Linear crystal oscillator with amplitude control and crosstalk cancellation

• US 5,047,734 A
• Filed: 05/30/1990
• Issued: 09/10/1991
• Est. Priority Date: 05/30/1990
• Status: Expired due to Term

• Current Assignee: BEI Sensors & Systems Company Incorporated (Schneider Electric SE)
• Original Assignee: New SD, Inc.
• Inventors: Bhardwaj, Pradeep, Nootbaar, Michael W., Newell, Gerald R.
• Primary Examiner(s): Grimm, Siegfried H.
• Application Number:
  US07/530,640
• Time in Patent Office: 468 Days
• Field of Search: 331/46, 331/65, 331/105, 331/109, 331/116 R, 331/116 FE, 331/158, 331/160, 331/175, 331/182, 331/183, 455/295
• US Class Current: 331/46
• CPC Class Codes: H03B 5/38 frequency-determining element being connected via bridge circuit to closed ring around which signal is transmitted
• CPC Class Codes: H03L 5/00 Automatic control of voltage, current, or power

Abstract

A series-feedback crystal oscillator has a crystal element in series with a buffer amplifier. The oscillator maintains a linear oscillation signal by using an amplitude control circuit to hold the oscillation signal to a certain level at which distortion, clipping, or saturation does not occur. A control signal indicative of the amplitude of the oscillation signal linearly multiplies the oscillation signal to control the amplitude of the oscillation signal. Shunt capacitance across the crystal is compensated for by connecting a compensating capacitor to the same input terminal of the buffer amplifier as is connected the crystal. A compensating signal, which is equal in amplitude but opposite in phase to the signal passing through the shunt capacitance of the crystal is fed to the input terminal of the buffer amplifier to cancel the effect of the shunt capacitance. A noninverted tracer signal is injected in the feedback loop and passes through the shunt capacitance. An inverted tracer signal passes through the compensating capacitor to cancel the noninverted tracer signal through the shunt capacitance and thereby compensate for the shunt capacitance of the crystal. Interfering crosstalk signals from one crystal oscillator circuit which are coupled into a second crystal oscillator circuit are reduced by feeding an oppositely phased interfering signal into the second crystal oscillator circuit.

Claims

1. A circuit compensating for the shunt capacitance of a frequency selective crystal element which is in series with a buffer amplifier having input and output terminals in a series-feedback oscillator circuit comprising:

a differential amplifier having an input terminal coupled to the output terminal of said buffer amplifier, said differential amplifier providing a noninverted signal at a noninverting signal output terminal and providing an inverted signal at an inverting signal output terminal;

said crystal element having first and second terminals and having a shunt capacitance between said two terminals, said crystal element having its first terminal coupled to the noninverting output terminal of the differential amplifier and having its second terminal coupled to the input terminal of said buffer amplifier to form said series feedback crystal oscillator circuit;

a compensating capacitor having first and second terminals, said compensating capacitor having its first terminal coupled to the inverting signal output terminal of said differential amplifier and having its second terminal coupled to the input terminals of said buffer amplifier such that the compensating capacitor passes a compensating signal, which is substantially equal in magnitude and opposite in phase to the signal passing through the shunt capacitance of the crystal element, to the input terminal of the buffer amplifier to thereby cancel the effect of the signal passing through the shunt capacitance of the crystal element.

2. The circuit of claim 1 including a variable voltage divider circuit having an input terminal to which the inverting signal output terminal of the differential amplifier is coupled and having a variable output tap terminal to which the compensating capacitor is coupled.

3. The circuit of claim 1 including means for automatically adjusting the amplitude of the compensating signal passing through the compensating capacitor, which automatic adjustment means includes:

means for injecting a tracer signal into said differential amplifier, said tracer signal having a frequency which is different from the series resonant frequency of said crystal element so that said tracer signal passes as a noninverted tracer signal through the shunt capacitance of the crystal element and so that the tracer signal passes as an inverted tracer signal through the compensating capacitor to combine with and cancel the noninverted tracer signal at the input terminal of the buffer amplifier, leaving a residual tracer signal; and

means for detecting the amplitude and phase of the residual tracer signal passing through said buffer amplifier with respect to the tracer signal to provide a tracer control signal indicative of the amplitude and phase of the residual tracer signal; and

means controlled by said tracer control signal for adjusting the amplitude of the inverted tracer signal passing through said compensating capacitor to minimize said residual tracer signal.

4. The circuit of claim 3 wherein the detecting means includes:

a synchronous demodulator for multiplying the residual tracer signal by the residual tracer signal to produce the control signal; and

a lowpass filter for said control signal.

5. The circuit of claim 3 wherein said amplitude adjustment means and said compensating capacitor include a voltage-variable capacitor which is controlled by said tracer control signal.

6. A circuit for reducing an interfering crosstalk signal coupled from a first crystal oscillator circuit to a second crystal oscillator circuit, comprising:

means for providing a crosstalk cancellation signal from the first crystal oscillator circuit, said crosstalk cancellation signal being oppositely phased with respect to the interfering crosstalk signal;

means for coupling said crosstalk cancellation signal into the second crystal oscillator circuit to cancel said interfering signal.

7. The circuit of claim 6 wherein said means for providing a cancellation signal includes means for summing two oppositely-phased signals from said first crystal oscillator to provide a cancellation signal having a selected amplitude and phase.

8. The circuit of claim 7 including a resistor connected between said oppositely-phased signals, said resistor having a variable tap at which is provided the cancellation signal.

9. A system for automatic cancelling of a crosstalk interference signal which is generated in a source circuit and which is coupled through a stray coupling impedance to interfere with a crosstalk-sensitive circuit, comprising:

means for injecting a crosstalk tracer signal into said source circuit, said crosstalk tracer signal having a frequency which does not interfere with operation of said crosstalk-sensitive circuit,

crosstalk-cancellation means for coupling an inverted tracer signal from an output terminal of said source circuit to said crosstalk-sensitive circuit, said crosstalk cancellation means including;

an inverter circuit coupled to the output terminal of said source circuit;

amplitude-control means for controlling the amplitude of said inverted tracer signal in response to an inverted-tracer control signal;

means for coupling said inverted tracer signal to an input terminal of said crosstalk-sensitive circuit so that said tracer signal coupled to said crosstalk-sensitive circuit is summed with the oppositely-phased inverted tracer signal to provide a residual tracer signal, which results from imperfect cancellation of said tracer signal and said inverted tracer signal;

means for detecting the amplitude of said residual tracer signal and for generating said inverted-tracer control signal;

wherein said amplitude-control means is controlled by said inverted-tracer control signal to minimize the amplitude of said residual tracer signal so that said crosstalk-cancellation means provides a path for a signal, which is approximately equal in amplitude and opposite in phase to said crosstalk interference signal, to said crosstalk-sensitive circuit to thereby provide for automatic cancellation of said crosstalk interference signal.

10. The system of claim 9 wherein said source circuit is an oscillator circuit providing an oscillation signal which is coupled through said stray coupling impedance to said crosstalk-sensitive circuit.

11. The system of claim 9 wherein said crosstalk-sensitive circuit is an oscillator circuit and wherein said tracer signal has a frequency different than the frequency of oscillation of said oscillator circuit.

12. The system of claim 9 wherein the detection and generation means includes:

a synchronous demodulator for multiplying the residual tracer signal by the tracer signal to produce said inverted-tracer control signal, and

a low-pass filter for said inverted-tracer control signal.

13. The system of claim 9 wherein the detection and generation means includes a voltage-variable capacitor which is controlled by said inverted-tracer control signal to control the level of said inverted-tracer signal.

14. In a linear oscillator:

an amplifier and a frequency determining element connected in a loop to form an oscillator for producing an output signal of predetermined frequency, means responsive to the output signal for providing a control signal corresponding to the amplitude of the output signal, and means for linearly multiplying the output signal by the control signal to maintain the amplitude of the output signal at a substantially constant level.

15. The oscillator of claim 14 wherein the frequency determining element comprises a series resonant crystal.

16. In a transducer circuit for measuring variations in a physical parameter:

an amplifier and a crystal having a series resonant frequency which varies with the parameter to be measured connected in a loop to form an oscillator for producing an output signal having a frequency corresponding to the parameter to be measured, means responsive to the output signal for providing a control signal corresponding to the amplitude of the output signal, and means for linearly multiplying the output signal by the control signal to maintain the amplitude of the output signal at a substantially constant level.