An interference canceling system in which a phase lock loop is utilized as narrow band tracking filter to adaptively lock to interference tones to be canceled by a process of signal subtraction.

Apparatus is described for eliminating monochromatic interfering frequency components of seismic signals detected during the course of seismic exploration comprising an electrical circuit for producing a monochromatic nulling signal that has the same frequency as the interfering signal. The monochromatic nulling signal is produced by generating a truncated triangular wave having the same period as the interfering signal and filtering the truncated triangular wave to remove all components thereof except those having the period of the interfering signal. Circuit means are provided for controlling the amount of truncation of the truncated triangular wave, and for controlling the time phase thereof. The filtered signal is combined with the geophone signal in such a manner that the filtered signal at least partially cancels out the interfering component of the seismic signal. Circuit means are provided for detecting the phase of the residual signal resulting from incomplete cancellation of the interfering signal to provide an indication of the amplitude of the nulling signal. Means are also provided for detecting the time phase of the nulling signal. The truncation of the truncated triangular wave, and the time phase of the truncated triangular wave are controlled so as to result in more complete cancellation.

Packaged signal routing circuits (e.g. on printed circuit cards or boards), route pulse signals with very short rise times from a lossy driver to multiple devices. In these routing circuits, a complex network of conductors branches from a common junction adjacent the driver output into multiple conduction paths of unequal length. In accordance with the invention, the internal impedance of the driver is matched to the aggregate characteristic impedance of the branch paths, and a lossless compensating circuit is attached to a shortest branch path. The compensating circuit is designed to transfer signal reflections of predetermined form to the branching junction at the driver via the shortest branch.

An improved adaptive notch filter for removing undesired cochannel FM interference includes in-phase and quadrature signal processing paths in which the undesired FM signal is translated to zero frequency, i.e. to DC. In each of the in-phase and quadrature phase signal paths, a first multiplier translates the input frequency spectrum in a dynamic manner so that the frequency of the undesired FM signal is always centered on DC. The undesired signal, now at DC, is filtered out by a high pass filter with a sharp rolloff. The output of the high pass filter is remultiplied in a second multiplier to translate the desired FM signal back to the original position in the frequency spectrum. In order to eliminate an unwanted difference frequency remodulation signal at the output of the second multiplier, the quadrature phase signal path performs a parallel operation on the same input signal, but with a quadrature phase (90 degree phase shift) control signal. The output the in-phase and quadrature signal processing circuits are combined in an adder circuit to cancel the unwanted difference frequency remodulation signal.

Packaged signal routing circuits (e.g. on printed circuit cards or boards), route pulse signals with very short rise times from a lossy driver to multiple devices. In these routing circuits, a complex network of conductors branches from a common junction adjacent the driver output into multiple (in the disclosed embodiment, three) conduction paths of unequal length. In accordance with the invention, the internal impedance of the driver is matched to the aggregate charateristic impedance of the branch paths, and a lossless compensating circuit is attached to a shortest branch path. The compensating circuit is designed to transfer signal reflections of predetermined form to the branching junction at the driver via the shortest branch. Without the compensating circuit, reflections presented to the branching junction from the shortest branch are dissimilar to reflections presented to that junction from other branch paths. Consequently, re-reflections return from the junction to the branches, causing distortions in signals sensed at the devices. However, with the compensating circuit connected in the shortest branch, reflections presented from that branch to the junction appear in a form matching reflections presented by the other branches; and the reflections from all branches then cancel at the driver junction. Consequently, signals sensed at the devices have considerably reduced distortions due to the absence of re-reflections. In a preferred embodiment, the compensating circuit consists of a printed circuit trace of predetermined length (representing a transmission line stub with predetermined phase delay characteristics) in series with a point capacitor (or several point capacitors) having predetermined capacitance (determining the shape of the compensating reflections). The compensating circuit, which extends beyond the end of the shortest branch, connects between the end of that branch and reference potential (e.g. ground). The end of the shortest branch is also attached to a device required to sense signals appearing at that point. A new method and polarized bridge device are disclosed for use in analyzing such networks in particular (and for analyzing transmission line effects in general). This method and device permit precise observation and comparison of reflections produced in branches of a network emanating from a common junction, and accurate determination of compensation suitable for modifying such reflections.