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EXTRA (11) 4,087,750 Allen Et Al * 5-28 SR 3 f. X 3 4 O 87, 75) United States Patent (19) EXTRA (11) 4,087,750 Allen et al. 45) May 2, 1978 54) RECEIVER FOR DETECTING AND Attorney, Agent, or Firm-Nathan Edelberg; Sheldon ANALYZING AMPLITUDE OR ANGLE Kanars; Jeremiah G. Murray MODULATED WAVES IN THE PRESENCE OF INTERFERENCE 57 ABSTRACT This receiver includes in its IF channel a summing net (75) Inventors: Joseph A. Allen, Eatontown; William work by means of which the output of a calibrated R. Fuschetto, Freehold, both of N.J. reference oscillator may be added to the received sig (73) Assignee: The United States of America as nals. The frequency and amplitude of the reference represented by the Secretary of the oscillator are adjusted to match those of a desired one of Army, Washington, D.C. several co-channel signals which may be simultaneously present in the IF channel. The combined signals are 21 Appl. No.: 731,662 amplitude limited and applied to a frequency deviation 22 Filed: May 23, 1968 detector in which the wide frequency deviations caused (51) Int. Cl” ........................ H04B 1/10; H04B 17/00 by the interaction of the reference and desired signal are (52) U.S. C. .................................... 325/363; 325/346; detected. The technique can be used to demodulate 325/473; 325/476; 325/482 angle or amplitude modulated signals obscured by jam (58) Field of Search ............... 325/344, 348, 346, 472, ming signals and to measure the frequency and ampli 325/476, 481,482, 473, 363; 324/77A, 77 B tude of signals which are similarly obscured. Primary Examiner-Howard A. Birmiel 7 Claims, 2 Drawing Figures 3. 9 5 25 27 l, F. VARABLE BAND-PASS SUMMING BAND-PASS AMPFER FTER NETWORK 7 LOCAL ATTENU OSC LATOR REFERENCE OSCATOR 23 -2 ANGE 39 MODULATOR DGA RECORDER 4,087,750 1 2 A further object of the present invention is to provide RECEIVER FOR DETECTING AND ANALYZNG a receiver adapted to receive and demodulate both AMP LTUDE OR ANGLE MODULATED WAVES amplitude and angle modulated signals in the presence IN THE PRESENCE OF INTERFERENCE of undesired co-channel signals. The invention described herein may be manufac 5 Still another object of the invention is to provide a tured, used, and licensed by or for the Government for novel AM-FM anti-capture receiver which includes governmental purposes without the payment to us of circuitry for analyzing received signals. any royalty thereon. A further object of this invention is to provide a The present invention relates to a radio receiver novel method for measuring the strength or power and which is adapted to receive and demodulate either am O frequency of a received signal in the presence of inter plitude or angle modulated signals even though the ference. signals may be obscured by a stronger co-channel sig These and other objects and advantages of the inven nal. In an ordinary FM receiver, the capture effect tion will become apparent from the following detailed causes the weaker of two co-channel signals to be sup description and drawing, in which: pressed or captured by a stronger signal. If the stronger 15 FIG. 1 is a block diagram of a receiver embodying signal happens to be an enemy jamming signal and the the principles of this invention, and weaker signal a friendly one which it is desired to re FIG. 2 is a modification thereof. ceive, the capture effect of the ordinary receiver will be In FIG. 1, the RF stage 3, mixer 5, local oscillator 7 a disadvantage. Specialized military receivers have and intermediate frequency amplifier 9 will all perform been designed which do not exhibit the capture effect or 20 in the usual manner of such components in a superhet which enhance the weaker signal to permit reception erodyne receiver to convert signals picked up over a thereof under jamming conditions. The present cir wide frequency range to a single intermediate fre cuitry is a receiver of this latter type which is capable of quency. The output of amplifier 9 is applied to variable receiving either frequency, phase, or amplitude modu bandpass filter 11, which has a narrower bandwidth lated signals in the presence of either modulated or 25 than IF amplifier 9 and the center frequency of filter 11 continuous jamming signals. The circuitry further in is variable over the frequency range of amplifier 9. Thus cludes means for measuring various parameters of in filter 11 permits the selection of any desired portion of coming signals, for example, the amplitude, carrier fre the passband of amplifier 9, which is useful in certain quency, and type of modulation thereon. The general applications for discriminating against undesired signals principle of operation involves inserting in or adding to 30 in certain frequency ranges. The switch 13 is provided the intermediate frequency (IF) channel of a superhet so that the filter 11 may be inserted in the IF channel or erodyne receiver a reference signal of controllable fre by-passed, as the conditions require. The summing net quency and amplitude which may be selectively angle work 15 is a means for combining in a linear manner the modulated or unmodulated. The combined reference received signals from the output of the IF channel with and received signals are then amplitude limited to pro 35 the reference signal derived from oscillator 19. A cali duce IF signals with only frequency or phase modula brated attenuator 17 permits the injected reference sig tion thereon. In detecting frequency or phase modu nal to be adjusted to the proper amplitude and the am lated received signals, the reference signal is unmodu plitude noted. The angle modulator 23 is connectible to lated, and is frequency or phase modulated in detecting oscillator 19 by means of switch 21. When this switch is amplitude modulated received signals. The deviation of 40 closed, the modulator 23 will either frequency or phase the reference oscillator is made significantly smaller modulate the output of reference oscillator. When the than that of the received signal to prevent undesired switch is open, the reference oscillator is unmodulated. interaction therebetween. The reference signal and the The term angle modulation is a generic one including desired received signal are approximately equalized in both frequency and phase modulation. The summing amplitude at the limiter input and the reference signal 45 network 15 may comprise merely a simple resistor ma carrier or center frequency is set to coincide with that trix for linearly adding or superimposing the reference of the desired received signal. The limiter output will signal and all of the simultaneously received signals. comprise a signal which has a theoretically infinite fre The output of the summing network is applied to an quency spectrum resulting from large frequency devia amplitude limiter 15, which may comprise one or more tions or excursions which recur at the same rate as the 50 stages and is preferably a so-called hard limiter, so that modulation of the desired incoming signal. By suitable all amplitude variations of all of the input signals are detection circuitry, this modulation rate can be ren substantially absent from the output. The limiter band dered audible to provide a readable but distorted ver width is made substantially wider than that of IF ampli sion of the desired modulation. Undesired received fier 9. The limiter output is applied to bandpass filter 27 signals are discriminated against by virtue of the fact 55 which is tuned to a band of frequencies either above or that they differ substantially in amplitude from the ref below the passband of the IF amplifier 9. The theoreti erence signal and for this reason the aforementioned cally instantaneous wide frequency deviations or excur frequency deviations are not produced with anywhere sions mentioned above, resulting from the interaction of near the same intensity for the undesired signals. When the reference and desired signal will sweep through the the desired signal is amplitude modulated, the reference passband offilter 27, thus producing at the output of this signal amplitude is set at approximately the average or filter a train of ac pulses similar to radar pulses. The unmodulated value of the desired signal. Since the de envelope detector 29 removes the carrier from the out sired signal will then pass through the reference signal put of filter 27 to yield a train of dc or video pulses amplitude at the modulation rate, the limiter output will which are applied to audio stage 31 wherein any audio comprise a signal with frequency deviations at the same 65 or voice modulation therein is rendered audible. The rate as the modulation on the desired AM signal. detector output is also passed through filter 33 which It is thus an object of this invention to provide an removes the frequency of angle modulator 23 from the improved anticapture receiver. signal, and passes the remainder in parallel to three 3 4,087,750 4. devices used for analyzing and measuring the parame amplitude signals, but is outside the passband of the ters of the selected or desired received signal. These intermediate frequency channel, so that none of the devices are the oscilloscope 35, frequency meter 37 and received or reference signals which have not been sub digital detector 39. stantially deviated in frequency can pass therethrough. The operation of the receiver of FIG. 1 is as follows: As stated above, the output of the filter 27 will comprise Assume that the switch 13 is closed to bypass filter 11, a series of alternating current pulses which recur at the and further that two co-channel signals are being re modulation frequency of the desired angle modulated ceived, one a frequency or phase modulated signal and received signals.
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