US 3174119 A
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March 16, 1965 R. J. JONES ETAL 3,174,119
MICROWAVE RECEIVER PROTECTIVE CIRCUIT Filed Aug. 29, 1962 5 f .4ax/L mer wm/sae 3- oz/Aafx'e r- ,el ,eraf/vf@ -nem/.ffwrTe-.c
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Syracuse, N.Y., assignors to the United States of America as represented by the Secretary of the Air Force Filed Aug. 29, 1962, Ser. No. 220,952 2 Claims. (Cl. S33- 17) It is the practice in radar systems for the transmitter and receiver to use the same antenna. In order to protect the receiver from damage during operation of the transmitter, a protective device called a duplexer or T-R network is used to couple the transmitter and receiver to the antenna. Nevertheless, in the case of powerful transmitters and microwave receivers having crystal mixers in their inputs, the leakage power from the duplexer may exceed the input capability of the receiver, making necessary an additional protective device or auxiliary T-R network between the duplexer and the receiver input.
The auxiliary T-R usually consists of a gas tube across the line which ionizes when `the power reaches a certain level. This places a short on the line which retlects the power back to the duplexer. In order to increase the sensitivity of this additional protective circuit, the gas tube is often placed in a high Q resonant cavity. In broad band protective devices of this type, two direct coupled cavities are usually employed with a gas tube in each. Since the cavities in the broad band circuit are necessarily low Q, the power leakage from the first cavity to the second after the first cavity tube has tired may not raise the voltage high enough to fire the tube of the second cavity but may still be great enough to damage the receiver.
The purpose of this invention is to provide a broad band T-R of the coupled cavity type whose effectiveness does not depend upon the firing of a gas tube in the second cavity. In accordance with the invention, the gas tube in the second cavity is replaced by a pair of oppositely poled variable reactance solid state diodes. Diodes of this type have a capacity across their terminals that varies with the applied voltage. With this arrangement the leakage from the first cavity into the second cavity, although insufficient to fire a gas tube, greatly increases the capacity of the diodes and detunes the cavity to such an extent that little power is passed to the receiver.
A more detailed description of the invention will be given with reference to the -accompanying drawings, in which:
FIG. 1 shows in block form the receiver protection scheme of a radar system, and
FIG. 2 illustrates an auxiliary T-R in accordance with the invention.
Referring to FIG. l, transmitter 1 supplies short high frequency high power pulses to antenna 2 through duplexer 3. During the intervals between pulses, echoes from reflecting objects illuminated by the radiation from antenna 2 are received by the same antenna and directed to the receiver 5 input through duplexer 3 and auxiliary T-R 4. The duplexer is responsive to the power level of the signal applied to it and operates in the presence of the high power level transmitted pulses to block the transmission of energy from the transmitter-antenna cir cuit to the receiver in order to protect the receiver from possible overload damage. Under the low power level conditions that exist during the reception of echoes, the duplexer permits the free iiow of energy from the antenna to the receiver. Circuits for performing the function of duplexer 3 are well known in the art.
Although receiver protection is the purpose of the duplexer, such devices are not completely eliective and some leakage power reaches the receiver during transmission. Microwave receivers which employ crystal mixers ICC in their inputs have very low signal power handling capability and an additional protective device, such as auxiliary T-R network 4, must be used to block the leakage power from the receiver input. These auxiliary protective circuits use a gaseous discharge tube in shunt to the transmission line which becomes conductive and short circuits the line when the voltage exceeds a certain level. To improve the sensitivity, the gas discharge tube is usually incorporated in a high Q resonant cavity to take advantage of the voltage gain inthe cavity. In broad band applications, however, it is customary to use two directly coupled low Q resonant cavities each incorporating a gas tube. Experience has shown, however, that, as already stated, the leakage power to the second cavity after the tube in the iirst cavity has tired may be insuiii cient to iire the tube in the second cavity because of the low Q of the cavity but still great enough to damage the receiver.
A broad band auxiliary T-R designed in accordance with the invention to overcome the above ditllculty is shown in FIG. 2. Power from duplexer 3 is applied over waveguide 6 and through coupling aperture 7 to low Q resonant cavity 8. This cavity incorporates a gas tube 9 having spaced electrodes 10 and 11 connected to points in the cavity having a high potential difference. This cavity is in turn coupled by aperture 12 to a second similar low Q cavity 13 which is coupled by aperture 14 and waveguide 15 to the receiver input. Cavity 13, however, differs from cavity 8 in that the gas tube 9 of cavity 8 is replaced by -a pair of oppositely poled variable reactance solid state diodes 16 and 17. Diodes of this type have a capacitance between their terminals that depends upon the voltage across the terminals. The cavity is designed so that it resonates at the transmitter frequency with the diode capacitances at their substantially zero voltage value, which is the situation when echo signals are being received. During transmission, however, the leakage power from cavity 8 in cavity 13 raises the voltage across diodes 16 and 17 enough to greatly increase their capacitances. This detunes cavity 13 to such a degree that little power is coupled to waveguide 15 and the receiver. Two oppositely poled diodes are used to insure detuning for both half cycles of the A.C. signal.
The protective circuit of FIG. 2 is more reliable and effective than in the case where a gas tube is employed in both cavities since its effectiveness is not dependent upon the tiring of a discharge tube in the second cavity.
1. A protective device for connection between the input of a microwave receiver and a source of radio frequency energy comprising a resonant cavity, a pair of similar solid state diodes of the voltage dependent variable reactance type in which the capacity between the diode terminals is dependent upon the voltage across the terminals, direct connections between the anode of one diode and the cathode of the other diode and one of two points in said cavity having a high radio frequency potential difference, and direct connections between the cathode of said one diode and the anode of said other diode and the other of said two points, said cavity being resonant at the frequency of said source when the capacitances of said diodes have their zero voltage values.
2. A broadband protective device for connection between the input of a microwave receiver and a source of radio frequency energy, said protective device comprising: a pair of directly coupled low Q resonant cavities connected in cascade between said source 'and said receiver input, a gaseous discharge device connected between points of high radio frequency potential difference in the cavity connected to said source, a pair of similar solid state diodes of the voltage dependent variable reactance type in which the capacity between the diode terminals is des, 1 74,1 1 9 3 i pendent upon the voltage across the terminals, direct con- References Cited by the Examiner nections between the anode of one diode and the cathode UNITED STATES PATENTS of the other diode and one of two points in the cavity connected to said receiver having a high radio frequency Re' 14585 1/19 Arnold n 322;171 potential difference, and direct lconnections between the 5 2'819425 1/58 Putt 333-13 cathode of Vsaid one `diode and the anode of said other 3067394 12/62 Llmmerman S33-17 diode and the other of said two points, said cavities being FOREIGN PATENTS resonant at the frequency of saidsource when said gaseous 877 040 9/61 Great B main discharge device is nonconductive and the capacities of said diodes have their zero voltage values. 10 HERMAN KARL SAALBACH, Primary Examiner.