VARIABLE SWITCHED-CAPACITOR FILTER CAPABLE OF CHANGING THE FILTER CHARACTERISTIC THEREOF

Abstract: The variable switched-capacitor filter (VSCF) comprises an input terminal (101), an output terminal (104) and first and second variable switched-capacitor filter blocks (102 and 103, resp.) each having a plurality of capacitors. In each of the filter blocks (102, 103) at least one of the filters is selected in response to a control signal for changing the filter characteristic based on the selected capacitor or capacitors. A first switch (SA) delivers during a first state a first output signal from the first variable switched-capacitor filter block (102) to the output terminal (104), and during a second state, prohibits the first output signal from reaching the output terminal (104). The function of the second switch (SB) is essentially opposite to that of the first switch (SA). The selection of at least one capacitor in the first and the second variable switched-capacitor filter block (102 and 103, resp.) is performed during the second and the first state, respectively. The VSCF accomplishes automatic line equalization without being affected by waveform distortions which result from switching the filter characteristic.

Description: The present invention relates to a switched-capacitor filter (SCF) including switches, capacitors and an operational amplifier or amplifiers and, more particularly, to a variable switched-capacitor filter (VSCF) capable of changing the filter characteristic thereof. A typical application of a VSCF is to an automatic line equalizer installed on a digital transmission path. Such a VSCF can readily be manufactured in a large-scale integrated circuit configuration so that the automatic line equalizer using the VSCF can be provided in desirably small dimensions. An example of such automatic line equalizers implemented by conventional VSCFs is disclosed in a paper entitled "A CMOS Switched-Capacitor Variable Line Equalizer", by T. Suzuki et al in the IEEE Journal of Solid-State Circuit, Vol. SC-18, No. 6, December 1983, pages 700 to 706. The disclosed conventional VSCF, however, has disadvantages that substantial waveform distortions take place at the time of selective capacitor switching because the variable filter characteristic of the VSCF is discrete. The distortions make error rate in data transmission great. In a two-wire communication system, for example, a transmitting mode and a receiving mode must be set up alternately with each other so that level detection is effected during the receiving mode operation and selective capacitor switching in each capacitor group is effected during the transmitting mode operation (in which a √f equalizer implemented by an SCF is not used). This frees the equalizer output from the influence of the waveform distortions attributable to the selective capacitor switching. In a four-wire communication system, however, the transmitting mode and the receiving mode occur simultaneously so that the SCF √f equalizer is always in operation. As a result, the equalizer output inevitably has the waveform distortions due to the selective capacitor switching. DE-A-29 50 433 describes an electronic circuit comprising at least two parallel-connected selective circuits, wherein the input signal of the electronic circuit is supplied to the input terminals of the selective circuit, wherein the output terminals of the selective circuits are connected to a switch means that delivers either the output of the first or the second selective circuit as the output signal of the electronic circuit. The object of this document is to sample an input signal of the frequency f e using firstly, the first selective circuit and secondly, the second selective circuit but in such a manner that the samples generated by the selective circuit are phase-displaced over the samples generated by selective circuit. By using two parallel-connected selective circuits the input signal can be sampled four times per period. Each selective circuit comprises switched-capacitors which are either connected to the input signal or to a reference voltage (ground) via switches A and B. The switches A and B of the first and the second selective circuit, respectively, are alternately closed and opened at a sampling frequency f e , so that during the time when one selective circuit is connected via switch to the output of the electronic circuit a changeover of the respective switch (A) occurs in order to sample the input signal. For changing the properties of the electronic circuits, such as the band widths or the amplification, each selective circuit may comprise a plurality of capacitors (C₀-C n ) which are connected in parallel and may be selected by respective switches (S₁-S n ). ELECTRONICS LETTERS, Vol. 16, No. 16, July (1980), pages 613/614, Hitchin Herts., GB; K. MARTIN, et al. "Easing Prefiltering Requirements of S.C. Filters" describes a first order switched-capacitor filter which includes at the output a sample-and-hold circuit to prevent signal feed-through between sampling periods. One object or the present invention is, therefore, to provide a variable SCF free from the above-mentioned disadvantages of the prior art VSCF. This object is achieved with a switched-capacitor filter comprising the features of the claim. The above and other objects, features and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram of a first embodiment of the present invention; Figs. 2A and 2B show waveforms of signals  A and  B , respectively; Fig. 3 is a circuit diagram of a VSCF block; Figs. 4A and 4B show waveforms of signals ₁ and ₂, respectively; Fig. 5 shows curves representative of a frequency-to-gain characteristic of the VSCF block; Fig. 6A is a circuit diagram of a second embodiment of the present invention; Fig. 6B is a circuit diagram of a signal generator; Fig. 7 is a block diagram of a line equalizer to which the present invention is applied; Fig. 8 is a circuit diagram of a prefilter; Fig. 9 is a circuit diagram of a roll-off filter; Fig. 10 is a circuit diagram of a peak detector or a data detector; Fig. 11 is a block diagram of a control signal generator; and Fig. 12 shows a timing chart demonstrating the operation of the line equalizer shown in Fig. 7. In the drawings, the same structural elements are represented by the same reference numerals. Referring to Fig. 1, a first embodiment of the present invention comprises input terminals 101, 105, 106, 107 and 108, a signal output terminal 104, a first VSCF block 102, and a second VSCF block 103. The terminal 101 is connected to the blocks 102 and 103, the terminal 105 to the block 102, and the terminal 106 to the block 103. In each of the blocks 102 and 103, at least one of a plurality of capacitors is selected responsive to a control signal S₁₀₅ or S₁₀₆ applied thereto through the terminal 105 or 106, for setting up a specific filter characteristic. Meanwhile, an input signal is applied to both the blocks 102 and 103 via the terminal 101. The block 102 is connected to a switch section S A which, during a first state, passes an output signal of the block 102 therethrough to the terminal 104 and, during a second state, does not pass it. On the other hand, the block 103 is connected to a switch section S B which, during the second state, passes an output signal of the block 103 therethrough to the terminal 104 and, during the first state, does not pass it. In the embodiment, the sections S A and S B each may be constructed by simple switch elements. Referring to Fig. 3, the block 102 is made up of an operational amplifier (op amp) 1, a capacitor 2, a first capacitor group 3, a second capacitor group 4, a third capacitor group 5, and switches S₁ and S₂. The capacitor group 3 has the construction in which four capacitors 31, 32, 33 and 34, whose both ends are connected to switches S₁₁, S₁₂, S₁₃ and S₁₄, respectively, are connected in parallel to one other. The other capacitor groups are constructed in the same manner as the capacitor group 3. Specifically, in the capacitor group 4, capacitors 41, 42, 43 and 44 are connected in parallel, while switches S₁₁, S₁₂, S₁₃ and S₁₄ are connected to opposite ends of the capacitors 41, 42, 43 and 44, respectively in the capacitor group 5, capacitors 51, 52, 53 and 54 are connected in parallel, while switches S₁₁, S₁₂, S₁₃ and S₁₄ are connected to opposite ends of the capacitors 51, 52, 53 and 54, respectively. The switches S₁ are opened and closed in response to a clock signal ₁ shown in Fig. 4A, and the switches S₂ are opened and closed in response to a clock signal ₂ shown in Fig. 4B. Each switch is closed when the associated clock signal ₁ or ₂ is a higher voltage and is opened when it is a lower voltage. The clock signals ₁ and ₂ have the same clock frequency, but their higher voltage periods do not overlap each other. The switches S₁₁ to S₁₄ are adapted to select their associated capacitors. Selection of any of the capacitors is effected by applying a selection signal having a higher voltage to the switches associated with the desired capacitor. The other block 103 has exactly the same construction as the block 102. Assuming that the capacitor 2 has a capacitance C₂, the capacitors 31 to 34 have capacitances C₃₁ to C₃₄, respectively, the capacitors 41 to 44 have capacitances C₄₁ to C₄₄, respectively, and the capacitors 51 to 54 have capacitances C₅₁ to C₅₄, respectively, then the transfer function H(Z) (where Z equals e jωT , ω represents an angular frequency, and T represents a sampling period) is expressed as: (i = 1 through 4) A specific example of the frequency-to-gain characteristic of the VSCF block is shown in Fig. 5. In Fig. 5, curves #1, #2, #3 and #4 represent characteristics which result from closing of the switches S₁₁ to S₁₄, respectively. Labeled f N (= l/T) in Fig. 5 is the Nyquist frequency. Referring again to Fig. 1, the sections S A and S B are supplied with signals  A and  B shown in Figs. 2A and 2B, respectively. When the signals  A and  B are the higher voltage and the lower voltage, respectively, the above-mentioned first state occurs. Contrary to this, in the case where the signals  A and  B are the lower voltage and the higher voltage, respectively, the above-mentioned second state occurs. The signals  A and  B have the same clock period t₂ which is longer than the clock period t₁ of the signals ₁ and ₂. The selection of the capacitor or capacitors in the block 102, i.e., the specific filter characteristics, is carried out during the second state. Likewise, the selection in the block 103 is carried out during the first state. It will therefore be seen that a transitional phenomenon due to switching of the capacitors inside the block 102 or 103 does not appear at the terminal 104. Referring to Fig. 6A, a second embodiment comprises sections S A and S B and a sample and hold circuit 113. The rest of the construction shown in Fig. 6A is identical with the first embodiment. As shown, the section S A comprises a capacitor 400, and switches S₁₁₁ and S₁₂₁, while the section S B comprises a capacitor 401, and switches S₁₁₂ and S₁₂₂. Referring to Fig. 6B, a signal generator 115 is for generating signals to control the switches. The generator 115, to which the signals ₁, ₂,  A and  B shown in Figs. 2 and 4 are applied, produces a signal ₁₁₁ from an OR gate 200, a signal ₁₁₂ from an OR gate 201, a signal ₁₂₁ from an AND gate 300, and a signal ₁₂₂ from an AND gate 301. Specifically, when the signal  A is the higher voltage and the signal  B is the lower one, there holds a condition wherein ₁₁₁ = ₁, ₁₂₁ = ₂, and ₁₁₂ are the higher voltage and ₁₂₂ is the lower voltage. When the signal  A is the lower voltage and the signal  B is the higher voltage, there holds a condition wherein ₁₁₂ = ₁, ₁₂₂ = ₂, and ₁₁₁ are the high voltage and ₁₂₁ is the lower voltage. The switches S₁, S₂, S₁₁₁, S₁₁₂, S₁₂₁, and S₁₂₂ are opened and closed in responce to the signals ₁, ₂, ₁₁₁, ₁₁₂, ₁₂₁, and ₁₂₂, respectively. In this construction, when the signal  A is the higher voltage, the output of the block 102 is sampled in the circuit 113, while the output of the block 103 is simply stored in the capacitor 401. This relationship will be reversed when the signal  B is the higher voltage. Referring to Fig. 7, an automatic line equalizer, to which the present invention is applied, comprises a prefilter 109 for removing a fold over nouse, automatic equalizer sections 702 and 703 (identical in construction with the blocks 102 and 103) each for equalizing a line loss, the switch sections S A and S B , the sample and hold circuit 113, a rolloff filter 114 for reducing intersymbol interference of data by wave-shaping, a peak detector 111, a control signal generator 112, and a data detector 116. Referring to Fig. 8, the prefilter 109 is made up of resistors 1091 and 1092 having resistance values R₁ and R₂, respectively, capacitors 1093 and 1094 having capacitance values C₁₀₉₃ and C₁₀₉₄, respectively, and an op amp 1095. The transfer function H(S) of the filter 109 (where S = jω) may be produced by: Referring next to Fig. 9, the filter 114 is composed of an ordinary second-order low-pass SCF. Assuming that capacitors 1141 to 1148 in Fig. 9 have capacitance values c₁₁₄₁ to C₁₁₄₈, respectively, the transfer function H(Z) of the filter 114 may be expressed as: Referring to Fig. 10, each of the detector 116 and the detector 111 comprises a comparator 1111 for comparing an input signal applied to a terminal 1114 with a reference voltage applied to a terminal 1113, and a flip-flop 1112 for producing a predetermined signal in response to an output of the comparator 1111 in synchronism with a sampling clock, which is applied to a terminal 1115. Referring to Fig. 11, the generator 112 includes an inverter 1121, an up-down counter 1122, and a decoder 1123. The equalizer shown in Fig. 7 changes its characteristic as will be described with reference to Fig. 12 as well. The detector 116 determines that an output of the filter 114 is a logical "1" or a logical "0" (hereunder referred to simply as "1" and "0", respectively.) Specifically, the comparator 1111 of the detector 116 compares an output V₁₁₄ of the filter 114 applied to the terminal 1114 with a threshold voltage V T and, if V T <V₁₁₄, produces "1" and, if V T >V₁₁₄, produces "0". The output of the comparator 1111 is sampled by the flip-flop 1112 at predetermined sampling timings. The sampling timings may be obtained from timing pulses provided by applying an output of the filter 114 to a tank circuit or like timing extractor means (not shown). The timing pulses have a period t₃ which is longer than the above-mentioned period t₁. The output V₁₁₄ of the filter 114 is also routed to the detector 111 and compared thereby with a reference voltage V ref . The reference voltage V ref is adapted to provide a reference level. The output of the detector 111 is applied to the generator 112. In the generator 112, the counter 1122 is decremented by one if the output of the detector 111 is "1" and incremented by one if it is "0", in synchronism with the data detection output from the detector 116 coming in through a terminal 1125. Counts "1" through "4", which are the contents of the counter 1122, correspond in one-to-one relation with the characteristic curves #1 through #4 of Fig. 5. The decoder 1123, decoding an output of the counter 1122, selects any of switches in the section 702 if the signal  B is high voltage and any of switches in the equalizer section 703 if the signal  A is the higher voltage. That is, the decoder 1123 delivers a signal for closing one of the switches S₁₁ through S₁₄ over one of signal lines 1124-1 through 1124-4. In this manner, automatic line equalization is accomplished without being affected by waveform distortions which result from switching of filter characteristic.

Claims: A switched-capacitor filter comprising: first and second variable switched-capacitor filter means (102 and 103, resp.), each having sampling switch means (S₁, S₂) responsive to a sampling clock for performing a sampling operation and a plurality of capacitors (3, 4 and 5) whose at least one is selected responsive to a control signal (S₁₀₅ and S₁₀₆, resp.) and each changing a filter characteristic thereof based an the selected capacitor or capacitors, the first and second filter means (102, 103) being applied with an input signal coming in through a common input point (101); first switching means (S A ) for, during a first state, delivering a first output signal from said first variable switched-capacitor filter means (102) to an output point (104) either directly or via a sample and hold circuit and, during a second state, prohibiting the first output signal from reaching said output point (104); and second switching means (S B ) for, during said second state, delivering a second output signal from said second variable switched-capacitor filter means (103) to said output point (104) directly or via said sample and hold circuit and, during said first state, prohibiting the second output signal from reaching said output terminal (104); the switched-capacitor filter characterized in that, said selection of at least one capacitor in said first variable switched-capacitor filter means (102) is performed during said second state and said selection of at least one capacitor in said second variable switched-capacitor filter means (103) is performed during said first state and a period of the switching between said first and second states is longer than periods of the sampling clock of said sampling switch means (S₁, S₂), such that the effect of transitional phenomena caused by switching of the capacitor or capacitors (3, 4 and 5) does not appear in the output point.

Inventors: NAKAYAMA, KENJI
Applicants: NEC CORPORATION
Application year: 1985
Main classification: H03H19
IPC codes: H04B3, H03H19