Oscillators – Relaxation oscillators
Reexamination Certificate
2000-06-30
2001-12-04
Grimm, Siegfried H. (Department: 2817)
Oscillators
Relaxation oscillators
C331S111000
Reexamination Certificate
active
06326859
ABSTRACT:
This application claims priority under 35 U.S.C. §§119 and/or 365 to 9915421.3 filed in Great Britain on Jul. 1, 1999; the entire content of which is hereby incorporated by reference.
1. Technical Field
This invention relates to an oscillator circuit, for example for generating a clock signal in an integrated circuit.
2. Background of the Invention
Oscillator circuits are commonly used in integrated circuits to provide clock signals. The frequency of the resulting clock signal depends on parameters of the oscillator circuit. In an integrated circuit, it is typically not possible to manufacture the oscillator circuit components with sufficient precision to provide a clock frequency with a desired accuracy.
Therefore, it is known to manufacture an oscillator circuit whose frequency is trimmable. That is, the frequency is adjustable, after manufacture, by means of an input signal, for example a multi-bit digital signal. Thus, the frequency can be brought accurately to the desired value.
U.S. Pat. No. 5,859,571 discloses a frequency trimmable oscillator, in which a current generator provides an output current, and also generates two threshold voltages. The current is switched into a delay unit, which includes two arrays of capacitors, one for providing coarse trimming of the output frequency, and one for providing fine trimming. The voltage on the capacitors is compared with the two threshold voltages, and the results of the comparisons are used to set and reset a flip-flop, with the time taken for each such cycle being the period of the output clock signal, such that the frequency is dependent on the value of the trimmed capacitance and is hence adjustable as desired.
However, as mentioned above, this circuit requires two arrays of capacitors. Moreover, each capacitor has an associated parasitic capacitance. It is an inherent feature of integrated circuit capacitors that the value of the parasitic capacitance is not as well controlled as the value of the intended capacitance. Therefore, while the output frequency of the oscillator should desirably vary linearly with the intended trimmed capacitance value, in practice this can only be achieved to a limited extent.
SUMMARY OF THE INVENTION
In accordance with a preferred aspect of the invention, an oscillator circuit includes a current generator producing a current which is trimmable by means of a resistor array, and thus includes only a single capacitor array.
In accordance with a further preferred aspect of the invention, an oscillator circuit includes a current generator which supplies current to input terminals of capacitors in a trimmable capacitor array. The input terminals of the capacitors are held at a relatively constant voltage, and thus all of the current from the current generator passes through the desired capacitors of the capacitor array, thus minimizing the effect of parasitic capacitance.
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Modern Electronic Circuits Reference Manual, McGraw Hill, 1980, p. 423, See Lower Circuit.
Cheng et al., “A low-frequency CMOS triangle wave generator”, IEEE Journal of Solid-State Circuits., vol. sc-20, No. 2, Apr. 1, 1985, pp. 649-652, XP002148814.
Goldman Richard
Wilson Robin
Burns, Doana, Swecker & Mathis, L.L.P.
Grimm Siegfried H.
Telefonaktiebolaget LM Ericsson (publ)
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