Abstract: The present invention provides an efficient point-to-multipoint microwave ATM network including a base station (BS) broadcasting a continuous transmission with a sector antenna. The system uses time division multiplex (TDM) for downstream transmission and time division multiple access (TDMA) for upstream transmission. The downstream transmission consists of ATM cells encapsulated in MAC protocol data units (PDUs) for forward error correction (FEC) and synchronization. Small Subscriber Terminals (STs), including Subscriber Radio Units (SRUs), receive the broadcast and pass it to a Subscriber Access System (SAS) that drops the ATM cells addressed only to them. To allow strong FEC protection and to maintain the same symbol rate as the downstream transmission without sacrificing bandwidth, a modified trellis code modulation technique, which incorporates Reed Solomon coding, is used during upstream transmission.

Abstract: In multiple-channel microwave transmitters and communications systems, such as multi-point video distribution systems operating at frequencies of around 29 GHz or 40 GHz, good frequency stability for each of the channel frequencies is achieved with a feed-back loop including an error detector circuit. The error detector circuit (39,29) is coupled between a sampler and an input circuit of the source. This detector circuit detects any drift or other error in the carrier frequency of the sample from the desired microwave frequency for that channel signal and provides a corrective signal to the input circuit. The part of the feed-back loop comprising at least a part of the detector circuit is common to a group of the channels. Switch means couple the common part of the feed-back loop between the sampler and the source input circuit of each channel, and so permit this common part to be time multiplexed between the respective feed-back loops of the group of channels.

Abstract: An automatic frequency control circuit measures the deviation in frequency of a selected signal from a reference frequency and when the deviation exceeds a selected threshold which is variable, steps the frequency of the selected signal back to the proper frequency. The selected signal is typically an intermediate frequency (IF) signal in a radio receiver. For detection of frequency deviation, an exclusive OR (EX-OR) circuit receives a reference signal having a frequency proportional to a desired IF carrier frequency and an input signal having a frequency proportional to an actual IF frequency. A divider coupled to a voltage controlled oscillator (VCO) used in demodulation of the IF signal provides the input signal. An output signal of the EX-OR circuit includes a component with a frequency proportional to the difference between the actual and desired IF frequencies. A low pass filter isolates that component, and a charge pump generates a control voltage proportional to the frequency of that component.

Abstract: In addition to a first opening (11) forming a primary output from a waveguide cavity (10) of a microwave oscillator, a coupling aperture (12) in a reflective wall (32) provides a secondary output which does not degrade the performance or tuning characteristics of the source. Less than -20 dB of the source power may be coupled out via the aperture (12) and is used in a feed-back loop (12,21,23,24,36) with an harmonic mixer (21) and frequency discriminator (24) to stabilize the source. The mixer comprises a further waveguide section (15) separated from the source cavity (10) by the reflective wall (32). An advantageous mixer assembly comprises a circuit substrate which is mounted across the further waveguide cavity (15) and which carries an inductive probe by which the signal from the coupling aperture (12) is fed to an antiparallel pair of mixer diodes on the substrate. The whole assembly may be bolted together through holes (34).