WO1992010881A1

WO1992010881A1 - Single frequency ring around transceiver

Info

Publication number: WO1992010881A1
Application number: PCT/US1991/009101
Authority: WO
Inventors: Thomas J. Warnagiris
Original assignee: Warnagiris Thomas J
Priority date: 1990-12-06
Filing date: 1991-12-04
Publication date: 1992-06-25
Also published as: AU9137691A

Abstract

The communications system of the present invention comprises a pair of transceivers (10, 10a) and an appropriate transmission medium. A 'ring-around' configuration provides a closed loop carrier between the transceiver pair (10, 10a). To exchange information a carrier may be modulated by conventional amplitude or frequency methods. Oscillation occurs between the pair of transceivers when the total round trip gain due to amplifiers (12) exceeds unity and the total phase shift due to the delay lines (16) and circuit delays is 360 degrees. Modulation applied to the carrier at either transceiver will be detectable at both. Demodulation can take place at any point on the signal path within one of the transceivers.

Description

SINGLE FREQUENCY RING AROUND TRANSCEIVER

Field of the Invention The present invention relates generally to communications systems and, more specifically, to communications systems which must be capable of operating on very low power and further capable of detecting errors when operating in noisy environments.

Background One of the problems which is often encountered in communications systems is loss of data due to environmental electromagnetic interference and other spurious ambient interference signals. This problem is especially acute when operating long wave communications systems in a noisy environment at very low power levels. In addition to the problem of loss of data due to interference, there is often concern over the security of data signal transmission - both in terms of ensuring that the data which is transmitted by a first transceiver is actually received at a second transceiver and also ensuring that transmitted data is not modified by an undesired third party. Thus far, the prior art has not provided an effective communications system which can provide adequate data security and accuracy when operating at very low power levels in a noisy environmenL The communications system of the present invention overcomes the difficulties of the prior art by providing a low power communications system which is capable of providing accurate and secure data communications even in extremely noisy environments. Summary of ² the Invention The communications system of the present invention comprises a pair of transceivers and an appropriate transmission medium. A "ring-around" configuration provides a closed loop carrier between the transceiver pair. To exchange information a carrier may be modulated by conventional amplitude or frequency methods. Modulation applied to the carrier at either transceiver will be detectable at both. Demodulation can take place at any point on the signal path within one of the transceivers. In the preferred embodiment, each transceiver consists of an amplifier, isolator, delay line, and bandpass filter. Oscillation occurs between the pair of transceivers when the total round trip gain due to the amplifiers exceeds unity and the total phase shift due to the delay lines and circuit delays is 360 degrees. Oscillation is initiated by noise either within the transceiver circuits or from external sources. The noise within the passband of the filter of one transceiver will be amplified and transmitted to the input of the other transceiver. Noise at exactiy the intended carrier frequency is incompletely cancelled by the negative feedback within each transceiver because the negative going correction signal is always 180 degrees in time behind the noise signal. There are various combinations of time delay and phase inversion between the transceiver pair that will sustain ring-around oscillation.

Brief Description 3 of the Drawings FIG. 1 is a schematic illustration of a pair of transceivers comprising the ring around transceiver system of the present invention. FIG.2 is a schematic illustration of a communication system comprising a ring around transceiver pair with masked one way transmission of information.

Detailed Description of ⁴ the Preferred Embodiment The communications system of the present invention comprises a pair of transceivers 10 and 10a, as shown in FIG. 1. Each transceiver consists of an amplifier 12, isolator 14, delay line 16, and bandpass filter 18. The possible feedback paths between each of the transceivers, which will be discussed in greater detail below, are shown by the arrows and dotted lines in FIG. 1. Any standard modulation technique can be employed in the communication system of the present invention. For example, an amplitude modulator (AM) can be placed in series with the input of the amplifier 12. Alternatively, frequency modulation (FM) can be accomplished by controlling the time constant of the delay Une 16. Demodulation can be accomplished by normal detection methods at any point on the signal path within each transceiver. Oscillation occurs between the pair of transceivers 10 and 10a when the total round trip gain due to the amplifiers exceeds unity and the total phase shift due to the delay lines and circuit delays is 360 degrees. For a single transceiver the input to output phase shift is only 180 degrees at the intended frequency of oscillation - degeneration. Oscillation is initiated by noise either within the transceiver circuits or from external sources. The noise within the passband of the filter of one transceiver will be amplified and transmitted to the input of the other transceiver. Noise at exactly the intended carrier frequency is incompletely cancelled by the negative feedback within each transceiver because the negative going correction signal is always 180 degrees in time behind the noise signal. An important feature of the present invention is the use of the delay line 16 to provide time separation between the positive and negative feedback. For the embodiment shown in FIG. 1, the delay line negative feedback (transmitted signal) is always one half cycle behind the positive feedback (received signal) from the other transceiver. This time separation provides isolation of the transmitted and received signals even though they are at the same frequency.

There are various combinations of time delay and phase inversion between a transceiver pair that will sustain ring-around oscillation. The primary requirements are that the overall (complete ring-around loop) phase change be 360 degrees and that at least unity gain exists between transceivers. Also, the enclosed signal loop within a single transceiver cannot have greater than unity gain at any frequency where the loop response provides positive in-pass feedback. The following table lists some of the possible time delay and inversion combinations for a pair of transceivers:

TABLE 1

TRANS CS g 1 TRANSCEIVER 2

90 degrees delay + 90 degrees delay + 180 degrees delay + no inversion

270 degrees delay + 90 degrees delay + no inversion no inversion

190 degrees delay + 180 degrees delay + no inversion no inversion

450 degrees delay + 270 degrees delay + 180 degrees inversion 180 degrees inversion

The amount of negative feedback supplied by the delay line not only provides isolation, but also acts as the key frequency determining element of the transceiver pair. Oscillation will occur only at the frequency where the total delay line and circuit delay provide a 360 degree phase shift. If the negative feedback due to the delay line did not exist, oscillation could occur, but the oscillation frequency would exist at any pass frequency of the bandpass filter and vary as a function of both loop gain and the separation distance between transceivers. The delay line negative and positive feedback establish the total loop response time and therefore the actual oscillation frequency of the complete loop. The oscillation frequency is independent of the spacing (spatial time delay) because when a signal from one transceiver is received at the other transceiver out of phase, the negative feedback loop within the receiving transceiver will attempt to cancel the receiver signal limited by the time delay response time so that the resultant output signal phase shift will change to be in phase with the received signal. The phase relationship between transceivers can change with spacing between transceivers, but the oscillation frequency will be the same within the limitations of the correction factor of the negative feedback. This ring-around scheme provides a closed loop carrier between a transceiver pair. To exchange information the carrier may be modulated by conventional amplitude or frequency methods, as discussed above. Modulation applied to the carrier at either transceiver will be detectable at both. Demodulation can take place at any point on the signal path within a transceiver. The isolators 14 shown are not essential to the basic operation of the ring- around loop, but allow greater transceiver gain and higher output. This is because, depending on their design, the delay lines 16 and amplifiers 12 might be damaged by high level input signals. Also, the isolators decrease the amount of out-of-band attenuation required from the bandpass filter to suppress oscillation at frequencies where a single delay line produces a 360 degree phase shift. There are many possible configurations of components using the single frequency ring around transceiver system of the present invention. The amplifier 12 could consist of a wide-band amplifier of constant time delay over the passband of the band-pass filter 18. The gain of the amplifier 12 should be sufficient to overcome the maxi um path and circuit loss for the intended signal path. The band pass filter 18 must provide sufficient bandwidth to accommodate the intended modulation rate. In addition, it should provide the minimum phase shift across the passband and attenuate the total loop gain within one transceiver sufficiently at points outside the passband so that the gain is less than unity at any frequency where the total phase shift within one transceiver is a multiple of 360 degrees. The delay lines 16 should provide a fixed (when not modulated) time delay for the intended oscillation frequency. It may be a passive time delay device with constant time delay at all frequencies, such as a transmission line delay, or it may be an active delay element such as an amplifier. The key consideration is that its delay characteristics complement the bandpass filter and amplifier delay characteristics so that at no frequency does the total delay within one transceiver reach 360 degrees at any frequency where the loop gain within one transceiver exceeds unity. The isolator 14, as previously mentioned, is not necessary for the basic function of the transceiver but does allow wider dynamic signal range. The isolation mechanism may be as simple as separate transmit and receive antennas oriented with respect to each other for minimal coupling, or as complex as an adaptive nulling scheme controlling a balanced output with single ended input. The amount of negative feedback through the isolator 14 affects the noise level and stability of the ring-around oscillation and should be controlled according to system needs. Large negative feedback provides the best stability and least noise, but makes realization of the filter and time delay components difficult. In addition to the embodiment shown in FIG. 1, it is apparent that it is possible to produce a corollary system with positive feedback within one transceiver and negative feedback between the two transceivers at the same frequency. This would result in frequency and amplitude lock between the two transceivers. FIG. 2 is a schematic illustration of the communication system of the present invention comprising a ring around transceiver pair with masked one way transmission of information. The system is illustrated generally with a first and second transceivers, 10 and 10a, with the output of the second transceiver 10a being detected by detector 20 which provides an input to adder 22. An input signal Z(t), which can be either code or noise, is used as an input to transceiver 10a and is also provided as an input to the adder 22. The form of the signal between the two transceivers is shown in FIG.2 as K{ 1 +[A(t) + Z(t)] } Cos α^t, where: = constant ϋύ --- carrier frequency t = time A = information Z = code or noise The signal transmitted by either of the transceivers 10 or 10a is essentially the same signal at two different places in the closed oscillating loop. If modulation (information) is injected into the loop at one transceiver, that modulation propagates to all other points in the loop, i.e., to both transceivers and to all points in the medium between the transceivers. Further, if modulation (code or noise) is injected at the other transceiver, it adds to the modulation placed on the carrier by the first transceiver. The code or noise injected by the second transceiver can be used by the operator of the second transceiver to recover the information placed on the closed loop oscillation at the first transceiver by simply subtracting it from a sample of the demodulated loop signal. If an observer external to the loop samples the signal, there would be no way for that observer to determine what part of the modulation [A(t) + Z(t)] is information and what part of it is code or noise injected at the second transceiver. Even if the observer sampled at several points within the loop, there is no way that the information can be separated from the code/noise unless the observer learns which transceiver is placing information on the carrier. Although the single frequency ring around transceiver of the present invention has been described in terms of the preferred embodiment, it is not intended to be limited to the specific form set forth herein. On the contrary it is intended to cover such modifications, alternatives and equivalents as can be reasonably included within the spirit and scope of the of the claims.

Claims

What is claimed is: 10 1. A communications system comprising: a first transceiver and a second transceiver, said first transceiver including: first modulation means for modulating a first signal at a first modulation rate to be transmitted to said second transceiver; first demodulation means for demodulating a second signal received from said second transceiver; a first amplifier for amplifying said first signal; a first band pass filter for filtering said first signal; and a first delay means to provide for time separation between said first signal and said second signal; said second transceiver including: second modulation means for modulating said second signal at a second modulation rate to be transmitted to said first transceiver; second demodulation means for demodulating said first signal received from said first transceiver; a second amplifier; a second band pass filter for filtering said second signal; and a second delay means to provide for time separation between said second signal and said first signal; wherein oscillation between said first and second transceivers occurs when the total gain due to said first and second amplifiers exceeds unity and the total phase shift due to the time separation by said first and second delay means is 360 degrees.

2. The communication system according to claim 1, wherein said first and second transceivers further include first and second isolator means, respectively, to protect said first and second amplifiers and said first and second delay means from damage from said second signal.

3. The communication system of claim 1, wherein the gains of said first and second amplifiers are sufficient to overcome the maximum path and circuit loss of the intended signal path.

4. The communication system of claim 1, wherein said first and second modulation means comprise first and second amplitude modulators placed in series with the inputs of said first and second amplifiers.

5. The communication system of claim 1, wherein said first and second modulation means comprise first and second means for controlling the time constant of said first and second delay means.

6. The communication system of claim 1, wherein said oscillation is initiated by noise within said first and second transceivers.

7. The communication system of claim 1, wherein said oscillation is initiated by noise from external sources.

8. The communication system of claim 1, wherein said first and second amplifiers comprise first and second wide-band amplifiers of constant time delay over the passbands of said first and second band-pass filters.

9. The communication system of claim 1, wherein said first and second delay means comprise passive time delay devices.

10. The communication system of claim 1, wherein said first and second delay means comprise active time delay devices.

11. The communication system of claim 2, wherein said first and second isolators comprise separate transmit and receive antennas oriented with respect to each other for minimal coupling.

12. The communication system of claim 1, wherein one of said two transceivers further includes means for masking said transmitted signal

13. The communication system of claim 12, wherein said masking means comprises a detector, an adder, and a signal source, said detector detecting the output of said one of said transceivers and providing a first input to said adder, and said signal source supplying an input to said one of said transceivers and supplying a second input to said adder.

14. A communication system comprising: a first transceiver; a second transceiver for transmitting information to said first transceiver, and for receiving information from said first transceiver in a ring around configuration; an adder; a detector for detecting the output of said second transceiver and for supplying a first input to said adder, and a signal source, said signal source supplying an input to said second transceiver and an input to said adder.

14

1 15. The communication system of claim 14, wherein said signal source

2 comprises code.

1 16. The communication system of claim 14, wherein said signal source

2 comprises noise.