Background of the Invention In radio frequency ("RF") transmission systems that utilize amplitude modulation ("AM") techniques to transmit information, an audio signal containing the information is used to modulate the amplitude of an RF carrier signal.

At the receiving end of a radio system, to demodulate the incoming composite signal (i. e., carrier and side-bands) and recover the audio information contained in the side-bands, the carrier signal must be replicated so that it can be removed from the incoming signal, leaving only the audio information.

Historically, phase lock loops ("PLLs") have been used in the carrier recovery process to lock a receive end local oscillator to the incoming signal through use of a phase comparison of the incoming signal and a locally generated carrier. In practice, PLLs behave similarly to flywheels and, like flywheels, have"inertia ;"that is, resistance to a change in energy level either in the form of phase or frequency. PLLs smooth out the carrier, creating a time-averaged model of the carrier. The same phenomenon occurs with high-Q filters, which is the other approach to recovering the incoming carrier. As a result, errors introduced into the composite signal during transmission are no longer common to both the recovered carrier and the sidebands. On the contrary, the recovered carrier is essentially error-free.

In addition, because the receive end carrier is generated locally, it is not perfectly synchronized with the original carrier on a cycle-by-cycle basis, thus

preventing the simultaneous bidirectional transmission of multiple signals via a single transmission medium because of the effects of heterodyning.

Therefore, what is needed is a method and apparatus for precisely locking a local oscillator onto a carrier on a cycle-by-cycle basis and to use the incoming recovered carrier as the carrier for modulation on the same frequency.

Summary of the Invention In a preferred embodiment, the invention comprises method and apparatus for recovering the incoming signal carrier in a radio frequency ("RF") transmission and using the recovered carrier to demodulate the information contained in the sidebands of the same RF transmission. In addition, the recovered carrier may be reused as the suppressed carrier for another RF transmission on the same frequency.

A technical advantage achieved with the invention is that it enables the recovery of an RF transmission signal carrier on a cycle-by-cycle basis.

Another technical advantage achieved with the invention is that use of the recovered carrier lowers the distortion and noise of a received signal, thus increasing its bandwidth and lowering bit error rates.

Another technical advantage achieved with the invention is that it facilitates low cost and easy to implement bi-directional communication on a single frequency.

Yet another technical advantage achieved with the invention is that it enables operation down to DC for AM, quadrature amplitude modulation ("QAM"), single side-band ("SSB"), and double side-band ("DSB").

Brief Description of the Drawings Fig. 1 is a system block diagram of a RF transmission system embodying features of the present invention.

Detailed Description of the Preferred Embodiment Fig. 1 is a system block diagram of a receive end portion of an RF transmission system 100 embodying features of the present invention. As shown in Fig. 1, an unsuppressed carrier transmission comprising modulated

sidebands (e. g., AM, QAM, SSB or DSB) transmitted via a transmission medium 102 is provided to a synchronous demodulator 104 through a bidirectional interface 106. In one embodiment, the transmission medium 102 is a wire, although it should be recognized that other types of transmission media, such as fiber optic cable or air, for example, may be used.

The carrier is extracted from the received signal using a carrier extraction device 108 that does not use resonance oscillator locking to replicate the carrier and that replicates the carrier on a cycle-by-cycle basis. In a preferred embodiment, the carrier extraction device 108 is best characterized as an electronic wave interferometric filter, such as that described in detail in the above-referenced U. S. Patent Application Serial No. 08/955, 480 (Atty. Docket No. 10393.27).

The carrier output from the carrier extraction device 108 is provided to the demodulator 104 for demodulating the received signal. Due to the characteristics of the carrier extraction device 108, as described above, the recovered carrier output from the device contains all of the same instantaneous phase and amplitude errors as the modulated sideband (s), due to mutually induced errors from both the transmission medium 102 and the transmission process Accordingly, a characteristic of the balanced demodulation process known as common-mode rejection reduces the effects of the errors on the modulated information, thus improving throughput, bandwidth, and error rate of the system 100. In addition, this exacting relationship between the carrier and the modulated information permits operation down to DC for DSB, SSB, AM and QAM.

On the transmission side, the carrier recovered by and output from the carrier extraction device 108 is input to a modulator 110 for use in a suppressed carrier modulation scheme to lay side-bands back through the bidirectional interface 106 to facilitate bidirectional communication in the same band space, i. e., on the same frequency. As the modulation performed by the modulator 110 is generated by the same carrier already on the transmission medium 102, the modulation products of this transmission will not beat or "heterodyne"against the modulation products of the unsuppressed carrier already on the line 102.

Although an illustrative embodiment of the invention has been shown and described, other modifications, changes, and substitutions are intended in the foregoing disclosure. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.