Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
1999-10-20
2001-03-27
Pascal, Robert (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S176000, C324S711000
Reexamination Certificate
active
06208215
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a VCO and filter, and in particular, to a circuit in which the output bandwidth of the filter and output frequency of the oscillator are controlled by the same control bias current signal. This control signal may be generated from a frequency reference formed from standard IC components utilizing the thermal time constant of silicon.
2. Description of the Related Art
Oscillators are well known in the art of solid state electronics. Oscillator circuits producing a constant frequency signal are extremely useful for performing vital system functions such as clocking. Any constant frequency oscillator requires: 1) a source of power; 2) an amplifying device; and 3) some form of resonant circuit to maintain the frequency of the output.
Many solid-state circuits include a separate crystal having an intrinsic vibrational frequency. Such a circuit utilizes the crystal's inherent vibration to generate a constant frequency output signal. Unfortunately, incorporating an external component such as a crystal into an IC creates additional complexity and expense in the manufacturing process.
Therefore, there is a need in the art for a structure formed from standard integrated circuit components that is capable of maintaining the output of an oscillator at a regular frequency.
Filters are also well known in electronics, and find use in a wealth of applications. Many filters include resistive and capacitive elements, with the resistance and capacitance exhibited by these elements determining bandwidth of the filter output.
Unfortunately, there has been long-standing difficulty in producing filters having precise characteristics for use in integrated circuits. This is attributable to the uncertainty in absolute values of resistance and capacitance exhibited by structures created by conventional silicon fabrication processes.
As discussed in detail below in connection with the operation of conventional switched capacitor filter, one way of surmounting this problem is to introduce an external signal of constant frequency into the circuit. However, this again entails substantial additional expense.
Therefore, there is a need in the art for a filter design that can be incorporated into an integrated circuit and which does not depend upon a constant-frequency clock signal provided by an external source.
SUMMARY OF THE INVENTION
The present invention relates to a frequency reference structure, a voltage controlled oscillator, and a filter, each of which may take advantage of the thermal resistance and capacitance of single crystal silicon to ensure uniformity of output.
The frequency reference is fabricated from a thermal RC network positioned in silicon in the form of a lateral array of bipolar transistors. Application of a clock signal from a voltage-controlled oscillator to the silicon produces a heat pulse which propagates through the silicon and across this thermal network.
Because the base-emitter voltage (V
be
) of the bipolar transistors is highly temperature dependent, propagation of the heat pulse through the silicon causes a fluctuation in V
be
of the arrayed transistors. Comparison of the V
be
appearing across two transistors in the array results in a combined voltage signal whose magnitude is determined by the distance between the transistors and the time constant &tgr; of the thermal RC network. This time constant &tgr; is solely a function of the volume of single crystal silicon present between the laterally-arrayed bipolar transistors. It is independent of the amplitude, frequency, and duty cycle of the original clock signal.
In accordance with the present invention, the original clock signal and the time-delayed voltage signal of the thermal RC network are compared, and the phase shift between these signals determined. A control bias current corresponding to this phase difference is then generated.
This control bias current can be fed back to a voltage-controlled oscillatory featuring a variable transconductance amplifier as the resistive element on the negative feedback loop. Control of negative feedback with the control bias current ensures generation of an output signal having a constant frequency. This control bias current can also be simultaneously fed to a transconductance amplifier making up the resistive element of a filter structure, thereby also permitting control over the effective bandwidth of the filter.
A method of calibrating a filter in accordance with one embodiment of the present invention comprises the steps of conveying a variable control bias current signal to a current input node of a first variable transconductance amplifier forming a resistive element of a filter, the first variable transconductance amplifier including a noninverting input node and positioned in series with a shunt capacitor, an output of the variable transconductance amplifier in negative feedback with an inverting input node of the variable transconductance amplifier. The variable control bias current signal is also conveyed to a current input node of a second variable transconductance amplifier forming a resistive element of a voltage controlled oscillator, whereby the variable control bias current signal calibrates a bandwidth of the filter with an output frequency of the voltage controlled oscillator.
An apparatus for producing an output signal with a substantially constant frequency in accordance with one embodiment of the present invention comprises a semiconductor workpiece having a thermal resistance and a thermal capacitance, and a voltage controlled oscillator formed in the semiconductor workpiece. The voltage controlled oscillator is configured to receive a control bias current signal and in response produce a clock signal having a first frequency. A heat source is positioned at a first point within the semiconductor workpiece, the heat source configured to receive the clock signal and in response generate a heat pulse in the semiconductor workpiece. A thermal network is formed within the silicon and has a thermal time constant, the thermal network configured to receive the heat pulse and produce a voltage signal based upon the thermal time constant. A limiting amplifier is configured to receive, amplify, and limit the voltage signal to produce a clipped output voltage. A comparator is configured to receive the clock signal and the clipped output voltage and produce the control bias current signal corresponding to a phase difference between the clock signal and the clipped output voltage, wherein the voltage controlled oscillator alters the first frequency of the clock signal in response to the control bias current signal.
A method for maintaining constant frequency output of a voltage controlled oscillator in accordance with one embodiment of the present invention comprises the steps of applying a clock signal from a voltage controlled oscillator to a first point of a semiconductor workpiece, and generating a heat pulse at the first point of the semiconductor workpiece based upon the clock signal. A voltage signal is produced based upon a thermal time constant of a thermal network formed within the semiconductor workpiece, and the voltage signal is amplified and limited to produce a clipped output voltage. The clock signal and the clipped output voltage are compared to produce a control bias current corresponding to a phase difference between the clock signal and the clipped output voltage. The frequency of the clock signal is altered in response to the control bias current.
A voltage controlled oscillator in accordance with one embodiment of the present invention comprises an oscillator input node configured to receive an input voltage signal, and a first operational amplifier. The first operational amplifier has a noninverting input node, an inverting input node, and an oscillator output node, the noninverting input node of the first operational amplifier configured to receive the input voltage signal from the oscillator input node, and the oscillator output node configured to produce a
Glenn Kimberly E
National Semiconductor Corp.
Pascal Robert
Stallman & Pollock LL:
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