Patent GB-1373062-A (GB)
Title: Method and apparatus for equalizing electrical signals
IPC: H04L25/03
Filed: 1972-03-07
Abstract: 1373062 Adaptive equalizers INTERNATIONAL BUSINESS MACHINES CORP 7 March 1972 [24 May 1971] 10459/72 Heading H4R In an equalizer for a system producing both pre-pulse and post-pulse interference the signal is applied first to an equalizer section, preferably of the type of Specification 1,357,609, having a number of cascaded stages each stage having a plurality of adjustable tap settings the equalizer section being capable of removing pre-pulse interference, after which the inverse of the postpulse interference is derived from the signal equalized by the first equalizer section, and this is used to adjust tap settings in a recursive equalizer to cancel the post-pulse interference. Fig. 1B shows the first equalizer section comprising a number of transversal equalizer stages 10. For setting up, a test sequence of isolated pulses is transmitted and the multipliers 14 are set to zero, except for # 0 <SP>(1)</SP>, # 0 <SP>(2)</SP> ÀÀÀ # 0 <SP>(n)</SP> which are set to 1. Circuit (2) averages the received pulses to remove the effects of random noise, and an average received pulse is applied to the equalizer input via an A.G.C. circuit 3. Due to the initial multiplier settings the signal passes straight through to the switch 23, being only subject to delay. With switch 23 on position 25 the pulse, 1, plus its distortion -A 0  is subtracted from 2 in summer 17 to provide the signal 1+A 0 . On the first passage through the equalizer this signal is applied via tap adjuster 16 to set the values 1+A 0  into the multipliers #<SP>(1)</SP>, so that, when the next pulse is applied to the equalizer, as the main component of the input pulse travels along the delay device 11 in the first transversal equalizer stage 10, it cancels, via the multipliers 14 and adder 13, the pre-pulse distortion feeding to the adder 13 via the multiplier # 0 <SP>(1)</SP>, which is set to a factor 1. The pre-pulse, and post-pulse, distortion feeding through the remainder of the multipliers # -1 <SP>(1)</SP> to # -N <SP>(1)</SP> is not, however, cancelled, but is of relatively low level, and this distortion is used, in a similar manner to that described above to set the multipliers #<SP>(2)</SP> of the second stage of the equalizer. Each subsequent iteration sets the subsequent equalizer stage until after the nth stage has been set the pre-pulse distortion has been reduced to a very low value. For the following pulse the switch 23 is switched to position 26 which feeds the signal from the front end equalizer 1b to the rear end equalizer Ic, Fig. 1C. At equalizer 1c the signal, comprising substantially only the main pulse 1 and the post pulse distortion -A r <SP>(n)</SP> is subtracted from 1 in adder 27 to leave only the inverted distortion components +A r <SP>(n)</SP> and these are set into the multipliers 14 # 1 <SP>(n)</SP> to # N <SP>(n)</SP>. Switch 23, Fig. 1B, is then operated to position 28 ready to receive communication pulses from the line. Equalization of the post pulse distortion is effected in equalizer 1c by feeding signals through the chain of adders 29 and delay elements 30 to a threshold circuit 31 which only responds to the main pulse. On response to the main pulse, multipliers # 1 <SP>(n)</SP> to # N <SP>(n)</SP> are simultaneously activated to feed to adders 29 signals which cancel the post-pulse response samples at those points. Between each iteration for setting up the multipliers of the equalizers 1B and 1C the A.G.C. circuit 3 can be adjusted to maintain the main pulse amplitude at the equalizer output at unity, or alternatively the settings of the multipliers # 0  can be adjusted to maintain the main pulse amplitude at unity.