US3803493A

US3803493A - Exciter frequency ability system

Info

Publication number: US3803493A
Application number: US00317320A
Authority: US
Inventors: T Barley; G Rast
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1972-12-21
Filing date: 1972-12-21
Publication date: 1974-04-09
Anticipated expiration: 1991-04-09

Abstract

An exciter frequency agility system is disclosed which provides broad separation of carrier sidebands, enhancing microwave system agility while eliminating the need for tunable filters. Preselected, precisely controlled frequencies are combined to provide a high megahertz stable oscillator frequency. This stable frequency is split and directed along three distinct channels for selected combination with an output signal of a variable high frequency generator. In combining the stable frequency with the generated variable frequency, the channels are isolated from adjacent channels and recombined to provide two synchronized output frequency ranges. One frequency is coupled to the receiver front end, the other is transmitted toward the target. The two frequency ranges or bands are maintained a fixed high frequency apart while being variable within their respective ranges. The two signals are combined when the transmitted energy is reflected back to the receiver. Frequency spacing is of such magnitude that the received reflected energy, when recombined, is easily filtered with a bandpass filter to obtain intelligence therefrom.

Description

United States Patent 1191 Barley et al.

[ 1 EXCITER FREQUENCY ABILITY SYSTEM [75] Inventors: Thomas A. Barley; Gustaf J. Rast,

Jr., both of Huntsville, Ala.

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

1221 Filed: Dec. 21, 1972 211 Appl. No.: 317,320

[52] U.S. Cl 325/444, 325/20, 331/22, 331/38, 343/175 [51] Int. Cl. H04b l/26, H04b l/30 [58] Field of Search 325/19, 20, 431, 432, 436, 325/442, 444; 331/2, 22, 37, 38; 343/l7.l R,

Primary ExaminerBenedict V. Safourek Assistant Examiner-Marc E. Bookbinder Attorney, Agent, or Firm-Edward J. Kelly; Herbert Berl; Jack W. Voigt 1 1 Apr. 9, 1974.

[57] ABSTRACT An exciter frequency agility system is disclosed which provides broad separation of carrier sidebands, enhancing microwave system agility while eliminating the need for tunable filters. Preselected, precisely controlled frequencies are combined to provide a high megahertz stable oscillator frequency. This stable frequency is split and directed along three distinct channels for selected combination with an output signal of a variable high frequency generator. ln combining the stable frequency with the generated variable frequency, the channels are isolated from adjacent channels and recombined to provide two synchronized output frequency ranges. One frequency is coupled to the receiver front end, the other is transmitted toward the target. The two frequency ranges or bands are maintained a fixed high frequency apart while being variable within their respective ranges. The two signals are combined when the transmitted energy is reflected back to the receiver. Frequency spacing is of such magnitude that the received reflected energy, when recombined, is easily filtered with a bandpass filter to obtain intelligence therefrom.

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06.3402 6 mwhzu w? OE EXCI'IER FREQUENCY ABILITY SYSTEM BACKGROUND OF THE INVENTION Frequency agility in a radar occurs when a radar changes transmission frequency rapidly. The ability to rapidly change frequency introduces tuning problems in transmitter and receiver filter circuits. At the higher frequencies (for example, 300 megahertz and up) there is little separation of intelligence carrying sidebands and carrier frequencies. When filtering or selecting one sideband, turning becomes difficult. A minimum shift in the transmitted signal can result in a substantial shift as seen by the finely tuned filters in the receiver, making problematical the selection of a desired sideband and suppression of the unused sideband. Thus, stringent and unrealistic demands upon filter design results in forcing selection of a filter which provides the best compromise.

SUMMARY OF THE INVENTION In the exciter frequency agility system, frequencies are selected and mixed for exclusive selection of the desired signal spectrum range. Undesired sidebands resulting from the mixing process are sufficiently displaced from the desired signal to be removed by conventional fixed tuned bandpass filters. The mixing and filtering process is arranged to allow a fixed frequency offset to exist between the transmitter and receiver local oscillator frequencies. By selective filtering, amplifying, and routing of the signals, the receiver and transmitter channels are segregated. A stable oscillator serves as a coherent oscillator source which is inserted into the respective channels. The respective channels are separated or offset by a predetermined fixed frequency difference obtained when the coherent oscillator frequency is mixed with other selected frequencies. The stable oscillator frequency is directed into a third channel and used as a local oscillator source for mixing with a selectable, programmable frequency source. The programmable mixed output is carefully isolated, split, and mixed with the respective transmitter and receiver channel frequencies. Therefore, the receiver and transmitter channels are always separated by a fixed high frequency band. This system allows sideband rejection with a fixed tuned broad band filter due to the separation, thereby eliminating any need for tunable filters.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a generalized block diagram of a preferred embodiment of the invention.

FIG. 2 is a single line schematic of the invention showing the changes in frequency through the system.

FIG. 3 is a block diagram of the up-converter network of FIG. 1

FIG. 4 is a block diagram of the directional circuit of FIG. 1.

FIG. 5 is a general block diagram of the isolation and amplifier network of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT In an exciter frequency agility system, high frequencies are selectively mixed to provide substantial spacing between interferring frequencies in the spectrum. This allows economical use of simple bandpass filters to pass the desired frequencies while blocking adjacent interference signals. No tunable filtering is needed to restrict unwanted signals or spurious radiation.

Referring now to the drawings wherein like numbers represent like parts in the several figures, FIG. 1 discloses apparatus representing a preferred embodiment of the exciter frequency agility system. In FIG. 1, an upconverter has respective fixed input frequencies f1, f2, f3 and f4 from a coherent oscillator source for selective mixing therein to provide output frequencies f7 and f8. Frequencies f7 and f8 are combined in coherent mixer 200 to provide a summed output f9 which functions as a stable coherent oscillator signal. F9 is coupled as an input to a signal direction circuit 300 which has plural output branches or channels for coupling coherent signal 19 to separate channels of an isolation and amplifier network 400. Isolation network 400 has an additional input of high frequency f4 coupled from the input of f4 to converter 100. A frequency synthesizer 410 or other high frequency generating means for providing a selectable or programmable radio frequency source provides an output band of frequencies around frequency fl0. This frequency band, referred to as 110, is capable of being swept rapidly between extremes for continuous control as is well known in the art. F10 is coupled as a variable input signal to isolation network 400. The sum and difference combination of respective input frequencies in isolation network 400 results in output frequencies f13 and fl4 from the exciter to using circuitry. In a radar having frequency agility, the frequency fl3, which varies with f.I0, is coupled to the receiver amplifier channel of the radar for combination with target reflected energy of fl4. Frequency fl4, also variable as fl0 varies, remains a fixed frequency apart from fl3 due to the mixing process and is directed to the radar transmitter intermediate power amplifier for radiation toward a target. When these two signals are combined in the receiver circuit, the difference frequency between the two is obtained for further processing.

In FIG. 2 a simplified single line diagram of the frequency agility system discloses the order of combination of the frequencies. In converter 100, f1 and f2 are combined in a mixer 120, producing the sum frequency f5 =fl +f2. Frequenciesf3 and f5 are then summed in a mixer 140, providing an output f6. Input frequency f4 is then summed with f6 in mixer 160, producing output f7. Frequency f4 is also coupled through a homodyne multiplier wherein thefrequency is mixed with itself to produce the fourth harmonic of f4 which is selected as f8. Operation of the homodyne multiplier is as disclosed in a copending application Ser. No. 289,025, filed Sept. 14, 1972 by Barley et al. and entitled Homodyne Multiplier. Frequencies f7 and 18 are summed in coherent mixer 200, providing the stable coherent oscillator frequency 19 which is coupled to director circuit 300. Directional couplers within the director circuit splitj9 into

channels

1, 2 and 3 for coupling into isolation and amplifier network 400. In channel 1, f9 is coupled to a mixer 430 where it is mixed with f4, one of the input frequencies of converter 100, producing a difference frequency output f9 f4 fl 1. In channel 3, f9 is coupled as an input to mixer 450 wherein it is summed with synthesizer output f10 to provide a varying output frequency band around fl2. The output signal, j12, is coupled into

mixers

470 and 490. In mixer 470, fl2 is summed-with'difference frequency fll to provide the summed output signal 114.

In channel 2, f9 is coupled to mixer 490 and summed with fl2 to provide output signal fl3.

FIG. 3 more particularly shows the circuitry of upconverter 100. The output of mixer 120 (I5) is serially connected through an attenuator pad 124, a fixedtuned bandpass filter 126, an amplifier 128 and another attenuator pad 124 to a first or high decibel (db) input of mixer 140. Attenuators 124, filter 126, and amplifier 128 form a typical fixed-tuned filter-amplifier circuit 130. Frequency f3 is coupled through an attenuator 132 to the second or low input of mixer 140. The output of mixer 140 (f6) is connected through a filteramplifier circuit 130 to the low input of mixer 160. Frequency f4 is connected through attenuator pad 134 and a fixed-tuned bandpass filter 136 to the input of a hybrid 150. Hybrid 150 splits f4 into separate paths, one output being connected through a filter-amplifier circuit 130 to the high input of mixer 160 and the other output being coupled to the input stage of an amplifier 152. The output of amplifier 152 is connected to a homodyne multiplier 180 which splits and mixes f4 with itself to produce an output spectrum therefrom. A filter-amplifier circuit 130 is connected to the output of multiplier 180 and selects f8 from adjacent harmonics. Frequency f8 is coupled with the low input of coherent mixer 200. The output of mixer 160 is connected through filter-amplifier circuit 130 to the high input of mixer 200.

Similar filter-amplifier circuits 130 in up-converter 100 are identical in function but may vary in components and the level of frequency response to provide predetermined db levels to the mixers. For example, the amplifier may have fixed-tuned filters on both the input and output stages and the attenuators would not be tuned to the same decibel level. Hence, filteramplifiers 130 are so designated because of the similar circuit function and need not be the same in structure.

As shown in FIG. 4 the output, f9, of coherent mixer 200 is coupled through a fixed-tuned isolator-amplifier circuit 320 to the input of a directional coupler 340. Isolator-amplifier circuit 320, similar to circuit 130, comprises a serially connected attenuator pad 322, fixed-tuned bandpass filter 324, isolator 326, amplifier 328, and another isolator 326. Directional coupler 340 has a first output coupled as channel 3 through an isolator-amplifier circuit 320 to the high input of mixer 450 (FIG. 5). Another output of coupler 340 is connected through an isolator circuit to amplifier 342 wherein )9 is amplified and connected to a second directional coupler 350. A channel 1 output of coupler 350 is connected as a low input through fixed-tuned filter 420 to mixer 430 shown in FIG. 5. Similarly, channel 2 output of coupler 350 is connected through an isolatoramplifier circuit 320 to the high input of the mixer 490 in FIG. 5. Direction circuit 300 and isolation amplifier network 400 include somewhat similar isolationamplifier circuits 320. These amplifier circuits can vary in structure by having more than one filter or attenuator pad for example and only one isolator, depending on the decibel level required for the particular frequency passing therethrough.

As further seen in FIG. 5, f4 is also coupled to the high input of mixer 430 for combination with )9. The output of mixer 430, fll, is connected through an isolator-amplifier circuit 320 to the high input of mixer 470. The output signal (flO) from frequency synthesizer 410 is connected as the low input for mixer 450 and is combined with 19 to produce output frequency fl2. The output of mixer 450 is connected through an isolator-amplifier circuit 320 to the input of hybrid 460. Hybrid 460 splits fl2 into first and second outputs, the first output being connected through isolator and fixed-tuned filter combination 462 to the low input of mixer 490 where it is combined with f9 to provide the output variable frequency fl3, the other output of hybrid 460 being connected through a fixed-tuned filter and isolator circuit 464 to the low input of mixer 470 where it is combined with fll to produce variable output frequency f14. These output frequencies are connected through respective fixed-tuned bandpass filters 472 and 492 to using circuitry, allowing efficient sideband rejection without the need of turnable filters.

System operation can be across the spectrum of microwave frequencies depending on the particular range of output frequencies desired. Directional circuit 300 and isolation and amplifier network 400 include appropriate isolation stages to segregate respective receiver and transmitter output channels (channels 1 and 2). Because of frequency spacing, filters for respective channels need not be tunable and may be fixed broad, or narrow bandpass filters for the frequency being separated. The channel serving the receiver is amplified, filtered and isolated to feed the radar receiver mixer circuit as a local oscillator frequency source. The channel for the transmitter is split off from the receiver channel, filtered, and mixed with the f4 frequency to provide an offset, difference, frequency for separating the transmitter and receiver channels by f4. This difference frequency provides the driving signal for the transmitter mixer circuit. The third channel being mixed with the programmable frequency source (synthesizer 410) produces a desired sideband range which is filtered to eliminate undesired mixer by-products and the power level is kept low to reduce undesired nonlinear effects from polluting the signal. The frequency selectable range, fl2, is then split, isolated, and mixed with respective transmitter and receiver channels. Thus, two variable frequency outputs are provided which always remain separated by the fixed frequency f4.

No tunable filters are required to reject sideband signals since a simple rf bandpass filter of fixed frequency will remove unused or undesired components. During mixing, the signal which is to dominate the output spectrum is coupled as a relatively high decibel level into the mixer first or high input while the least dominate signal is coupled at a lower decibel level as the mixer low input. This dictates the number of amplifier stages and level of attenuation which may be provided between mixers as well as controlling the frequencies present in progressive stages of the system.

Obviously many modifications and variations of the exciter frequency agility system are possible in the light of the foregoing disclosure. It is therefore understood that with the scope of the appended claims the invention may be practiced otherwise than as specifically described herein.

We claim:

1. An exciter frequency agility system comprising: an up-converter network having a plurality of input frequencies and first and second frequency outputs; a coherent frequency mixer for combining the output frequencies of said up-converter and providing a coherent frequency output; a direction circuit coupled for receiving the coherent output of said frequency mixer and having a plurality of common coherent frequency outputs; an isolation and amplifier network having a plurality of frequency inputs and first and second outputs, said outputs having simultaneously varying frequencies thereon which remain spaced apart by a fixed frequency difference; said plurality of direction circuit coherent frequency outputs being coupled to respective isolation network inputs; a frequency synthesizer having a high frequency variable output coupled to a first input of said isolation network; and a second input of said isolation network being coupled in common with one of said plural inputs of said converter network.

2. An exciter frequency agility system as set forth in claim 1 wherein said up-converter network comprises first, second and third series coupled mixers, each of said mixers having first and second inputs and an output; the output of said first mixer being coupled as a first input to said second mixer, the output of said second mixer being coupled as a second input to said third mixer; the output of said third mixer being coupled as a first input to said coherent frequency mixer, said first mixer being coupled to receiver first and second frequencies of said converter plurality of input frequencies, said second mixer having the second input coupled to receive a third frequency of said converter plural input frequencies, said third mixer having the first input coupled to receive a fourth frequency of said converter plural input frequencies; and a multiplier coupled between a second input of said coherent frequency mixer and said fourth input frequency of said converter.

3. An exciter frequency agility system as set forth in claim 2 and further comprising a hybrid having an input responsive to said fourth input frequency of said converter, said hybrid having a first output coupled to the first input of said third mixer for coupling said fourth frequency thereto; and wherein said multiplier is a homodyne multiplier having an input coupled to a second output of said hybrid, an output of said multiplier being coupled to the second input of said coherent mixer for producing the output spectrum of said converter fourth input frequency.

4. An exciter frequency agility system as set forth in claim 3 and further comprising a fixed-tuned filteramplifier circuit in each of the respective inputs for said coherent mixer and said third mixer and in the first input of said second mixer for filtering and controlling input signal levels coupled to said mixer.

5. A frequency agility system as set forth in claim 4 wherein said isolator and amplifier network further comprises third, fourth and fifth inputs from said common coherent frequency; fourth, fifth, sixth and seventh mixers for selectively combining said network input frequencies to provide first and second spaced apart variable output frequencies; said fourth mixer having the output coupled as a first input to said fifth mixer and having first and second inputs respectively coupled to said fourth frequency as said second input to said isolation network and to said third input for providing a fixed output frequency difference signal to said fifth mixer, said sixth mixer having a first input coupled to said coherent fourth input, said seventh mixer being coupled to said first and fifth network inputs for receiving and mixing said synthesizer variable output signal and said coherent frequency; a hybrid junction responsive to the output of said seventh mixer for splitting and coupling said variable output signal as respective second inputs to said fifth and sixth mixers; and first and second fixed-tuned bandpass filters coupled respectively to the output of said fifth mixer and said sixth mixer for coupling said spaced apart, variable output frequencies therefrom.

6. A frequency agility system as set forth in claim 5 and further comprising respective fixed-tuned filters and isolators connected between said hybrid outputs and said fifth and sixth mixer inputs for selectively isolating the variable frequency input thereto.

7. In an exciter frequency agility system, the method of generating a variable output signal for transmission which is selectively spaced apart from another variable output signal for combining with the first variable signal to provide a known difference frequency therebetween and comprising the steps of: mixing first and second high frequencies to obtain a third frequency sum output; mixing a fourth high frequency with the third frequency to provide a fifth frequency sum output; mixing a sixth high frequency with the fifth frequency for providing a seventh high frequency sum output; homodyne multiplying said sixth frequency for producing the fourth harmonic of said sixth frequency; mixing the fourth harmonic of said homodyne sixth frequency with said seventh frequency for providing an eighth frequency, stable, coherent output signal; and selectively mixing individual ones of said sixth, eighth, and ninth frequencies with mixed combinations of said sixth, eighth and ninth frequencies to provide and selectively spaced apart variable output frequencies.

8. The method as set forth in claim 7 and wherein said selective mixing comprises the steps of: mixing said eighth frequency with said sixth frequency for providing a tenth frequency output difference signal, mixing said eighth frequency with said ninth frequency for providing summed eleventh frequency output signal, mixing said tenth frequency and said eleventh frequency to provide the first variable output signal for transmission, and mixing said eighth frequency with said eleventh frequency for providing the second variable output signal.

9. The method as set forth in claim 8 and further comprising the step of selectively varying said ninth frequency across a broad high frequency band for shifting said variable output frequency while maintaining a fixed frequency separation therebetween.

10. The method as set forth in claim 9 further comprising the step of selectively isolating said variable ninth frequency and said sixth frequency from feedback into adjacent channels during mixing with said eighth, tenth and eleventh frequencies.

Claims

1. An exciter frequency agility system comprising: an upconverter network having a plurality of input frequencies and first and second frequency outputs; a coherent frequency mixer for combining the output frequencies of said up-converter and providing a Coherent frequency output; a direction circuit coupled for receiving the coherent output of said frequency mixer and having a plurality of common coherent frequency outputs; an isolation and amplifier network having a plurality of frequency inputs and first and second outputs, said outputs having simultaneously varying frequencies thereon which remain spaced apart by a fixed frequency difference; said plurality of direction circuit coherent frequency outputs being coupled to respective isolation network inputs; a frequency synthesizer having a high frequency variable output coupled to a first input of said isolation network; and a second input of said isolation network being coupled in common with one of said plural inputs of said converter network.

4. An exciter frequency agility system as set forth in claim 3 and further comprising a fixed-tuned filter-amplifier circuit in each of the respective inputs for said coherent mixer and said third mixer and in the first input of said second mixer for filtering and controlling input signal levels coupled to said mixer.