Method and circuit arrangement for multiplying frequency

[0002] The invention relates to a method and a circuit arrangement for frequency multiplication.

[0003] From telecommunications and computer engineering, one knows of analog frequency multiplication methods used for various purposes. Circuit arrangements designed for these various purposes are also known, which multiply the frequencies of sinusoidal and cosinusoidal oscillations. In this context, the circuit expenditure is considerable, particularly when working with multiples, which are not a square (power of two) of the output frequency, since, depending on the particular realization, additional division circuits might even be necessary. Circuits of this kind, designed, for example, as PLL (phase-locked loop) circuits, are described, for example, in the book by U. Tietze, Ch. Schenk, Halbleiterschaltungstechnik [Semiconductor Circuit Engineering], Springer Publishers, 1980. The Chebyshev polynomials of the nth order are defined by the equation T_n(cos(ψ))=cos(nψ).

[0004] The Chebyshev polynomials are described for example in I. Schur, “Arithmetisches über die Teschebyscheffschen Polynome” [Arithmethic Aspects of Chebyshev Polynomials], Collection of Treatises vol. III, pp. 422 to 453, Springer Publishers 1973 which is hereby incorporated by reference herein.

[0005] An object of the present invention is to provide a method and a circuit arrangement for performing analog frequency multiplication using simple and easily combined modules, which will eliminate, in particular, the need for division circuits used in existing methods, when it is required to produce multiples of a fundamental frequency which are not a power of two thereof.

[0006] The present invention provides a method of frequency multiplication, the method including providing a plurality of circuit modules for realizing Chebyshev polynomials of the nth order T_n(x)), the Chebyshev polynomials having arithmetic properties and being defined by T_n(cos(ωt))=cos(nωt). The circuit modules are interconnected to form a modular circuit array or a modular circuit structure using at least one of the relations T_nm(x)=T_n(T_m(x)) and T_n+m(x)=T_n(x)T_m(x)−T_n−m(x). A cosinusoidal oscillation having a first frequency is applied to an input of the circuit module for the Chebyshev polynomial (T_n(x)) so as to generate a cosinusoidal oscillation having a second frequency at an output of the circuit module, the second frequency being a factor of n times the first frequency.

[0007] The present invention also provides a circuit arrangement for frequency multiplication, the circuit arrangement including a first circuit module for realizing a first Chebyshev polynomial T_m(x) defined by T_m(cos(ωt))=cos(mωt), the first circuit module being capable of accepting a first sinusoidal/cosinusoidal oscillation input having a first frequency and outputting a third sinusoidal/cosinusoidal oscillation having an third frequency, the third frequency being a factor of m times the first frequency. A second circuit module is provided for realizing a Chebyshev polynominal T_n(x) defined by T_n(cos(ωt))=cos(nωt), the second circuit module being capable of accepting a second sinusoidal/cosinusoidal oscillation input having a second frequency and outputting a fourth sinusoidal/cosinusoidal oscillation having fourth frequency, the fourth frequency being a factor of n times the second frequency, the second circuit module being connected to the first circuit module to form a modular circuit array or a modular circuit structure using at least one of the relations T_nm(x)=T_n(T_m(x)) and T_n+m(x)=T_n(x)T_m(x)−T_n−m(x).

[0008] The present invention also provides a circuit arrangement for frequency multiplication where the circuit arrangement includes a first circuit module for realizing a first function T_m(x) defined by T_m(x)=(½)((x+(x²−1)^½)^m+(x−(x²−1)^½)^m), m being a rational or a real number, the first circuit module being capable of accepting a first sinusoidal/cosinusoidal oscillation input having a first frequency and outputting a third sinusoidal/cosinusoidal oscillation having an third frequency, the third frequency being a factor of m times the first frequency. A second circuit module is provided for realizing a second function T_n(x) defined by T_n(x)=(½) ((x+(x²−1)^½)ⁿ+(x−(x²−1)^½)ⁿ), n being a rational or a real number, the second circuit module being capable of accepting a second sinusoidal/cosinusoidal oscillation input having a second frequency and outputting a fourth sinusoidal/cosinusoidal oscillation having fourth frequency, the fourth frequency being a factor of n times the second frequency, the second circuit module being connected to the first circuit module to form a modular circuit array or a modular circuit structure using at least one of the relations T_nm(x)=T_n(T_m(x)) and T_n+m(x)=T_n(x)T_m(x)−T_n−m(x).

[0009] An advantage of the method and device of the present invention is that, by using structures derived from Chebyshev polynomials, a frequency multiplication is able to be achieved using simple and easily combined modules or modular circuit structures.

[0010] If a cosinusoidal oscillation is expressed as an input variable in terms of T_n(x), then a cosinusoidal oscillation with an n-fold frequency is obtained at the output. Information on Chebyshev polynomials can be found, for example, in Abramawitz, Stegun: Handbook of Mathematical Functions. The first Chebyshev polynomials are expressed as: T_o(x)=1, T₁(x)=x, T₂(x)=2x²−1, etc. in accordance with T_n+1(x)=2xT_n(x)−T_n−1(x). They can be produced using multiplier circuits and adder or subtracter circuits. To realize any n, the following relations are particularly helpful:

[0011] T_n−m(x)=T_n(T_m(x))

[0012] T_n+m(x)=2·T_m(x)·T_n(x)−T_n−m(x).

[0013] The Chebyshev polynomials, as well as the multiplier circuits and summing circuits, i.e., adder or subtracter circuits required for an implementation in terms of circuit engineering, are able to be realized using integrated circuit engineering, and are then able to perform the most widely varying, desired functions, depending on the external wiring or interconnections. Other functions mentioned here which can be easily realized with such a chip are the synthesis of any desired functional progression by expressing the function as a Chebyshev series, or using the function T_n(x) as an amplifier having a gain n for small x where sin(nx)≈nx, and for uneven (odd) n.

[0014] The present invention will be elucidated in the following on the basis of embodiments depicted in the drawings. The terms specified in the list of reference symbols at the end of this document and the associated reference symbols are used in the Specification, in the Claims, and in the Abstract.

[0015] FIG. 1 shows a circuit arrangement for realizing T_nm(x);

[0016] FIG. 2 shows a circuit a arrangement for realizing T_n+m(x);

[0017] FIG. 3 shows a circuit arrangement for realizing T₂(x); and

[0018] FIG. 4 shows a circuit arrangement for realizing T₃(x).

[0019] First, the mathematical fundamentals of the method and circuit arrangement will be described. According to the present invention, structures derived from Chebyshev polynomials are used, which make it possible to realize any desired frequency multiplication by means of simple and easily combined circuit modules. The Chebyshev polynomials of the n-th order T_n(x) are defined by the equation

[0020] T_n(cos(ψ))=cos(nψ),  (1)

[0021] i.e., if one specifies a cosinusoidal oscillation as an input variable in terms of T_n(x), then a cosinusoidal oscillation with an n-fold frequency is obtained at the output. Information on Chebyshev polynomials can be found, for example, in Abramawitz, Stegun: Handbook of Mathematical Functions which is hereby incorporated by reference herein. The first Chebyshev polynomials are expressed as: T_o(x)=1, T₁(x)=x, T₂(x)=2x²−1, etc. They can be produced using multiplier circuits and adder or subtracter circuits. To realize any n, the following relations are particularly helpful:

[0022] T_n−m(x)=T_n(T_m(x))  (2)

[0023] T_n+m(x)=2·T_m(x)·T_n(x)−T_n−m(x)  (3)

[0024] Circuits for equations (2) and (3) are illustrated in FIGS. 1 and 2.

[0025] The circuit for equation (2), constructed and realized on this basis, is comprised of two series-connected Chebyshev modules 1 and 2, the input variable being applied with the fundamental frequency at the input of Chebyshev module 1, and the output variable being applied with the frequency multiplied by the factor n.m at the output of Chebyshev module 2. A realization of equation (3) is shown in FIG. 2. This circuit is comprised of Chebyshev modules 3, 4 and 5, whose inputs are all fed the input variable with the fundamental frequency. The outputs of Chebyshev modules 3 and 4 lead to the input of a multiplier circuit 7, at whose other input a 2 is applied for multiplication purposes. The output of multiplier circuit 7, together with the output of Chebyshev module 5, leads to a subtracter circuit 8, at whose output the function T_n+m(x) is applied when the function T_n−m(x) is realized in Chebyshev module 5, and T_n−m(x), when the function T_n+m(x) is realized in Chebyshev module 5.

[0026] At this point, the Chebyshev module T_N(x) is able to be constructed from the circuits of FIGS. 1 and 2, for any desired value N. In this context, different possible realizations result depending on N. One skilled in the art will select the particular realization as a function of the costs. For the sake of simplicity, a realization based on operational amplifiers is assumed in the following. The circuits indicated are not necessarily equally suited for each application case. Depending on the costs of the components needed, in some instances, other realizations would be easily possible and more cost-effective. With the aid of the equations indicated here, the design can be easily modified and tailored to the particular application case.

[0027] Using the operational amplifier circuits known in circuit engineering, circuits shown in FIGS. 3 or 4 may be implemented for the first non-trivial polynomials T₂(x) and T₃(x). If K_ndenotes the costs of realizing function T_n(x), it can then be inferred from FIG. 3 that polynomial T₂, implemented with operational amplifiers, results in the costs of a squaring circuit 9, an operational amplifier 10, two resistors 11 and 12, as well as of a constant voltage source 13. It is, thus, apparent that a circuit of this kind renders possible an inexpensive realization of frequency-multiplication circuits.

[0028] The circuit according to FIG. 4 for realizing the function T₃(x) is comprised, in turn, of a squaring circuit 9, a subsequent multiplier circuit 7, as well as of an operational amplifier 10, whose output signal is fed back via a voltage divider, comprised of resistors 14 and 15, to its input. The corresponding electrical variables of function T₃(x) are available at the output of this circuit.

[0029] The indicated formulas (2) and (3) are the formulas which are useful for the synthesis of any T_n(x).

[0030] These formulas can be schematically represented, as already mentioned, as circuits in accordance with FIGS. 1 and 2. As a special case, from equation (3), for m=1, one obtains the well known formula T_n+1(x)=2x·T_n(x)−T_n−1(x), and for m=n, the formula T_2n(x)=2·T_n(x)²−1. Transposing equation (3), one also obtains T_n−m(x)=2·T_m(x)·T_n(x)−T_n+m(x), which can likewise be realized by a circuit in accordance with FIG. 2. Assuming that K(S₁) and K(S₂) denote the costs for circuits S₁and S₂according to FIG. 1 and FIG. 2 without the Chebyshev modules contained therein. Depending on the circuits, one obtains in the case of a realization with operational amplifiers:

[0031] K(S₁)=0

[0032] K(S₂)=costs for multiplier circuits, 1 OV, 2 resistors

[0033] A cost calculation can now be drawn up for each of these circuits. One example best illustrates the procedure. To determine T₁₇(x), one can formulate:

[0034] T₁₇(x)=2·T₈(x)·T₉−x=2·T₂(T₂(T₂(x)))·T₃(T₃(x))−x,

[0035] obtaining k₁₇=3K₂+2K₃+K(S₂) since the costs K(S₁) for realizing equation (2) are set here to equal zero.