US2422069A - Automatic gain control for pulseecho systems - Google Patents

Description

June 10, 1947. A. v. BEDFORD 2,422,069

AUTOMATIC GAIN CONTROL FOR PULSE-ECHO SYSTEMS.

Filed Dec. 30, 1942 2 Sheets-Sheet 2 Patented June 10, 1947 UNITED STATES PATENT OFFICE AUTOMATIC GAIN CONTROLFUR PULSE- ECHG SYSTEMS Alda V. Bedford, Frinccton, NZ J., assi'gfior t6 Radili cfilpdl'afiiofi f Alfiiic'al, 2i, GOYfidfatibfi of Delaware Application December 30, 1942', Serialrio. 470,689

5 Claims.

My invention relates to pulse-echo systems and particularly to systems wherein a directiveantenna system having a plurality ofoverlappingdirectional patterns is mounted for movement to a position where it is directed at a target or other reflecting object. The invention will be described specifically as applied to a pulse-echo system in which the directive antenna system is utilized for both transmission and reception, although either the transmitter antenna or the receiver antenna alone may be directive if preferred.

lhe invention is applied to a Well known type of system in which the antenna radiation pat-' terns may be rotated in both a horizontal plane and a verticalplane to search for an object such as an enemy airplane. In this system four directiveantennas are switched in sequence whereby they radiate pulses in radiationpatterns displaced slightly up, to the right, down and to the left fromthe-meanaxis of the antenna assembly. In the-particular example, which will be described for the purpose of illustration, this switchingcycle occurs sixty times per second. Thus the received signal contains a step-like modulation- Withthe amplitude of each step depending upon theposition of the antenna radiation pattern with respect to the reflecting object.

It has been foundthat the received signal tends to fade or vary at a rather high frequency because of changes in the standing wave pattern and because of variations: in the reflecting efiiciency of the target as it changes position. These effects are especially noticeable when the target is an airplane which is twisting? and turning in response to maneuvering.- or because of rough air. Such fading makes it more diliicult to obtain proper operation of the several circuits to be controlled by the received pulses. The results o'f'the fading cannot be overcome by tneusc of a can'- ventional automatic gain control" circuit because if it is madesuffi ciently fast in its" response to eliminate t-nefading, it will tend to remove or distort the up; right, down, left "step" modulation which is' to be utilized for" control or" metcation purposes All object of the present" invention is lid pro-- vi'de: an improved object locating systemi A further object of the invention is to provide animproved method: of and means for providing automat-icgain control for aradio syst'emutilizing reflectedradio energy.

In accordance with the present invention the above-mentioned difliculty" in obtaining suitable automatic gain control is avoided by employing 2; an automatic gain control circuit which obtains all of its control voltage for high speed response during only one of the four-above-mentioned up, right, down, left intervals, for example during the up intervals only. The control voltage the following description: taken in connection with the accompanying drawing in which Figure 1 is a circuit diagram of one preferred embodiment of the invention; Figures and 3are circuit diagrams illustrating changes that may be made in the system of Fig. 1 in practicing other embodiments of the invention; Figure 4 is a graph showing: the" modulated radio frequency signal that is'appl-iedto the receiver; Figure 5 isa graph showing the signal: of Fig 4 after it is demodu lated by the second detector and then passed through a low-pass filter; and Figure 6 is a graph showing the automatic gain control or AVG voltage that is obtained by my improved circuit of Fig. In the several figures, similar parts are indicated-by similar reference characters.

Fig. 1 shows a radio pulse-echo system com: prising four directive antennas II); II', I2-, and I3 for radiating slightly upward, downward, to the right and to the left; respectively, with respect to a centrat common axis asindicated by the letters U, 1D,:R and L; The antenna system, which is indicated only schematically in the drawing, may consist of four directive antennas that may be keyed successively as described for example in application Serial No.- 259,057, filed February 28,

1939-, inthe name of Irving Wolff and entitled Object detection and location, or an antenna systern may be employed which is keyed-by means of shorting capacitors as described and claimed in application serial No. 412,943, filed September 30,1941, in the name of George H. Brown and entitled Antenna systems-r It will be understood that the radiation patterns forthese antennas preferably are overlapping conical patterns and that the antenna radiation is greatest at the center and weakens toward the edges of the cones.

Highfrequency radio pulses aresupplied successivel-y to the antennas In, H, l2 and l3from a transmitter 15- through a" rotating switch arm I 6- and associated commutator segments l1, l8,

l9 and 2|, respectively. The switch arm I6 is rotated by a motor 22 which is mechanically coupled thereto as indicated by the broken line. The radio pulses may be obtained by modulating the high frequency carrier wave of the transmitter I?) by means of keying pulses from a multivibrator 23 or the like supplying pulses having a recurring rate of about four kilocycles per second, for example.

The reflected signal is picked up by the di-' rective antenna system H), II, l2 and I3 and is;

supplied through the rotating switch arm |6..to. the first detector and signal channel selecting portion 24 of a superheterodyne receiver. If the antenna system is pointed to the right and above the target, the signal will have the step modulation as shown in Fig. 4. The intermediate frequency signal is amplified by I.-F. amplifier tubes 25 and 21 an'd is then supplied to a second detector comprising a diode 28 having the usual output resistor 29 shunted by a capacitor 3|. There will appear across the output resistor 29 the demodulated signal consisting of narrow l-kc. pulses having step-like variations in peak amplitudes corresponding to up, right, down, left. The second detector output will also include a direct current, component corresponding to average carrier and this component may be applied through a conductor 32 to an average carrier AVC circuit 33 for controlling the gain of the I.-F. amplifier in a conventional manner to eliminate comparatively slow variations in the signal strength. As previously stated, however, this AVC system cannot be made sufilciently fast operating to eliminate the effects of fast fading since such an adjustment would distort or elimihate the up, right, down, left modulation.

In accordance with the present invention the second detector output is applied to an amplifier tube 31 through a suitable filter 34-35 for filtering out the l-kc. pulses. The signal appearing on the grid of tube 37 is shown in Fig. 5. The output of amplifier tube 3'! is applied to a rotatable -switch arm 38 which makes contact periodically with a commutator segment 39. The switch arm 38 is driven by the motor 22 as indicated by broken line and is rotated in synchronism with'the switch arm l6 of the antenna switch. The two switch arms I6 and 38, in the particular example illustrated, are so phased that during the interval of transmission and reception ofthe up group of pulses a charging current is applied through the switch arm 38 and commutator segment 39 to a capacitor 4| that is connected in the input circuit of one or more I.-F amplifier tubes for controlling their gain in accordance with the charge on the capacitor 4|. In the circuit illustrated, the high side of capacitor Al is connected by lead it to supply a variable bias to the grid of the tube 21.

It will be seen that the capacitor 4| receives a charge only during the up interval and that it holds this charge until the next up interval occurs. At this time the charge of the condenser is changed immediately to a new value if there has been a change in the amplitude or voltage level of the signal during the up interval. This is illustrated by the graph of Fig. 6. 'Thus the gain of the I.F. amplifier tube 21 is controlled by the voltage level corresponding to one switching interval only (the up interval in this instance) and the time constant of thi s'gain control circuit may be made in effect fast enough to remove the effects of fast fading without removing the up, downright, left switching modu- 4 lation. The switching sequence for the gain control capacitor with respect to the second detector output is indicated by the legends in Fig. 5. The way in which the fast gain control circuit functions will be seen more clearly by comparing the four up, right, down, left switching cycles (shown in Figs. 4. and 5) with the gain control voltage shown in Fig. 6. The intervals U1, R1, D1, L1 and U2, R2, D2, L2 in the first and second switching cycles, respectively, contain signal of the same amplitude in the second cycle as in the first cycle since there has been no fading and since the position of the antenna with respect to the target has not changed. Therefore, the gain control or AVC voltage (Fig. 6) has not changed. The voltage levels in the intervals U3, R3, D3, L3 in the third switching cycle have the same relative amplitude as before, since the relative positions of antenna and target have not changed, but the amplitude of all pulses has decreased due to fading. Therefore, the signal during the up interval U3 applies less charging voltage to capacitor 4| and the negative polarity gain-control voltage drops to and holds at a lower value as indicated by the first step in the graph of Fig. 6. Thus the gain of the L-F. amplifier is increased. The signal during the intervals U4, R4, D4, L4 of the fourth switching cycle illustrate a case where the incoming signal has increased in strength because of fading and where the antenna position with respect to the target has changed. The increase in strength of the signal during the up interval U4 causes the AVG voltage to become more negative as shown by the second step in the graph of Fig. 6 whereby the gain of the I.-F. amplifier is decreased. As a result, in the ouput of the second detector, only slight variations will occur in the voltage level during the up interval (or in the voltage level during any other interval that. may be selected for use in providing AVC voltage). This will be seen by comparing the voltage levels during intervals U1, U2, U3 and U4 in Fig. 5, the amount of departure from equal amplitudes depending upon the fiatness of the AVG charac teristic. It will be apparent that violent fast fluctuations in the amplitude of received signals is thus prevented and that the control information or U, R, D, L modulation has not been lost since ineach switching cycle the relative amplitudes of the signal during the U, R, D and L intervals are not affected by the AVG circuit.

As indicated by the block 42, the second detector output may be supplied to range, train and elevation control or indicating circuits.

Figure 2 illustrates a different embodiment of the invention wherein the up pulses control only the relatively high frequency components of the automatic gain control signal, this being accomplished by including a high pass filter 46-41 in the circuit between the capacitor 4| and the input circuit of one or more of the I.-F. amplifier tubes such as tube 26 (Fig. 1). The extreme low frequency and direct current components of the automatic gain control signal are derived from all of the up, right, down, left pulses, these components being supplied through a low pass filter 4849 to the input circuits of one or more I.-F. amplifier'tubes. With this circuit, the average signal supplied to the control or indicator circuits 42, is not changed by the automatic gain control action to the same extent as in the firstdescribed circuit. Neither fast fading, slow fading nor slow changes in target position will cause the average value of the signal to change appreciably.

Figure 3 shows another embodiment of the invention in which a fast automatic gain control or AVC action is provided by the up pulses only and in which the low frequency AVC action is provided by all the groups of pulses. In this embodiment, separate amplifier tubes 5| and 52 are provided so that by biasing them differently one of them is made to respond to peak Values of the l-kc. pulses while the other is made to respond to the area of these pulses. The two resulting AVC signals may be combined and supplied to the input circuit of one or more of the I.-F. amplifier tubes for controlling the gain.

I claim as my invention:

1. An object locating system including means for receiving successivel in two directions. an automatic gain control circuit which includes a capacitor and means for charging said capacitor in accordance with the amplitude or intensity of the signal received from one of said directions only, means for controlling the gain of said receiving means in accordance with the charge on said ca acitor whereby there is obtained a fast gain control action, an additional automatic gain control circuit having a comparatively slow gain control action. said last circuit includingmeans for producing a gain control voltage which has a value which changes in accordance with the average amplitude of the signals received from both directions, and means for also controlling the gain of said receiving means in accordance with said last gain control voltage.

2. An object locating system comprising a transmitter for the transmission of radio waves to said object and a receiver for the reception of said radio waves after reflection from said object, an assembly of directive antennas having a pair of radiation patterns, means for switching said antennas successively to said system to radiate said waves in one radiation pattern during one switching interval and to radiate said waves in the other radiation pattern during another switching interval, said receiver including an amplifier for amplifying said radio waves, means for obtaining a signal having voltage levels which are a measure of the amplitudes of the reflected waves which are received during said switching intervals, respectively, an automatic gain control circuit including a capacitor for storing energy in accordance with one of said voltage levels, switching means operated in synchronism with said antenna switching for applying said signal to said storage capacitor only while it is at said one voltage level, and circuit connections for applying the voltage of said storage capacitor to said amplifier to control its gain.

3. .A pulse-echo system comprising a transmitter and a receiver for the transmission and reception of periodically recurring groups of radio pulses, an assembly of directive antennas having overlapping radiation patterns, each pattern including a group of said pulses, means for switching said antennas successively to said system, said receiver including an amplifier for amplifying said groups of pulses, means for obtaining a signal having voltage levels which are a measure of the amplitude of the two groups of reflected pulses, respectively, an automatic gain control circuit including a capacitor for storing energy in accordance with one of said voltage levels, switching means operated in synchronism with said antenna switching for applying said signal during said one voltage level only to said storage capacitor, and circuit connections for applying the voltage of said storage capacitor to" said amplifier to control its gain.

4. An object locating system comprising a transmitter and a receiver for the transmission of radio waves to said object and for the reception of said waves from said object after reflection, an assembly of directive antennas having a pair of radiation patterns in the horizontal plane and having a pair of radiation patterns in the vertical plane, means for switching said antennas successively to said system to radiate radio waves during successive switching intervals, means for obtaining a signal having two voltage levels which are a measure of the amplitude of the reflected waves during said switching intervals, respectively, in the horizontal plane for utilization in control or indication circuits, means for obtaining a signal having two voltage levels which are a measure of the amplitude of the reflected waves during said switching intervals, respectively, in the vertical plane for utilization in control or indication circuits, an automatic gain control circuit which includes a capacitor for storing energy in accordance with one of said voltage levels, switching means operated in synchronism with said antenna switching for applying one of said signals during only one of said four voltage levels to said storage capacitor, and circuit connections for applying the voltage of said storage capacitor to said amplifier to control its gain.

5. A pulse-echo system comprising a transmitter and a receiver for the transmission and reception of groups of radio pulses, an assembly of directive antennas having overlapping radiation patterns in the horizontal plane and having overlapping radiation patterns in the vertical plane, each pattern including a group of said pulses, means for switching said antennas successively to said system, means for obtaining two voltage pulses which are a measure of the am plitude of the two groups of reflected pulses, respectively, in the horizontal plane for utilization in control or indication circuits, means for 010- taining two voltage pulses which are a measure of the amplitude of the two groups of reflected pulses, respectively, in the vertical plane for utilization in control or indication circuits, an automatic gain control circuit which includes a capacitor for storin one of said four voltage pulses, switching means operated in synchronism with said antenna switching for applying only one of said four voltage pulses to said storage capacitor, and circuit connections for applying the voltage of said storage capacitor to said amplifier to control its gain.

ALDA V. BEDFORD.

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

UNITED STATES PATENTS Number Name Date 2,189,549 Hershberger Feb. 6, 1940 2,240,600 Appelgarth May 6, 1941

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