Abstract: FM receivers for communications at UHF frequencies often use automatic frequency control (AFS) or phase lock to precisely control a local oscillator. By causing the local oscillator to track errors and slow variations in the frequency of the received signal, these control loops suppress DC and low-frequency modulation components in the intermediate frequency signal and interfere with demodulation of low frequency information, such as digital signalling data. This invention permits recovery of the full modulation spectrum with flat frequency response. A conventional FM demodulator recovers the high frequency modulation components; the frequency control signal for the local oscillator provides the low frequency components. Combining the components with proper weighting produces recovered modulation with bandwidth limited only by the IF bandpass filter of demodulator.

Abstract: A trunked radio subscriber requesting dispatch service transmits a request for a channel and immediately begins to speak without waiting to receive permission to access the channel. The subscriber radio locally records the information to be transmitted and, upon being assigned a channel, begins to reproduce and transmit the information. Recording and reproduction provides delay to span the time between request for and assignment to a channel and eliminates the need for the subscriber to wait to transmit or to quickly seize a channel assigned after an initial refusal. This enhances overall response time and increases time available for transmitting information signals. During interconnect calling, a trunked system alternately assigns inbound and outbound frequencies only as required for back-and-forth communication between simplex radio and duplex wireline subscribers. Voice buffering allows the parties to begin talking during the delay between request for and assignment of frequencies.

Abstract: A modulation limiter that eliminates over-deviation otherwise caused by asymmetrical modulation signals in a frequency modulated (FM) transmitter. The output signal from a conventional modulation clipper is integrated, multiplied by a gain constant, and subtracted from the input, which causes the clipper output to have zero average and a high-pass frequency response with a time constant determined by the feedback gain. The zero average value allows the clipped modulation signal to be coupled to a modulator without over-deviation caused by shifts in the average value of the clipped signal, and the high-pass response can be used to give the modulation signal pre-emphasis.

Abstract: A logic interface signal input with a high degree of electrical noise immunity, particularly suited for use in an automative environment for detecting an on/off control signal multiplexed with an information signal such as a voice audio signal. The invention provides a fast logic output signal in response to the level of the control signal, which is suitable for driving higher speed circuitry and which permits detection to be highly immune from electrical transients or variations in power supply voltage. The invention permits the information signal to be combined with the control signal through polarized means, such as an electrolytic capacitor.

Abstract: A predistortion network preceding a continuous-time active MOSFET-C lowpass filter cancels the inherent distortion of the filter over a large portion of its passband. The lowpass filter has resistive feedback and input devices whose ratio controls its transfer function at DC and low frequencies. The predistortion network has resistive feedback and input devices that control its transfer function and that match the input and feedback devices, respectively, of the active filter. Cancellation of distortion results in low distortion operation for signals in the frequency range from DC to a substantial fraction of the passband. As the filter response rolls off at higher frequencies, imperfect cancellation results in distortion performance similar to that without predistortion. The invention reduces large signal distortion of active MOSFET-C filters, which enhances their use in monolithic system with switched capacitor circuits.

Abstract: A detector locates a shift register sequence within a digital data stream by correlating the data stream with a sequence generated locally from a portion of the data stream. Error correction circuitry estimates errors that may have corrupted the sequence during transmission across a noisy channel and corrects them to the extent possible. The data stream and local sequence are correlated during an interval that is shifted either ahead or behind the portion of the error-corrected data stream used to initialize the local sequence generator, thereby avoiding the region during which short-term correlation between the data stream and local sequence would otherwise cause false indications of detection when only noise or random data is being received.

Abstract: An improved power controller and protection circuit to regulate the RF output power of a radio transmitter and to protect its RF amplifier from overload damage. The controller features a floating input, voltage controlled current source with high common-mode rejection to provide feedback of the DC input current drawn by the RF power amplifier. The controller can use an arbitrary reference voltage, which allows its application universally with power amplifiers designed for different rated output powers and frequency ranges.

Abstract: A receiver for frequency modulated signals, having down-conversion to a baseband, zero intermediate frequency for selectivity, followed by up-conversion to a non-zero IF for amplification, limiting, and demodulation.A phase-lock loop locks the frequency of the down-conversion source to the center frequency of the signal coupled to the baseband IF, converting the signal to precisely zero frequency. This avoids the beat note often found in direct conversion systems. The phase-lock loop additionally provides inherent demodulation of the FM signal.The received FM signal is coupled to the baseband IF through a radio frequency amplifier for improved sensitivity and local oscillator isolation, or through a first IF comprising a mixer, local oscillator, filter, and amplifier.

Abstract: A multiplexing arrangement is described for use in a digital communications system, in which it is desired to multiplex plural user data inputs into a single serial digital communications channel and then to decode the data to the appropriate user data outputs. The arrangement contemplates the transmission of data in blocks of n.sup.2 bits, and provides framing in the form of a frame alignment word having the value of F for n bits, followed by n-1 subsequences each of n bits duration and each initiated by a framing bit having the value of F-bar, with the last subsequence terminating with a framing bit having the value of F-bar. The implementation is simple and provides rapid acquisition of the framing. In principle, the framing is error free in not being affected by the values assigned to the data.

Abstract: A modified duobinary filter in which an analogue data signal is translated to an intermediate frequency, filtered by a surface acoustic wave filter having a modified duobinary response, and retranslated in the opposite direction to the first frequency translation.