1 2
carried out by the use of zero forcing algorithm without
EQUALIZER MODIFYING A PHASE OF A GAIN any superfluous circuitry.
CONTROL SIGNAL TO CARRY OUT An equalizer to which this invention is applicable is
EQUALIZATION responsive to an input signal subjected to modulation at
5 a modulation rate for producing an output signal. The BACKGROUND OF THE INVENTION input signal is accompanied by a carrier wave of a carThis invention relates to an equalizer for use in equal- "er frequency. The equalizer includes delay means izing an input signal of an intermediate frequency band responsive to the input signal for successively delaying in a digital radio transmission system. the input signal in relation to the modulation rate to In general, an equalizer of the type described is sup- 10 produce a central tap signal and additional tap signals, plied through a transmission path with an input signal control signal generating means for generating a central subjected to modulation at a modulation rate. The mod- control signal and complex control signals which are in ulation may be, for example, quadrature amplitude one-to-one correspondence to the central and the addimodulation. Degradation of a characteristic in the trans- tional tap signals, respectively, and output signal promission path and interruption in the transmission path 15 ducing means responsive to the central and the addigive rise to a distortion of a waveform of the input tional tap signals and to the central control signal for signal. The equalizer serves to equalize the distortion of producing the output signal. The complex control sigthe waveform, as well known in the art. nals have different phases relative to the central control A conventional transversal equalizer comprises a signal. According to this invention, the equalizer corntransversal filter having a central tap, a plurality of taps 20 prises phase modifying means for modifying a specific on both sides of the central tap, and a plurality of delay pnase of one of the complex control signals into a modiunits which are connected between two adjacent ones fle(i phase in consideration of the modulation rate and of the taps and each of which provides a predetermined t{,e carrjer frequency, delay which is substantially equal to a reciprocal of the
modulation rate. The input signal is derived from the 25 BRIEF DESCRIPTION OF THE DRAWING central tap and the other taps as a central tap signal and FIG j snows a block diagram of a conventional additional tap signals, respectively. The central and the equalizeradditional tap signals are multiplied by a central control FIQ 2'shows a Wock dia of an ^ accord. signal and complex control signals supphed from a con- m tQ a fim embodiment of this mvention. tool signal circuit and are sent as gam con rolled signals 30 mQ g ^ for ^ ^ ...
to a pair ol adder circuits. Anyway, equalization of the .. r., ,. .,, . . , . _ r
. . ,. ■ , .I. t it-it J ation of the equalizer illustrated m FIG. 2; input signal is earned out in the transversal filter and an . , M ., , .. r .. , r J • i • A A fi* 4. * • i FIG. 4 shows a block diagram of an equalizer accordequalized signal is produced as a filter output signal. ^ , .. ° , x,. . M ^.
In order to produce the central and the complex mgoa second embodiment of this invention; control signals, zero forcing algorithm is often used in 35 . FIG- 5 shows a block diagram of an equalizer accordthe transversal equalizer. With the zero forcing algo- m8 t0 a tlurd embodiment of this invention; rithm, each of the additional tap signals should fall FIG. 6 shows a block diagram of an equalizer accordwithin a predetermined range relative to the central tap mS t0 a fourth embodiment of this invention; signal on a vector plane. The predetermined range is, mG-7 shows a block diagram of an equalizer accordfor example, between minus ir/2 and plus It/2, both 40 lnS to a fifth embodiment of this invention; and exclusive. Otherwise, the transversal equalizer can FIG; 8 shows a Clrcult diagram of a multiplier illusnever carry out desired operation. trated in FIG. 7.
Phases of the other tap signals are often located out- DESCRIPTION OF THE PREFERRED
side of the predetermined range in dependency on a EMBODIMENT relationship between a carrier frequency of the input 45
signal and the predetermined delay. Referring to FIG. 1, a conventional equalizer will be
In another conventional transversal equalizer pro- described for a better understanding of this invention,
posed in U.S. Pat. No. 4,453,256 issued to T. Ryu and Tne illustrated equalizer is supplied with an input signal
assigned to the instant assignee, supplementary delay IN of an intermediate frequency band. The input signal
lines are added to a transversal filter to give supplemen- 50 IN is assumed to be subjected to quadrature amplitude
tary delays to a preselected one of the additional tap modulation at a predetermined modulation rate. Thus,
signals and to a preselected one of the gain controlled the input signal is accompanied by a carrier wave of a
signals. With this structure, it is possible to locate each preselected frequency modulated at the predetermined
phase of the preselected tap signal and the preselected modulation rate.
gain controlled signal within the predetermined range. 55 The equalizer comprises a transversal filter 20 respon
The proposed equalizer, however, becomes intricate sive to the input signal IN for producing an output
in structure because superfluous supplementary delay signal OUT which is also subjected to quadrature am
lines are added to the equalizer. In addition, inasmuch as plitude modulation like the input signal but which is
each length of the supplementary delay lines should equalized by the transversal filter 20. The illustrated
precisely be controlled or adjusted in order to locate 60 transversal filter 20 has three taps. A central one of the
each phase within the predetermined range. Such ad- three taps may be called a central or a first tap 21. The
justment of each length requires much labor and time. remaining ones of the taps are depicted on the lefthand
TM~„r*„*r ^ T»„r^»^,x^r and the righthand sides of the central tap 21 and may be
SUMMARY OF THE INVENTION called JQnd ^ third taps 22 md £ respectivyeIy.
It is an object of this invention to provide an equalizer 65 The second and the third taps 22 and 23 may often be
which is simple in structure. referred to as first and second additional taps.
It is another object of this invention to provide an First and second delay units 26 and 27 are placed
equalizer of the type described, wherein equalization is between the second and the first taps 22 and 21 and
