The present invention relates generally to the field of integrated circuits, and more particularly to a system and method of providing an oscillator having an oscillating output signal associated therewith, wherein a frequency thereof may be adjusted via a trimming circuit.
Oscillator circuits are implemented in many different applications in the field of electronics and telecommunications. Various classes of oscillator circuits exist; one type of oscillator is an RC oscillator circuit. RC oscillator circuits typically include control circuitry which is coupled to a series resistor-capacitor (RC) network. The control circuit alternately charges or discharges the voltage across the capacitor through the resistor to generate an oscillating signal across the capacitor. The frequency of the oscillating signal is dictated by the time constant of the RC network (often times referred to as the RC time constant).
One exemplary conventional RC oscillator circuit is illustrated in prior art FIG. 1, and designated at reference numeral 10. The prior art oscillator circuit 10 includes two comparator circuits 12 and 14 that compare the output voltage across an output capacitor 16 to respective reference voltages, and generate control output signals to a latch 18. The latch 18 outputs a charge or discharge voltage which feeds the resistor 20 that together with the capacitor 16 forms the RC network. Thus a rate at which the voltage varies across the capacitor 16 is a function of the RC time constant of the resistor 20 and the capacitor 16.
The two comparator circuits 12 and 14 vary between a top and bottom threshold value (e.g., VH and VL). Thus when the output voltage has increased to VH as the capacitor 16 charges, the output of the comparator 12 trips, thus signals that a discharge operation should commence. Similarly, when the output of the capacitor 16 falls below VL as the capacitor discharges, the output of the comparator circuit 14 trips, thus signaling that a charging operation should commence.
Although the prior art oscillator circuit 10 of FIG. 1 is acceptable for many applications, the circuit 10 requires use of two comparators to control the charge/discharge of the capacitor 16 within a given voltage range. In addition, the oscillation frequency dictated by the RC time constant in circuit 10 is fixed. Consequently, if process variations, supply variations, or varying application needs require a variation in the oscillator output frequency, the circuit 10 proves unacceptable.
Thus, there is a need in the art for a method of providing high speed sampling without harmonic distortion on the output of sampling circuits.
The present invention relates generally to an RC type oscillator circuit, wherein a rate of charge and discharge of the output capacitor may be controlled, thereby allowing for frequency trimming thereof.
According to one aspect of the present invention, a trimmable oscillator circuit comprises a selectively adjustable charge/discharge circuit which is operable in a charging mode and a discharging mode. The rate of charging or discharging of the capacitor (which dictates the output oscillation frequency of the oscillator) may be adjusted in accordance with one or more user selectable control signals which operate to adjust a charging resistance or a discharge resistance associated with the capacitor, thereby allowing the frequency of the oscillator circuit to be trimmed.
According to another aspect of the present invention, the charge/discharge circuit comprises a selectively trimmable charge circuit portion. The charge circuit comprises a plurality of differing value resistances which are selectively coupled to the output capacitor through a base charge resistance. One of the resistances is then coupled to the capacitor through the base charge resistance in response to the one or more user selectable control signals, thereby coupling a unique resistance value to the capacitor to control a charging rate of the capacitor via the unique RC time constant.
According to still another aspect of the present invention, each of the plurality of differing value charge resistances are coupled in series with a transistor and each serially coupled resistance/transistor pair are coupled together in a parallel component arrangement and in turn coupled to the base charge resistance. The transistors are controlled by the one or more user selectable control signals and thus operate to selectively couple one of the differing value charge resistances to the base charge resistance. In addition, the parallel arrangement reduces the number of series switches needed to trim the charging resistance to one, thereby reducing the total RDS(ON) of the charge circuit portion, and making the circuit advantageous for low supply voltage applications.
According to another aspect of the present invention, the charge/discharge circuit comprises a selectively trimmable discharge circuit portion. The discharge circuit comprises a plurality of differing value discharge resistances which are selectively coupled to the output capacitor through a base discharge resistance. One of the discharge resistances is then coupled to the capacitor through the base discharge resistance in response to the one or more user selectable control signals, thereby coupling a unique resistance value to the capacitor to control a discharge rate of the capacitor via the unique RC time constant.
According to yet another aspect of the present invention, each of the plurality of differing value discharge resistances of the discharge circuit portion are coupled in series with a transistor and each serially coupled resistance/transistor pair are coupled together in a parallel component arrangement and in turn coupled to the base discharge resistance. The transistors are controlled by the one or more user selectable control signals and thus operate to selectively couple one of the differing value discharge resistances to the base discharge resistance. In addition, the parallel arrangement reduces the number of series switches needed to trim the discharge resistance to one, thereby reducing the total RDS(ON) of the discharge circuit portion, and making the circuit advantageous for low supply voltage applications.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.