Abstract: A communication network for an area such as an underground mine. A plurality of cascaded base stations (13) are serially interconnected in a ring structure (11) to form a network backbone. One of the base stations (13) at one end of the network backbone forms a network controller (15) for controlling the network. A plurality of portables (17) are adapted for communication with any of the base stations via a common air interface. A backbone network protocol (19) is provided for communicating between the network controller (15) and all of the base stations (13) and between the base stations (13) along the network backbone themselves. A common air interface protocol (21) is provided for communicating between a portable (17) and a base station (13). The other base stations (13) function as slaves relative to the network controller (15).

Abstract: A frequency difference detector (FDD) implemented as a balanced quadricorrelator has an automatic gain control system (68) having inputs capacitively coupled to the quadrature related, baseband frequency paths from the front end mixers (12,14). The a.g.c. system (68) has quadrature related mixers (78,80) coupled to said inputs for frequency up-converting the baseband signals to an intermediate frequency. The IF signals from the mixers are summed in a summing circuit (82) and subsequently squared in a squaring circuit (86). Any ripple in the output of the squaring circuit is removed by low pass filtering to produce a d.c. control signal which is applied to gain controlled amplifiers (22,24) present in said baseband frequency paths in order to render the output (V.sub.o) of the FDD less sensitive to variations in the input signal (V.sub.i).

Abstract: A dual branch receiver consists of first and second branches (12, 14) respectively comprising first and third mixers (16, 20) and second and fourth mixers (17, 21). An input terminal (10) for a signal having a carrier frequency (.omega..sub.c) is connected to the first and second paths. A first local oscillator frequency (.omega..sub.o) is supplied in quadrature to the first and second mixers (16, 17), where .omega..sub.c -.omega..sub.o =.DELTA..omega. and .DELTA..omega. is of the order of 2.pi..times.100 radians/sec. A second local oscillator (30) frequency is supplied in quadrature to the third and fourth mixers (20, 21). An output signal is derived by connecting a sum circuit (22) and a difference circuit (24) to the first and second branches. The outputs of the sum and difference circuits (22, 24) are also used to provide gain and phase correction signals. It has been found that if the correction signals are based on signals centered on 2.DELTA..omega. then they are substantially free of ripple.

Abstract: A divider circuit arrangement in which in order to avoid dividing by zero the divisor (V.sub.d) is modified by the addition of an extra signal (X.sub.a) to form a modified divisor V'.sub.d =V.sub.d +X.sub.a and the dividend (V.sub.i) is modified by the addition of the product of the quotient (V.sub.o) and the extra signal (X.sub.a) to form a modified dividend V'.sub.i =V.sub.i +V.sub.o X.sub.a. A particular but not exclusive application of this divider circuit arrangement is in normalizing an ouptut signal from a dual branch receiver (not shown).

Abstract: A dual branch receiver in which the frequency of the first local oscillator (32) is offset by a few hundred hertz from the input carrier in order to provide separate wanted and image components. Gain and phase control are provided to compensate for differences between the two branches (12, 14), the image signal being used to provide the required error signals. However in the event of the modulation frequency being equal to the offset frequency, the image signals cannot be distinguished from the wanted signals thus inhibiting gain and phase control. This drawback is overcome in the receiver in accordance with the present invention by wobbling the frequency provided by the first local oscillator (28). The wobbling frequency is preferably sub-audible so as not to be discernable to the ear.

Abstract: An arrangement for generating an angle-modulated carrier signal of a constant amplitude and continuous phase .phi.(t) comprises a control circuit which in response to a given number of consecutive data symbols of symbol frequency 1/T produces addresses at a clock frequency 4q/T, where q is an integer greater than 1, and further produces phase state numbers for characterizing the value modulo-2.pi. or .phi.(t) at the boundaries of the symbol intervals. Connected to the control circuit is a signal processor comprising a read-only memory for storing values representing cos .phi.(t) and sin .phi.(t) at the clock instants, which values are processed after reading to form the analog angle-modulated signal. An arrangement having a comparatively small capacity of the read-only memory is obtained by providing that storage in this read-only memory is only effected for non-decreasing phases .phi.

Abstract: An arrangement for recovering a clock signal of frequency 1/T from an angle-modulated carrier signal having a modulation index m=0.5 comprises a frequency doubler, a circuit for generating a clock signal component from the frequency-doubled signal, a clock filter for selecting the generated clock signal component and a pulse shaper for producing a clock pulse signal.The arrangement produces a reliable clock signal within a very short time after the start of the signal reception, because the frequency doubler comprises a hard limiter, a first bandpass filter connected thereto and having a Q.ltoreq.10, a balanced modulator connected to the limiter and to the first bandpass filter and a second bandpass filter having a Q.ltoreq.10 tuned to twice the carrier frequency 2f.sub.c ; and because the generating circuit comprises a frequency converter for converting the frequency-doubled signal to a frequency band around a carrier frequency f.sub.

Abstract: An arrangement for generating an analog angle-modulated carrier signal of a substantially constant amplitude in response to data signals having a symbol rate 1/T comprises a read-only memory for storing modulation parameters, an addressing circuit responsive to a predetermined number of consecutive data symbols for producing read addresses with a rate q/T, where q is an integer greater than 1, and a signal processor for processing read values to form the analog angle-modulated signal.

Abstract: A transmitter for transmitting a carrier whose frequency is modulated by pseudo five-level signals. In order to obtain a simple receiver and an improved signal-to-noise ratio, a substantially three-level signal is generated in the transmitter at the detection instants t=(2m-1)T/2 by means of a pre-modulation filter.

Abstract: Transmitter for angle-modulated signals having an input for binary signals, a premodulation filter and a frequency modulation arrangement.In order to improve the error rate of the system of transmitter and receiver a premodulation filter is used having a pulse response h(t): ##EQU1## wherein g(t) is the pulse response of a Gaussian low-pass filter and T the duration of a binary signal element.The postdemodulation filter 6-4 (6-5) has a pulse response of the same general form for optimum results.