The present invention is directed generally to a variable impedance integrated circuit and, more particularly, to a unary-switched variable impedance integrated circuit.
Since the advent of radio circuits and other circuit control applications, it has been desirable to provide controlled input signals such as those signals used to control frequency band selection and maintain specific frequencies. Control circuits such as these are particularly useful in applications such as communications devices, guidance systems, and feedback control systems such as phase-lock-loops (PLL) employing voltage controlled oscillators (VCO).
Controlling frequency has often proved difficult, since electronic operation of most equipment produces heat, friction and other environment altering factors causing frequency to shift unpredictably. These factors are often addressed by utilizing a VCO in a PLL to continuously compare the VCO output signal with an incoming reference signal, and correct for undesirable frequency-shifts.
A standard PLL usually includes a VCO, a loop filter (LPF), a phase comparison circuit (COMP), a reference frequency signal input, and an oscillation signal output. The output of the VCO is fed back into the input of the COMP along with a reference signal. The output of the COMP is fed into the loop filter. The output of the loop filter is connected to the input of the VCO.
As is well known, the operation of the phase locked loop is such that the phase comparison circuit compares the phase of the oscillation of the VCO output with the phase of the reference frequency signal, outputs an error signal indicating the error between the phases of the oscillation signal and the reference frequency signal, and supplies the error signal to the loop filter. The loop filter smoothes the error signal, outputs it as a control voltage, and supplies the control voltage to the VCO. In the VCO, the resonance frequency of an LC resonator circuit is controlled in correspondence to the control voltage supplied by the loop filter, and the frequency of the VCO output signal is adjusted to eliminate the error between the VCO output and the reference signal.
In a high frequency VCO, a band-switch is used to improve the oscillator performance. The band-switch adds discrete values of capacitance to the frequency-tuning element in the oscillator's LC tuning circuit. The number of steps is traditionally powers of 2(e.g., 2, 4, 8, 16, 32, 64 . . . ). The switching is done in a binary way. From band 31 to 32, 31 capacitors are switched off and another 32 are switched on. Any inaccuracy in the capacitor is added up and is clearly visible as an inaccurate frequency selection. The manufacturing process for integrated circuits (ICs) limits the matching of capacitors within the IC, and mismatch creates errors during band switching. The errors are often corrected by using a continuous-voltage controlled capacitor (varicap) or they can be accepted, and contribute to manufacturing yield losses. Both situations are undesirable. Extra tuning in the varicap impacts performance, again resulting in yield losses that ultimately affect production cost.
Therefore, it would be advantageous to provide voltage control circuits that do not have band switching errors leading to high production yield losses. It would further be advantageous to provide an improved capacitance switching-network that does not suffer from capacitor-matching induced switching errors.
Various aspects of the present invention are directed to ICs configured and arranged in a manner that addresses and overcomes the above-mentioned issues for radio circuits, guidance circuits, lock-in amplifiers, and other applications benefiting from the use of variable impedance circuits.
In one embodiment of the present invention, a voltage control circuit is provided, the circuit including an array having a plurality of cells, at least one output, a plurality of coarse-setting inputs and a plurality of fine-setting inputs. The coarse-setting and fine-setting inputs are adapted to enable selectable combinations of the cells. The voltage control circuit is adapted to operate at a selected one of a plurality of bit-addressable reference frequencies ranging over a plurality of frequency bands. An address control circuit is adapted to establish one of the plurality of frequency bands by controlling the plurality of coarse-setting inputs of the array and adapted to establish a reference frequency in one of the plurality of frequency bands by controlling the plurality of fine-setting inputs of the array.
In another embodiment, A VCO circuit includes a VCO adapted to operate at a selected one of a plurality of bit-addressable reference frequencies ranging over a plurality of frequency bands, and as defined by a data-programming circuit. An array having a plurality of equally-weighted cells is provided, having at least one cell with selectable lesser-weighted circuits, and having at least one output. A plurality of coarse-setting inputs enable selected ones of the plurality of equally-weighted cells, and a plurality of fine-setting inputs enable ones of the lesser-weighted circuits, the coarse-setting and fine-setting inputs being adapted to provide an array output responsive to selected combinations of the enabled equally-weighted cells and the enabled lesser-weighted circuits. An address control circuit is responsive to the data-programming circuit for controlling the array and selecting one of the plurality of frequency bands. By controlling the plurality of coarse-setting inputs of the array and controlling the plurality of fine-setting inputs of the array, the reference frequency is established.
Consistent with the above example embodiments, in another embodiment an analog circuit provides an extra-fine tuning control as another input to the voltage-controlled target circuit.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.
The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:
While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
The present invention is directed generally to a variable impedance integrated circuit and, more particularly, to a unary-switched variable impedance integrated circuit. Control circuits in accordance with the present invention may be used to control frequency-band selection and maintain specific frequencies, as well as perform other circuit control. Control circuits such as these are particularly useful in applications such as communications devices, guidance systems, and feedback control systems such as phase-locked loops (PLL) employing voltage controlled oscillators (VCO).
In accordance with the present invention, a first embodiment of the present invention is directed to an impedance-dependent circuit that has a voltage-controlled input at which a selected impedance is coupled for controlling an output of the circuit. The impedance-dependent circuit controls a voltage-controlled target circuit by way of an array having a plurality of cells, at least one output, and a plurality of coarse-setting and fine-setting inputs for enabling selectable combinations of the cells. The voltage-controlled target circuit operates at a selected one of a plurality of data-addressable reference frequencies, for example, ranging over a plurality of frequency bands. For driving the array, the impedance-dependent circuit also includes an address control circuit that controls the coarse-setting and the fine-setting inputs of the array. With these settings, a reference frequency is established at the output of the voltage-controlled target circuit.
In a more specific example embodiment, the present invention is directed to a VCO-based PLL that employs switched variable capacitor for frequency control of a VCO. A unary switchable variable capacitor circuit, in accordance with the present invention, is a capacitor circuit that switches a capacitor between an “on” state and an “off” state to either enable or disable the capacitor's effective capacitance value through a summation of respectively enabled capacitors. In accordance with the present invention, an array of such selectable switchable capacitors is provided for enabling any combination of capacitors to provide an effective capacitance value at an output of the array. The array includes a plurality of commonly-weighted capacitor cells with each such commonly-weighted cell providing the same capacitance value, and also includes at least one cell having different lesser-weighted capacitor cells with each such lesser-weighted cell providing respective capacitance values that are less than the capacitance value of the commonly-weighted cell. In this manner, any incremental increase or decrease in desired capacitance can be presented to the VCO (for voltage bias) by enabling the appropriate combinations of capacitor cells.
It is understood that manufacturing processes produce variation within elements designed to have a common value, and that the capacitors described herein will have values ranging within the available manufacturing tolerances. By providing a sufficiently-sized array for a given application (e.g., whether 7-cell array or a 25-cell array), the manufacturing-process variation is mitigated by the selectability of the designed capacitance values. For example, in certain VCO applications, the reference frequency is coarsely selected by using certain bits to address and enable commonly-weighted cells. Finer tuning is provided by using additional bits to address and enable certain of the different lesser-weighted cells.
For many applications (VCO and otherwise), with a sufficient number of such combinable capacitance cells, this above-described coarse/fine tuning approach adequate to control the voltage signal at the input of the impedance-dependent circuit. In other applications requiring more precision, also in accordance with the present invention, this coarse/fine tuning approach is complemented with an analog extra-fine voltage-adjustable input for more precisely tuning the VCO.
In a particular embodiment (not shown in
Also shown in
In accordance with one embodiment of the present invention, the capacitors are arranged in a multidimensional array to reduce the complexity associated with the conversion from a first received format to a second desired format, such as, for example, converting from a binary format to a unary format. In a binary format, for example, switching from a binary 0111 to a binary 1000 involves turning off three capacitors, and turning on one capacitor. In a unary format, only a single capacitor is switched into the circuit (i.e. 01111111 to 11111111).
As a more particular example embodiment,
As shown in
As shown and described in connection with the circuits of
With this type of encoding scheme for the encoder 302 of
Within each of the 15 unary (commonly-weighted) cells, at each capacitor the corresponding row bit and column bit are decoded using simple local logic, that is based on 2 row inputs and 1 column input. Basically, the cell logic is a function of the numerically-corresponding column and of the input from its numerically-corresponding row and the next row: when this row is active, use the column bits to select the capacitor; when the next row is active, make all capacitors active (ignore the column bit); and when no row is active, no capacitor is active (ignore the column bit).
The following truth table illustrates the overall translation of the this 7-bit to 9-bit encoding scheme for the example embodiment discussed above involving 7 bits, four of which are unary plus three bits that are weighted:
The above table assumes that the 7 bits of the databus, for 128 possible values (0-127), are feeding the inputs (B7-B1) to the encoder as if these 7-bits are weighted. Also, for those rows representing 8 values (e.g., 8-15), “R” is used in the above table to denote that the LSB values are repeated (no change) from the first 8 values (0-7) as shown in the table. In this manner, it is appreciated that the row corresponding to value 127 would, in each entry, be “1”. Further, it is appreciated that the header term “Encoded” at the top line of the truth table shows an intermediate step for addressing all 15 unary cells but with only 6 lines instead of 15. Effective encoding increases with the number of unary cells (now 4 bits). However, decoding at each unary cell is needed. But this decoding is repeated in a similar way for all unary cells. The “Bw” bits are the “weighted bits” for the uniquely weighted 16th cell (for which the 3 LSB's just pass through the encoder). The “row” and “col” bits are the row and column bits. The 4 bits unary coding (bits 4. . . 7) creates 6 new encoded bits (row 1.3 and col 1.3). The header term “Decoded at capacitors matrix” denotes where the real capacitors are switched and with the LSB's again connected straight through the decoder with no decoding needed.
Various modifications and additions can be made to the preferred embodiments discussed hereinabove without departing from the scope of the present invention. Accordingly, the scope of the present invention should not be limited by the particular embodiments described above, but should be defined only by the claims set forth below and equivalents thereof.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB04/51708 | 9/7/2004 | WO | 11/1/2006 |
Number | Date | Country | |
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60501905 | Sep 2003 | US |