Phase Lock Loop Multitone Interference Canceling System

This invention relates to interference canceling systems and, more particularly, to such a system in which continuous wave, or slowly varying continuous wave, interference is significantly reduced in communications applications.

As is well known and understood, perhaps the most common method of reducing continuous wave interference is through the use of band pass notch filters. However, as applied in audio communications systems-- and, especially, in audio avionics defense communication-- the use of such filters are disadvantageous in certain respects. First of all, relatively large, and therefore expensive, inductors and capacitors are required for the passive, low frequency filtering. Secondly, the filtering also distorts the voice signal and deteriorates its intelligibility. Thirdly, the filtering is done around a pre-set frequency, affording the possibility of signal jamming merely by changing frequencies.

As will become clear hereinafter, the interference canceling system of the present invention employs a phase lock loop as a narrow band filter which, through its inherent utilization of resistors and capacitors, substantially reduces problems associated with any requirement for large value inductors. The phase lock loop, additionally, is used to permit voice signal information, for example, to be stripped from an interfering tone in yielding a signal-free duplicate of the interference; this duplicate can then be readily adjusted in amplitude and phase to permit linear subtraction of the interference from a composite input signal. Because the input voice signal is not filtered, it remains essentially free of distortion, to enhance the intelligibility of the output signal. As the phase lock loop can be set for individual interfering tones or can be swept, it can find extensive use in avionics communication-- both where there is known multitone interference (as with acoustic helicopter noise having tendencies to degrade performance of on-board communications) and where the interference frequency can be changed, as in the context of signal jamming.

These and other features of the present invention will be more clearly understood from a consideration of the following description, taken in connection with the accompanying drawing, in which:

FIG. 1 is a block diagram of a portion of an audio communications system helpful in an understanding of the invention; and

FIG. 2 is a block diagram of a multitone interference canceling system constructed in accordance with the present invention, and useful in the communications system of FIG. 1.

In FIG. 1, assumed to represent an audio communications system aboard a tactical defense helicopter, for example, voice intelligence from a speaker 10 (e.g. the pilot) is mixed within a microphone 14 with acoustic interfering tones (e.g. ambient rotor noise) from a source 12, for subsequent communication, either by cable 16, aboard the aircraft, or by antenna 18, to a ground station. An amplifier 20 amplifies the detected microphone signals, which then modulate a carrier prior to further amplification and transmission. The carrier source 22, modulator 24 and power amplifier 26 may be of appropriate design. As will be apparent, the acoustic interfering tones tend to interfere with the voice intelligence whether the interference be continuous wave or slowly varying continuous wave, as long as it is within the same frequency band as the voice signal. Using a band pass notch filter to eliminate such acoustic interference would deteriorate the intelligibility of the voice signal.

However, in accordance with the invention, there are provided n interference canceling phase lock loops in tandem between the demodulated signal output terminal 28 of a receiver 30 and the sound reproduction apparatus, typically a listener's earphones 32. Each phase lock loop, e.g. 34, 36, 38, etc., can be utilized as needed, and will independently synchronize as a very narrow band phase lock loop to one of the interfering tones. In applications where only a single interfering tone is to be canceled, only the phase lock loop 34 need be utilized. As the output of each interference canceler serves as the input to the next interference canceler (except for the nth canceler which couples to the earphones 32), optimum performance will follow if the number of phase lock loop arrangements equal the number of different interfering tones. To provide maximum anti-jamming capabilities, the number of interfering cancelers should be one more than the number of interfering tones, to protect against signal jamming which could result if yet an additional jamming tone were introduced.

In the direction that follows, it will be understood that each tone interference canceler is of identical construction, differing, if at all, substantially only when the individual loops are preset to search for known frequency tones, instead of being adaptively arranged to sweep to lock to unknown interference frequencies. In these arrangements, the voltage controlled oscillator 40 of each tone interference canceler would be set to operate at a different frequency.

Referring more specifically to the tone interference canceler 34, it will be seen that the voltage controlled oscillator 40 is part of a phase lock loop also including a phase detector 42 and a loop filter 44. When the loop locks to the interfering tone present in the signal coupled to the receiver 30, the output from oscillator 40 is applied, via a 90.degree. phase shifter 46 to a second phase detector 48, whereat the output from terminal 28 is also applied. When the oscillator 40 locks to the interfering tones, the output of phase detector 48 will be a voltage, which is further filtered by an integrator 50 to provide a direct voltage output at terminal 52 indicating the loop has locked to an interference signal.

Also illustrated in the tone interference canceler 34 are a variable amplifier 54, a subtractor 56 and a null detector 58. As shown, the intelligence signal along with the interference tone is applied to the subtractor 56, to which is also coupled the output of amplifier 54 when a switch 60 is in the closed position. Such switch can be manually closed by the helicopter pilot upon noting the output voltage indication at terminal 52 that the loop is in lock (e.g., by an illuminated bulb), but can also be automatically closed in response to such direct voltage, as well. The output from the phase shifter 46 is applied to the variable amplifier 54 as an input, with the null detector 58 serving to adjust the setting of the variable amplifier until a null indication is presented with respect to the interference tone. In other words, the null detector 58 serves to adjust the amplitude of the supplied voltage controlled oscillator signal after it is locked in frequency and phase to the interference tone. The output from subtractor 56 will thus be devoid of the interference tone to which the loop 34 has locked, and can thereafter be applied as an input to the phase lock loop system 36 for subsequent extraction of any second, different interfering tone that might be present at the output terminal 28 of receiver 30.

As will be readily apparent, the foregoing discussion followed on the assumption that the voltage controlled oscillator was initially set to lock onto a known interfering tone. However, the phase lock loop and its oscillator could be arranged to be manually, or automatically, swept across the spectrum in search of an unknown frequency tone, as well. For example, any appropriate arrangement could be employed to vary the voltage on a capacitor of the voltage controlled oscillator, to vary its tank circuit and resultant frequency of oscillation in an attempt to lock onto an interfering tone.

To optimize the described operation, the design parameters should be selected to make the phase lock loop very narrow band, so as to reduce its lock-in, or pull-in, range and to further insure that little voice intelligence would be filtered out along with the interfering tone. If a wider bandwidth were employed, there would be a comparable reduction in the required sweep of the oscillator, but this would be at the sacrifice of performance because added intelligence information could be extracted from the received signal, as well as the interference tone. With present day designs, phase lock loops are readily available with closed loop bandwidths of less than 10 hz, so as to insure retaining synchronization in the presence of the intelligence information and such that substantially all of the voice signal energy would be filtered out in the phase lock loop. For the same reason only a small amount of information would be extracted, that being around the interference signal, so that the ultimate signal coupled to the listener's earphones would be essentially free of distortion.

It will also be apparent that the phase lock loop could be used for slowly varying continuous wave signals in addition to a continuous wave signal, whether the interference be audio in nature, electronic, or a jamming signal. As to the variations in continuous wave frequency, once the rapidity of the frequency variations falls within the frequency of the audio or other measured spectrum, some of the intelligence information could be lost--but this would be minimal if the rate of change of the interference falls outside the frequency of the audio spectrum. This is so even though the slow variations be large in magnitude. If the rate of change of frequency becomes too great, however, an increased bandwidth would be required in order to maintain lock, causing part of the signal intelligence to be lost and audio distortion in the output would result.

While there has been described what is considered to be a preferred embodiment of the present invention, it will be apparent that modifications may be made by those skilled in the art without departing from the scope of the teachings herein of utilizing a phase lock loop as a very narrow band tracking filter to enable stripping the voice signal from an interfering tone to yield a signal free duplicate of the interference, which could then be readily adjusted in amplitude and phase to enable linear subtraction from the composite signal. For at least those reasons, the scope of the invention should be read in light of the claims appended hereto.