Patent GB-1525706-A (GB)
Title: Automatic passband equalizer for data transmission system
IPC: H03H15/00
Filed: 1975-12-17
Abstract: 1525706 Adaptive equalizers TELECOMMUNICATIONS RADIOELECTRIQUES ET TELEPHONIQUES 17 Dec 1975 [20 Dec 1974] 51626/75 Heading H4R In an adaptive transversal equalizer for equalizing pass band signals at the input of a receiver for phase modulated data signals the input signals and a 90 degree phase shifted version of the input signal are applied to respective transversal filters L and L', Fig. 1, the outputs of which are combined at 10 to provide the equalizer output signal Y(t) to be fed to a detector 2. In order to obtain adjusting signals for the filter, multiplier coefficients a(m), b(m), the equalizer output and a 90 degree phase shifted version thereof are sampled at clock frequency by samplers 12 and 13 and fed to difference circuits 17 and 18 both direct, at 15, 16, and via a one clock period delay 19, 20, and a phase shifter 23, which provides a phase shift proportional to the sum of the phase shift due to the transmitted signal, derived from the detector 2, and the phase shift of the carrier between successive samples at clock frequency. The difference signals, e(nT) and Û(nT) respectively, at the outputs of difference circuits 17 and 18 correspond to the in-phase and quadrature components of the error vector between the actual equalizer output and the estimated correct equalizer output. An equalizer multiplier coefficient updating circuit is provided for each tap of the equalizer filter and comprises a pair of difference circuits 29, 30, fed on one input 31, 32, with the signal on the corresponding tap of the filter, sampled at clock frequency, and on the other input 35, 36, the signal from the following tap of the equalizer after it has been subject, in-phase shifter 39, to a phase shift equal to that in-phase shifter 23, i.e. the phase shift due to the estimated signal plus the phase shift of the carrier between successive sampling instants, so that the outputs, d(nT-mT) and #d(nT-mT) respectively, of the difference circuits 29, 30, are the in-phase and inquadrature components of the error signal between the signal at the respective tap and the estimated correct signal at that tap. This tap error signal is applied to a calculating circuit 41 where the inphase and in-quadrature components are multiplied and cross multiplied by the in-phase and inquadrature components of the equalizer output error signal from difference circuits 17, 18 and the signals combined to give factors e(nT).d(nT-mT) + Û(nT.#d(nT-mT) and e(nT).#d(nT-mT) - Û(nT).d(nT-mT) which are respectively the tap multiplier adjustment factors for the m<SP>th</SP> tap of the in-phase and in-quadrature equalizer filters. These factors are each multiplied by a small constant -# and combined with the existing tap multiplier coefficients, held in stores 6 and 7, to increment the coefficients in the appropriate direction. The phase shifting circuits 23 and 39 are described with reference to Fig. 8 (not shown) and the calculating circuit 41 with reference to Fig. 9 (not shown). Also described are modifications and simplifications of these circuits which can be adopted if the signs only of the various error signals are used in the adjustment, so that the magnitudes are ignored, or if the adjustments are restricted to particular phase changes or phase positions in the transmitted signal, in which latter case the difference circuit 18 can be eliminated. Also described, and shown dotted on the drawings, are the modifications required if the transmission system uses both phase and amplitude modulation of the carrier, a detector 97 being added to supply an estimated amplitude change signal. Also described, with respect to Fig. 18 (not shown) are modifications to adapt the equalizer for systems in which the received carrier may be different from the transmitted carrier due to frequency offset in the transmission, the detector 2 supplying, in addition to the detected phase jump, a phase deviation which modifies the phase shift provided by the shifter 23.