US2698933A

US2698933A - Cathode-ray tube apparatus with partial direct current restoration

Info

Publication number: US2698933A
Application number: US186039A
Authority: US
Inventors: Charles E Moore
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1950-09-21
Filing date: 1950-09-21
Publication date: 1955-01-04
Anticipated expiration: 1972-01-04

Description

Jan- 4, 1955 c. E. MOORE 2,698,933 CATHODEI-RAY TUBE APPARATUS WITH PARTIAL DIRECT CURRENT RESTORATION Filed Sept. 21. 1950 4 Sheets-Sheet 1 hanna/VMM P0255 l 50u/Toor@ 1/ INVENTOR CharlafZ/NMI@ mf. im

ATTORN EY l Jan. 4, 1955 C. E. MOQrRE CATHGDE-RAY TUBE APPARA US WITH PARTIAL DIRECT CURRENT RESTORATION Filed Sept. 2l, 1950 Tic Jan. 4, 1955 c. E. MOORE 2,698,933

CATHODE-RAY TUBE APPARATUS WITH PARTIAL DIRECT CURRENT RESTORATION Filed Sept.. 21, 1950 4 Sheets-Sheet 3 0 I l l A D l /fmz fm@ 1 i i .5 @anx/5 .ZEHM CURRENT nl i V INVENTOR ATTO RN EY Jan. 4, 1955 C E MOORE 2,698,933

CATHODE-RAY TUE PPARATUS WITH PARTIAL DIRECT CURRENT RESTORATION Filed Sept. 2l, 1950 4 Sheets-Sheet 4 u/vz/wm/ Pz/Lff /ZZV T ifm( A El Z0 M/ff mam/a5. c

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(harlam ATTO R N EY United States 2,598,933 Patented Jan. 4, 1955 ice CA'E'HDE-RAY TUBE APPARATUS WITH PAR= T IAL DIRECT CURRENT RESTORATIN Application September 21, 1959, Serial No. 186,039

Claims. (Cl. 343-11) My invention relates to cathode ray tube indicators and pulse-echo radar systems and particularly to systems in which means is provided for switching to diierent range scales, such as from a twenty mile scale to a forty mile scale.

It has been found in practice that the picture intensity on the cathode ray indicator tube tends to change when the radar system is switched from one range scale to a different range scale. The reasons for this will be dis cussed hereinafter.

An obect of the present invention is to provide an improved method of and means for obtaining substantially uniform picture intensity in a radar system as the system is switched to different range scales.

Another object is to provide an improved method and means for providing substantially uniform image intensity on a cathode ray tube screen with variation of the cathode ray scanning speed.

A further obiect of the invention is to provide an improved plan-position-indicator (PPI) radar system of the type 1lllaving different range scales that may be selected at wi In practicing a preferred embodiment of the invention, the invention is applied to a circuit wherein an unblanking pulse is supplied to the cathode ray indicator tube through capacity coupling. According to said preferred embodiment, instead of operating the circuit either with no direct current restoration or with full direct current restoration, the circuit is operated with partial direct current restoration. ln this way, as will be apparent hereinafter, it is possible to avoid changes in picture intensity when switching from one range scale to another.

The invention Will be better understood from the following description taken in connection with the accompanviug drawing in which.:

Fig. 1 is a block and circuit diagram of a radar system 'l' of the plan-position-indicator (PPI) type embodying the invention;

Figs. 1A and 1B are circuit diagrams showing modications of a portion of the circuit shown in Fig. l;

Fig. 2 is a circuit and block diagram showing in more detail a portion of the system of Fig. l;

Figs. 3. 4 and 5 are graphs that are referred to in explainng the invention;

Fig. 6 is a group of graphs showing a cathode ray tube characteristic and showing unblanking pulses for two scale ranges as they would be applied to the cathode ray tube grid without any direct current restoration;

Figs. 7, 8 and 9 are further graphs that are referred to in' explaining the invention; and

Figs. l0, ll and l2 arev groups of graphs that illustrate for comparison purposes the three cases of unblanking pulses without D.C. restoration, with full D.C. restoration, and with partial D.C. restoration, respectively.

Fig. l shows a pulse-ocho radar system of the planposition-indicator or PPi type comprising a radio puise transmitter l@ which supplies` pulses of radio energy through a T-R box 1l to a directive antenna 12. Reected pulses are received by antenna l2 and passed through the T-R box to a pulse receiver 13. The re ceiver 13 dernodulates and amplities the signal and supplies it through a coupling capacitor 1d to the cathode Bti of a cathode ray indicatory tube 17. The TS1?. box may be any suitable duplexer.

The cathode ray4 tube is of the well known type having a long persistence phosphorescent screen i3 ,commonly employed in PPI radar systems. The tube further comprises the cathode 16, a control grid 19 and an anode 21.

A rotatable deecting yoke 20 is mounted on the tube 1S and provided with rings 22 and brushes 23 so that sawtooth deecting current may be supplied thereto from a sawtooth deection and unblanking circuit 24. Thel sawtooth waves are made to occur in synchronism with the transmitted pulses by means of trigger pulses supplied over leads 25 to the deilection circuit 24.

The deiccting yoke 20 is rotated in synchronism with.

the antenna 12 by any suitable means. In the example shown, this is accomplished by means of a motor 26, gearing 27, and mechanical drive connections. The de.- iiecting coil 20 makes one rotation for each rotation of the antenna 12.

It will be apparent that in operation the usual PPI deiiection pattern is produced, i. e. the electron beam is deflected radially from the center of the screen 18 eachtime a sawtooth wave is produced. One radial deflection occurs each time a radio pulse is transmitted. This occurs 1000 times per second in the present example. Meanwhile the antenna 12 and yoke 20 make about one rotation every three seconds.

The cathode ray tube 17 is biased substantially to beam cut-ofi between the radial deflections by a biasing Voltage indicated at C--. During each radial deection the cathode beam is unblocked by an unblanking pulse sup-- plied from the circuit 24 through a coupling capacitor 31. to the control grid 19.

Referring more specifically to the input circuit of the cathode ray tube, it will be seen that the reected pulse is impressed upon the cathode 16 by means ofthe coupling capacitor 14 and a cathode resistor 32.y The biasing voltage (C-) is applied to the grid 19 through

re sistors

33 and 34. v

Partial direct current restoration is provided by means of a diode 36 connected across the resistor 33 withv its cathode at the grid end of resistor 33. The diode 3d conducts following the positive portion ofthe unblanking pulse signal to put a charge on capacitor 31. This charge leaks oil slowly through resistors 33v and 34. .Full D.C. restoration, which is not desired, would be obtained if the diode 36 were connected across both of the

resistors

33 and 34, i. e., with the diode anode connected as shown and with the diode cathode connected to the grid 19.

With the partial D.C. restoration circuit illustrated, only part of the unblanking pulse Voltage is impressed across the diode 36 and thus only partial D.C. restoration is obtained. The desired percentage offull D.C. restoration may be obtained by selecting suitable relative values of the

resistors

33 and 34.

The diode 36 need not be connected eiactly as shown in FigAl. For example, it ma'y be connected as shown in Fig. l

Also, it will be apparent that the circuit of Fig. lll may be employed. Here the unblanking pulses are applied to the cathode of the cathode ray tube with negative polarity. Also, the diode connection is the reverse of that used in Fig. 1.

Before describing the circuit operation more completely reference will be made to Fig. 2 which shows one suitable circuit for the sawtooth deflection and unblanking circuit 24.

The trigger pulse from transmitter 10 is impressed upon a one-shot multivibrator comprising vacuum tubes 37 and 38. This multivibrator is of conventional design.

The switch S1 controls the unblanking pulse' width and the sweep rate. For one sweep rate the switch S connects the anode ofv tube 38 to the grid of tube 37 through capacitor C1. For the other two sweep rates the said anode-to-grid connection is through C2 and C5, respectively. The changes in sweep rates result, of course, from the fact that capacitors C1, C2 and C3 have different capacity values.

The positive unblanking pulse is' taken off` the anode circuit of the tube 37 as shown and applied over a conductor il and through capacitor 31 to the grid 19 ot' the cathode ray tubeV 17.

The sawtooth generating circuit may be any one of several well known circuits. in' the example shown, three capacitors 42 d3A and d4' of different capacity values are provided for the three range scales, respectively. A switch S2 switches in the correct capacitor for the desired range.

The selected one of the

capacitors

42, 43 and 44 is charged from a B-isource through a selected one of the

resistors

46, 47 and 48, the switch S3 and the resistor 49.

A diode 52 is connected to be normally conducting and to prevent the selected one of

capacitors

42, 43 and 44 from acquiring a charge until a positive pulse is put on the cathode of the diode 52. It will be seen that diode 52 is connected across the selected capacitor 43, in the example shown, by way of the cathode resistor 53 of the tube 38.

In operation, a positive pulse, corresponding to the unblanking pulse in time, is impressed upon the cathode of diode 52 at the instant a radio pulse is transmitted from the transmitter 10. This blocks the diode 52, the capacitor 43 begins to charge, and the useful sloping portion of a sawtooth wave appears thereacross. At the end of the positive pulse, diode 52 becomes conducting and discharges the capacitor 43. Thus, a sawtooth wave of the duration of the unblanking pulse is supplied to a sawtooth voltage amplifier 54 each time a radio pulse is transmitted. This time relation is illustrated in Fig. 3.

The

resistors

46, 47 and 48 and the switch S3 are not essential but preferably are provided for sweep length control.

The amplified sawtooth wave from amplifier 54 is applied to a sweep driver tube 56 which supplies a sawtooth current wave to the deflecting yoke 20.

Reference will now be made to the graphs of Figs. 4 to 12 for a more complete explanation of the partial D.C. restoration operation.

Figs. 4 and 5 show sawtooth waves for two different sweep rates or range scales together with the corresponding unblanking pulses. The point to note in these figures is that with respect to the alternating current axis the negative portion of the unblanking signal is less negative for the fast sweep (Fig. 5) than for the slower sweep.

Fig. 6 shows the grid volts-beam current characteristic of theA cathode ray tube. It also shows the relation between the unblanking pulses applied to the cathode ray tube grid and the tube characteristic for the case of no D.C. restoration. Unblanking pulses for both a fast sweep and for a slower sweep are shown for comparison purposes. It will be noted that the fast sweep unblanking pulses drive the cathode ray tube grid the more positive (i. e., less negative) so that the result would be a picture of greater intensity for the short range switch setting than for the longer range switch setting.

Referring next to Figs. l0, 11 and l2, these graphs areof the same kind as shown in Fig. 6 but for comparison purposes shown in Fig. no D.C. restoration, in Fig. 1l full D.- C. restoration and in Fig. 12 partial D.C. restoration. It is found that with full D.C. restoration as shown in Fig. 11, the picture intensity is less for the short range scales (faster sweeps) than for the longer range scales. This is because the screen is brightened less by a beam sweeping quickly across it than by a beam sweeping slowly across it.

Partial D.C. restoration as shown in Fig. 12 provides substantially uniform picture intensity for the several range scales. It will be seen that for the faster sweeps the unblanking pulses hold the grid of the cathode ray tube more positive than in the case of the slower sweeps, but not to the extent that exists in the case of no D.C. restoration.

Figs. 7, 8 and 9 illustrate a specific example of partial D.C. restoration for three different range scales in accordance with the present invention. ln these graphs the broken line A represents the A.C. axis and the broken line B represents the reference axis or base line for the partial D.C. restoration. The bottom of the graphs at C is the reference axis or base line for full restoration. The voltage values on the graphs indicate, by way of example, how far suitable partial D.C.. restoration departs from full D.C. restoration. It will be seen that in the example shown there is only alittle difference for the faster sweep but a substantial difference for the slower sweeps.

It should be noted that partial D.C. restoration 0P- eration requires less D.C. restoration than might be expected because on long ranges the pulse receiver is operated at higher gain and the general noise level thus produces an increase in intensity level on the longer ranges. more signal on the cathode 16 of the cathode ray tube 17 which causes more beam current to brighten the screen 1S.

From the foregoing it will be apparent that the present invention provides a simple and erfective way or maintaining the picture intensity constant as cathode ray tube apparatus such as a radar receiver is switched from one sweep rate or range scale to another.

What I claim to be my invention is:

l. in combination, a cathode ray tube having input electrodes and a phosphorescent screen, means for bias ing said tube substantially to beam current cut-ott', generator means for producing a sawtooth deflecting wave and for producing an unblanking pulse during each detlecting wave, means for deecting the cathode ray of said tube by said detlecting wave, means for applying4 said unblanking pulses through a capacitor coupling to one of said input electrodes to unblock the cathode ray tube during deflection of the cathode ray, range scale switching means for increasing the slope or" the sawtooth deflecting wave and for decreasing the duration of the unblanking pulse, and means for maintaining the picture intensity on said screen substantially constant as the range scale switching means is operated to provide different range scales, said last means including a direct current restorer circuit connected to the input electrode to which said unblanking pulses are applied, said restorer circuit comprising a diode and circuit means for applying only a portion of the unblanking pulse voltage to said diode whereby only a portion of the direct current component of the unblanking pulses is restored.

2.1n pulse-echo distance determining apparatus, a cathode ray indicator tube having input electrodes and a phosphorescent screen, means for biasing said tube substantially to beam current cut-ott", dellecting means for deflecting the cathode ray of said tube, generator means for supplying to said deecting means a sar/tooth deflecting wave, means for generating an unblanking pulse and for applying it through a capacitor coupling to one of said input electrodes to cause flow of beam current during the occurrence of said sawtooth detlecting wave, means comprising range scale switching for increasing the slope of the sawtooth dci'lecting Wave to thereby increase the speed of the cathode ray sweep and for decreasing the duration of the unblanking pt and means for maintaining the picture intensity on sind screen substantially constant when the apparatus is switched from one scale range to a diterent scale range, said last means including a direct current restoration circuit connected to the input electrode to which unblanking pulses are applied, said restoration circuit including a diode and circuit means for applying only a portion of the full unblanking pulse voltage to said diode whereby the direct current component of the unblanking pulses is only partially restored.

3. ln cathode ray tube apparatus wherein means is provided for switching from one sweep rate to a diierent sweep rate, a cathode ray indicator tube having input electrodes and a phosphorescent screen, means for proc ducing deilecting waves and for deliecting the cathode ray of said tube by said waves, means for producing unblanking pulses simultaneously with said deiiecting waves and for applying them to one of said input electrodes. and means for maintaining the picture intensity on said screen substantially the same for one sweep rate as for a different sweep rate, said last means including a direct current restorer circuit connected to said one input electrode, said restorer circuit including a rectifier and circuit means for only partially restoring the direct current component of said unblanking pulses by an amount such as to maintain said same picture intensity for said different sweep rates.

4. In a radar system of the plan-pcsinon-indicator type and wherein means is provided for switching from one range scale to a diierent range scale, a cathode ray indicator tube having input electrodes and a phos phorescent screen, means for producing deiiecting waves and for deflecting the cathode ray of said tube by said waves, means for producing unblanking pulses simultaneously with said deflecting waves and for applying ln this example, the receiver noise provides them to one of said input electrodes, and means for maintaining the picture intensity on said screen substantialiy the same for one range scale as for a different range scale, said last means including a direct current restorer circuit connected to said one input electrode, said restorer circuit including a rectier and circuit means for only partially restoring the direct current component of said unblanking pulses by an amount such as to maintain said same picture intensity for said different range scales.

5. In a puise-echo radar system of the plan-positionindicator type, means for radiating radio pulses from a directive rotating antenna, means for receiving said pulses after reflection, a cathode ray indicator tube having input eiectrodes and a phosphorescent screen, means for biasing said tube substantially to beam current cut-off, means for producing a sawtooth deecting Wave and for deflecting the cathode ray of said tube by said Wave radially from the center of said screen in synchronism with the pulse radiation, means for rotating said radial deflection in synchronism with said directive antenna, means for generating an unblanking puise and for applying it through a capacitor coupling to one of said input electrodes to cause iiow of beam current during the occurrence or". said sawtooth deflecting wave, means for applying said retiected pulses to said other input electrode, means comprising range scale switching for increasing the slope of the sawtooth deflecting Wave to thereby increase the speed of the cathode ray radial sweep and for decreasing the duration of the unblanking pulses, and means for maintaining the picture intensity on said screen substantially constant when the system is switched from one scale range to a diierent scale range, said last means including a direct current restoration circuit connected to the input electrode to which unblanking pulses are appied, said restoration circuitincluding a diode and circuit means for applying only a portion of the full unblanking pulse voltage to said diode whereby the direct current component of the unblanking pulses is only partially restored.

References Cited in the tile of this patent UNITED STATES PATENTS 2,414,486 Rieke Ian. 21, 1947 2,421,747 Engeihardt June 10, 1947 2,466,711 Kenyon Apr. 12, 1949 2,481,045 Schroeder Sept. 6, 1949 2,515,763 Downie July 18, 1950 2,525,103 Sprecher Oct. 10, 1950 2,525,106 Wendt Oct. 10, 1950 2,535,912 Frank Dec. 26, 1950 2,548,436 Loughren Apr. 10, 1951 FOREIGN PATENTS 510,715 Great Britain Aug. 4, 1939