together with the magnified timebase generator 18, is shown in detail in Figs. 4A and 4B.

Referring now to Fig. 4A the potential divider consisting of resistors R1 and R2 and also the potentiometer P1 consisting of resistors R3, R4 and R are again shown, the junction of resistors R1 and R2 being connected to the control grid of the thermionic tube VT1. The thermionic tubes VT1, VT2 and VT3 form a DC. coupled amplifier, tubes VT1 and VT3 being normally conducting and taking grid current while tube VT2 is normally cut off. The values of the resistors R1 and R2 are sufiiciently high to limit the grid current flow in tube VT1. The output from tube VT3- is connected to the control grid of the gas-filled tetrode VK1 which will normally be non-conducting so that the capacitor C2 will be charged. The tube VK1 is of the type known by the registered trademark Thyratron. Tubes VT4 and VTS are connected to form a toggle circuit i.e. a circuit in which either one or the other of the tubes, but not both, is conducting at any one time. Normally tube VT4 is conducting. Tube VT6 (Fig. 4B) is a cathode follower stage terminating the gate or comparator circuit and tubes VT7 and VT8 form the magnified timebase generating circuit. This circuit is a variant of the wellknown Miller timebase circuit.

The gate or comparator operates in the following manner. When the main timebase voltage has fallen to a value which is equal and opposite to the potential obtaining at the slider of the potentiometer P1 (Fig. 4A), the potential applied to the control grid VT1 falls to zero and tube VT1 ceases to draw grid current and its anode potential starts to rise. This potential change is amplified by tubes VT2 and VT3 so that a rapid rise of potential occurs on the control grid of the gas discharge tube VKl. Tube VK1 thereupon conducts and capacitor C2 discharges through resistor R7. This produces a negative potential on the control grid of tube VT4 which triggers the toggle circuit to the condition in which VT4 is cut off and VTS conducts. As is well known this changeover of the toggle circuit occurs rapidly and consequently a positive steep-fronted waveform is applied from the anode of VT4 through the cathode follower VT6 (Fig. 4B) to the suppressor grid of tube VT7. This application of the positive waveform to the suppressor grid of tube VT7 initiates the run-down operation of the Miller timebase circuit and the magnified timebase Waveform obtained from the cathode of tube VT8 is applied to the timebase amplifier over terminal 20.

Before the onset of the positive potential at the suppressor grid of the tube VT7, its anode current is held at a very low level by a negative bias applied to the suppressor grid from the potential divider consisting of resistors R8 and R10 (Fig. 4A) through the cathode follower VT6 (Fig. 4B). The space current of tube VT7 thus flows mainly to the screen grid through R11. When the negative bias applied to the suppressor grid of tube VT7 is removed, current flows to the anode thus reducing the flow to the screen grid causing a potential increase which is applied to the control grid of tube VTS. At the end of the magnified timebase sweep, the space current of tube VT7 is again transferred to the screen grid causing a negative'potential to be applied to the grid of tube VT5 (Fig. 4A). The circuit consisting of tubes VT4 and VTS is thus returned to its initial condition since the control grid of tube VT4 is raised to a positive potential by the potential divider consisting of resistors R9 and R7. The gas-filled tube VK1 is still conductive but with a very low anode potential. This condition is maintained until the termination of the main timebase sweep when the control grid of tube VK1 is driven negative. The settings of the potentiometer P1 and the strobe ratio control R12 (Fig. 4B) can be such that the termination of the magnified timebase sweep occurs after the termination of the main timebase sweep. When this condition obtains the positive fiy-back of the main timebase causes the control grid of tube VT1 (Fig. 4A) to be driven positive. Thus, the DC. amplifier consisting of tubes VT1, VT2 and VT3 is returned to its initial condition applying a negative bias to the control grid of tube VK1. Tube VK1 de-ionises and the capacitor C2 charges through resistors R7 and R9 causing a positive potential to be applied to the control grid of tube VT4. The anode potential of tube VT4 falls causing the suppressor grid of tube VT7 (Fig. 413) to be biased to a negative potential thus terminating the magnified timebase sweep.

As shown in Fig. 3A, a brightening pulse is obtained from the magnified timebase generator and is applied to the grid of the cathode ray tube CRT1. This brightening pulse has the same duration as the magnified time base waveform and serves to brighten that portion of the trace on the screen of tube CRTl which is displayed on tube CRT2. The derivation of the brightening pulse has not been shown in Figs. 4A and 4B but it could be obtained from the anode of tube VT4.

As regards the practical details of the circuit, the resistors R1 and R2 (Fig. 4A) should be of great stability and their nominal value should be accurate. Resistors R3 and R5 of the potentiometer P1 are variable to enable the limits of the magnified timebase to be varied and resistor R4 is varied to enable the point on the main timebase waveform at which the magnified timebase waveform begins to be adjusted. The DC. coupled amplifier has been designed to provide a gain of 5000 and assuming a 5 volt change on the control grid of tube VK1 to be sufiicient to cause the tube to conduct, then the potential change at the control grid of tube VT1 necessary to produce such a change at the grid of tube VK1 would be 0.001 volt. The swing of the main timebase waveform is 200 volts so that the comparator will gate the main timebase waveform with an accuracy cathode ray tubes, this is not essential.

of 0.0005%. In actual fact the accuracy is normally limited only by the linearity of the potentiometer P1.

The duration of the magnified timebase waveform is determined by the value of the capacitor C3 (Fig. 4B), the resistor R13, the variable resistor R12 and the potential at the slider of the preset fine control potentiometer P2 which consists of the three resistors R14, R15 and R16. The capacitor C3 must have different values for different durations of the main timebase. The preset fine control potentiometer- P2 is adjusted during manufacture so that the strobe ratio control, which is the duration of the magnified timebase relative to the duration of the main timebase, can be adjusted during operation to any ratio between l/l and 1/20.

Referring to the waveform diagrams of Figs. 1 and 2, Fig. 1A shows a typical complex waveform which may, for instance, result from the periodic switching of a hydraulic system. The waveform as a whole will be displayed on the screen of tube CRTl. If now it is required to examine the waveform 5 on the magnified timebase, the potentiometer P1 (Fig. 4A) will be adjusted to give a voltage which is equal and opposite to the j voltage V shown in Fig. 1B which is the main timebase waveform. Figs. 10 and 1D show the magnified timebase waveform and brightening pulse respectively which are generated by the circuit of Figs. 4A and 4B. By varying the position of the slider of the. potentiometer P1 any desired portion of the complex waveform can be madeto appear in magnified form on the screen of the tube CRT2.

v The waveforms shown in Fig. 2 illustrate the use of the invention for examining recurrent transient pulses and I in this case the recurrence frequency of the main timebase is adjusted to be half that of the reccurrence frequency of the transient pulses. Again the waveform of Fig. 2B is applied to. the comparator and the potentiometer P1 is adjusted so as to display the intermediate transient pulses along the magnified timebase. Figs. 2C and 2D show respectively the magnified timebase waveform and the brightening pulse.

The arrangement of Figs. 3A and 3B can also be employed to determine the time elapsing between, for instance, the initiation of the waveform S in Fig. 1A and the edge T. For this purpose the potentiometer is provided with a dial which is time calibrated. The dial is adjusted so that the start of the magnified timebase is made to coincide with the start of the waveform S and then with the start of the waveform T, the difference between the two dial readings giving the time delay between the start of the two waveforms.

Although the preferred embodiment employs two It will be apparent that if the strobe ratio control is adjusted so that the ratio ofthe magnified to the main timebase is 1/ 1, the complete signal waveform will be displayed on the screen of the tube CRT2. Adjustment of the potentiometer P1 and the strobe ratio control will then enable the operator to select any desired portion of the complete signal waveform for display on tube CRT2. The use of two tubes is however to be prepared since the brightening pulse facilities the selection of the desired portion of the complete signal waveform.

It will therefore be seen that the present invention provides a very simple method of obtaining a magnified timebase and it is applicable to any range of frequencies subject only to the limitations of the band pass characteristics of the amplifiers feeding the Y-deflection plates of the cathode ray tubes. For low frequency operation, the performance of the apparatus can be made satisfactory by employing a cathode ray tube whose screen has a long afterglow characteristic.

It will also be understood that the invention is not limited to the particular method described of comparing the DC potential with the main timebase waveform. While the null method described is preferred, other methods may be employed.

Iclaim:

1. An indicating arrangement for the observation of source of direct potential feeding said potentiometer,

means to which the main timebase waveform and the direct potential from said potentiometer are applied and responsive when the instantaneous potential of the main timebase Waveform is equal to the direct potential to trigger said magnified timebase generator to generate a magnified timebase waveform for application to the second deflecting means of said second cathode ray tube and means for adjusting said potentiometer to vary the direct potential obtained therefrom whereby the, position of the trace on said tiecond cathode ray tube relative to the trace on said first cathode ray tube is capable of variation.

2. An indicating arrangement as claimed in claim 1.

wherein a dial provided for adjusting said potentiometer is time calibrated to enable the time elapsing between two points on the trace on said first cathode ray tube to be determined.

References Cited in the file of this patent UNITED STATES PATENTS 2,443,634 Morgan June 22, 1948 2,454,782 La Rosa Nov. 30, 1948 2,703,401 La Rosa Mar. 1, 1955 2,731,583 Ellis Jan. 17, 1956 2,752,527 Ropiequet June 26, 1956 2,855,593 Gloess Oct. 7, 1958

Images