Abstract: Circuits are described which optimize the signal-to-noise ratio of a signal amplifier by means of active circuits which simulate low-noise resistors. When such "absorbers" or "desorbers," as they are termed, are used in amplifier circuits with signal sources characterized by equivalent thermal noise resistances they provide low-noise means for matching the source and amplifier for optimum noise behavior. In accordance with the invention therefore, it is possible to utilize active circuit elements such as transistors or op-amps in such a way as to realize amplifiers which have better noise figures than conventional amplifiers fabricated of the same circuit elements.

Abstract: An improved homodyne receiver for use in wideband optical communications systems is disclosed. The receiver utilizes quadrature demodulation of the optical signal in the manner of a two-phase receiver. A beam-splitting/phase-splitting optical assembly is used to channelize the received and the local oscillator beams to a pair of optical detectors. Squaring amplifiers in the quadrature channels followed by a summing network provide an output signal which is proportional to the square of the modulation of the input signal. This receiver is especially well-suited to the reception of Pulse Code Modulated optical signals.

Abstract: An improved multiplexer especially adapted for combining a plurality of microwave signal channels for transmission over a common transmission path is disclosed. A single "directional filter" multiplexer structure is utilized to combine contiguous or adjacent channels without resort to odd-even channel separation or additional equalization and/or compensation networks. By purposely extending the bandpass characteristics of preselected channel filters so that either or both of their cutoff frequencies extend substantially past their associated channel band edges and well into the reflection loss regions of the adjacent channel or channels the adjacent channel mutual interaction effect is exploited.

Abstract: Single frequency signaling system which simulates the low level tone during the idle circuit condition even though the radio frequency carriers in the transmission link through a repeater have been turned off. Said system utilizes 2600 Hz detectors having long time constants to turn the transmitters on and off and to connect 2600 Hz oscillators to the receive lines to simulate the idle circuit condition.

Abstract: An improved homodyne receiver for use in wideband optical communications systems is disclosed. The invention provides phase-locked homodyne reception of non-suppressed carrier or suppressed carrier input signals. Novel means are provided in the receiver for generating an error signal to provide phase-locked operation in the presence of noise and frequency excursions of the received signal.