Complex filter circuit

Abstract

Disclosed is a complex elliptic filter having an order of three or higher which receives two differential signals that differ in phase from each other by 90 degrees are applied and outputs two differential signals that differ in phase from each other by 90 degrees. The complex filter circuit has internally at least two circuit blocks that include a capacitor connected in series with a coupler (gyrator). The complex filter is a third-order inverse Chebychev filter having an equiripple stopband of 40-dB attenuation amount. Alternatively, the coupler (gyrator) between elliptic capacitors is removed. Alternatively, the elliptic capacitors are made substantially equal to the capacitor arranged in parallel therewith. Alternatively, the gm value of an OTA and the capacitance value are each in an integral ratio represented substantially by a geometric progression of 2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the structure of a first-order gm-C LPF circuit used in a complex filter according to the present invention;

FIG. 2 is a diagram illustrating the structure of a biquad second-order elliptic gm-C LPF circuit for describing a complex filter according to the present invention;

FIG. 3 is a diagram illustrating the structure of a third-order elliptic gm-C LPF circuit for describing a complex filter according to the present invention;

FIG. 4 is a diagram illustrating an equivalent circuit of a complex third-order filter for describing the present invention;

FIG. 5 is a diagram illustrating an equivalent circuit of an admittance-transformed complex third-order filter used in a complex filter circuit according to the present invention;

FIGS. 6A, 6B, 6C and 6D are diagrams illustrating imaginary-resistance implementation circuits;

FIG. 7 is a diagram illustrating the structure of a first complex third-order elliptic gm-C filter circuit according to the present invention;

FIG. 8 is a diagram illustrating the structure of a second complex third-order elliptic gm-C filter circuit according to the present invention;

FIG. 9 is a diagram illustrating the structure of a third complex third-order elliptic gm-C filter circuit according to the present invention;

FIG. 10 is a diagram illustrating the structure of a first complex fourth-order elliptic gm-C filter circuit according to the present invention;

FIG. 11 is a diagram illustrating the characteristic of a complex third-order inverse Chebychev filter circuit according to the present invention and the characteristic of an former third-order inverse Chebychev filter circuit;

FIG. 12 is a diagram illustrating an equivalent circuit of a complex third-order filter from which a complex inductance has been eliminated for describing the present invention;

FIG. 13 is a diagram illustrating an equivalent circuit of an admittance-transformed complex third-order filter used in a complex filter circuit from which a complex conductance has been eliminated according to the present invention;

FIG. 14 is a diagram illustrating the structure of a simplified first complex third-order elliptic gm-C filter circuit according to the present invention;

FIG. 15 is a diagram illustrating the structure of a simplified second complex third-order elliptic gm-C filter circuit according to the present invention;

FIG. 16 is a diagram illustrating the structure of a simplified third complex third-order elliptic gm-C filter circuit according to the present invention;

FIG. 17 is a diagram illustrating the structure of a simplified fourth complex third-order elliptic gm-C filter circuit according to the present invention;

FIG. 18 is a diagram showing one characteristic of a simplified complex third-order inverse Chebychev filter circuit according to the present invention;

FIG. 19 is a diagram illustrating another characteristic of a simplified complex third-order inverse Chebychev filter circuit according to the present invention;

FIG. 20 is a diagram illustrating the structure of a further simplified complex third-order filter circuit from which an elliptic capacitor has been eliminated according to the present invention;

FIG. 21 is a diagram illustrating the structure of a further simplified first complex third-order gm-C filter circuit from which an elliptic capacitor has been eliminated according to the present invention;

FIG. 22 is a diagram illustrating the structure of a further simplified second complex third-order gm-C filter circuit from which an elliptic capacitor has been eliminated according to the present invention;

FIG. 23 is a diagram illustrating the structure of a further simplified third complex third-order gm-C filter circuit from which an elliptic capacitor has been eliminated according to the present invention;

FIG. 24 is a diagram illustrating the structure of a further simplified fourth complex third-order gm-C filter circuit from which an elliptic capacitor has been eliminated according to the present invention;

FIG. 25 is a diagram illustrating a characteristic of a further simplified complex third-order gm-C filter circuit from which an elliptic capacitor has been eliminated according to the present invention;

FIG. 26 is a diagram illustrating the structure of a third-order elliptic RLC ladder filter useful in describing a circuit according to the prior art;

FIG. 27 is a frequency characteristic diagram useful in describing a complex filter characteristic;

FIG. 28 is a diagram illustrating the structure of a third-order elliptic gm-C filter circuit (single-ended) useful in describing the conventional circuit;

FIGS. 29A, 29B and 29C are an explanatory views for describing methods of implementing a complex filter;

FIG. 30 is a diagram illustrating the structure of a complex third-order elliptic gm-C filter circuit (single-ended) according to the prior art;

FIG. 31 is a diagram illustrating an equivalent circuit of a complex third-order elliptic filter using a gyrator;

FIG. 32 is a diagram illustrating the structure of a complex third-order elliptic gm-C filter circuit (fully differential) according to the prior art; and

FIG. 33 is a diagram illustrating the structure of a complex fifth-order elliptic gm-C filter circuit according to the prior art.

Claims

1. A complex filter circuit acting as a complex elliptic filter with an order of three or higher which receives two differential signals that differ in phase from each other by 90 degrees and outputs two differential signals that differ in phase from each other by 90 degrees, said complex filter circuit comprising at least two circuit blocks, each including a capacitor connected in series with a coupler.

2. The circuit according to claim 1, wherein said coupler comprises a gyrator.

3. The circuit according to claim 1, wherein said complex elliptic filter is a third-order inverse Chebychev filter having an equiripple characteristic in a stopband with a prescribed stopband attenuation factor.

4. The circuit according to claim 3, wherein the stopband attenuation factor is about 40 dB.

5. The circuit according to claim 1, wherein a coupler between elliptic capacitors of said complex elliptic filter is removed.

6. The circuit according to claim 5, wherein said capacitor is arranged in parallel with said elliptic capacitor and the capacitance value of said elliptic capacitor is made substantially equal to that of said capacitor.

7. The circuit according to claim 5, wherein said elliptic capacitor is removed.

8. The circuit according to claim 1, wherein a value of mutual conductance gm of an OTA (Operational Transconductance Amplifier) constituting the circuit block and a capacitor value are each in an integral ratio represented substantially by a geometric progression of 2.

9. A complex filter circuit which receives a signal of an in-phase component and a signal of a quadrature component, and which has a circuit on the in-phase side and a circuit on the quadrature side being identically constructed, each circuit comprising: a first OTA (Operational Transconductance Amplifier) which receives differential signals differentially;a second OTA which receives differential outputs of said first OTA differentially;a first capacitor connected between commonly connected differential outputs of said first and second OTAs;a third OTA having differential inputs connected via second capacitors of positive and negative phases to respective ones of the commonly connected differential outputs of said first and second OTAs; anda third capacitor connected between differential inputs of said third OTA;the differential outputs and differential inputs of said third OTA being commonly connected to differential output terminals;wherein said complex filter circuit further comprises first to sixth OTA pairs as a coupler of the in-phase and quadrature sides, each OTA pair having two terminals at which differential inputs of one OTA thereof and differential outputs of the other OTA thereof are respectively connected, and another two terminals at which differential outputs of the one OTA and differential inputs of the other OTA are respectively connected;the two terminals of said first OTA pair are connected to both ends of the first capacitor on the in-phase side, and the two other terminals of said first OTA pair are connected to both ends of the first capacitor on the quadrature side;the two terminals of said second OTA pair are respectively connected to one end of one of said first and third capacitors on the in-phase side via a capacitor and to one end of the other of said first and third capacitors on the in-phase side directly, and the other two terminals of said second OTA pair are respectively connected to one end of one of said first and third capacitors on the, quadrature side via a capacitor and to one end of the other of said first and third capacitors on the quadrature side directly;the two terminals of said third OTA pair are respectively connected to the other end of one of said first and third capacitors on the in-phase side via a capacitor and to the other end of the other of said first and third capacitors on the in-phase side directly, and the other two terminals of said second OTA pair are respectively connected to the other end of one of said first and third capacitors on the quadrature side via a capacitor and to the other end of the other of said first and third capacitors on the quadrature side directly;the two terminals of said fourth OTA pair are connected to both ends of the positive-phase second capacitor on the in-phase side and the other two terminals are connected to both ends of the positive-phase second capacitor on the quadrature side;the two terminals of said fifth OTA pair are connected to both ends of the negative-phase second capacitor on the in-phase side and the other two terminals are connected to both ends of the negative-phase second capacitor on the quadrature side; andthe two terminals of said sixth OTA pair are connected to both ends of the third capacitor on the in-phase side and the other two terminals of said sixth OTA pair are connected to both ends of the third capacitor on the quadrature side.

10. A complex filter circuit which receives a signal of an in-phase component and a signal of a quadrature component, and which a circuit on the in-phase side and a circuit on the quadrature side being identically constructed, each circuit comprising: a first OTA (Operational Transconductance Amplifier) to which differential signals are differentially input;a second OTA to which differential outputs of said first OTA are differentially input;a first capacitor connected between commonly connected differential outputs of said first and second OTAs;a third OTA having differential inputs connected via second capacitors of positive and negative phases to respective ones of the commonly connected differential outputs of said first and second OTAs; anda third capacitor connected between differential inputs of said third OTA;the differential outputs and differential inputs of said third OTA being commonly connected to differential output terminals;wherein said complex filter circuit further comprises first to fourth OTA pairs as a coupler of the in-phase and quadrature sides, each OTA pair having two terminals at which differential inputs of one OTA thereof and differential outputs of the other OTA thereof are respectively connected, and another two terminals at which differential outputs of the one OTA and differential inputs of the other OTA are respectively connected;the two terminals of said first OTA pair are connected to both ends of the first capacitor on the in-phase side, and the two other terminals of said first OTA pair are connected to both ends of the first capacitor on the quadrature side;the two terminals of said second OTA pair are respectively connected to one end of one of said first and third capacitors on the in-phase side via a capacitor and to one end of the other of said first and third capacitors on the in-phase side directly, and the other two terminals of said second OTA pair are respectively connected to one end of one of said first and third capacitors on the quadrature side via a capacitor and to one end of the other of said first and third capacitors on the quadrature side directly;the two terminals of said third OTA pair are respectively connected to the other end of one of said first and third capacitors on the in-phase side via a capacitor and to the other end of the other of said first and third capacitors on the in-phase side directly, and the other two terminals of said second OTA pair are respectively connected to the other end of one of said first and third capacitors on the quadrature side via a capacitor and to the other end of the other of said first and third capacitors on the quadrature side directly; andthe two terminals of said fourth OTA pair are connected to both ends of the third capacitor on the in-phase side and the other two terminals are connected to both ends of the third capacitor on the quadrature side.

11. The circuit according to claim 10, wherein said second capacitors of positive and negative phases are removed.

12. The circuit according to claim 9, wherein values of mutual conductance of said first, second and third OTAs are set to be equal, and each mutual conductance of said second and third OTA pairs coupling said first and third capacitors is made a whole-number multiple of mutual conductances of said first, second and third OTAs.

13. The circuit according to claim 10, wherein values of mutual conductance of said first, second and third OTAs are set to be equal, and each mutual conductance of said second and third OTA pairs coupling said first and third capacitors is made a whole-number multiple of mutual conductances of said first, second and third OTAs.

14. The circuit according to claim 12, wherein the capacitance value of a capacitor connected to one end of one of said first and third capacitors on the in-phase side and to one end of one of said first and third capacitors on the quadrature side in said second OTA pair and the capacitance value of a capacitor connected to the other end of one of said first and third capacitors on the in-phase side and to the other end of one of said first and third capacitors on the quadrature side in said third OTA pair are made a whole-number fraction of capacitance values of said first and third capacitors that have been set to be equal to each other.

15. The circuit according to claim 13, wherein the capacitance value of a capacitor connected to one end of one of said first and third capacitors on the in-phase side and to one end of one of said first and third capacitors on the quadrature side in said second OTA pair and the capacitance value of a capacitor connected to the other end of one of said first and third capacitors on the in-phase side and to the other end of one of said first and third capacitors on the quadrature side in said third OTA pair are made a whole-number fraction of capacitance values of said first and third capacitors that have been set to be equal to each other.