Method and apparatus for MEMS phase locked loop
The proper operation of a phase locked loop is determined by monitoring certain signals within the loop for their phase relationship or duty cycle. If a malfunction of the loop is detected, proper operation may be imposed or restored by resetting a phase-frequency detector, or by flipping the output of the phase-frequency detector.
1. A method of driving an oscillating member using a phase locked loop, comprising:
applying a drive signal having a drive frequency to the oscillating member;
providing a reference signal indicative of the phase shift imparted on the drive signal by the oscillating member;
comparing the phase of the drive signal to the reference signal in a phase detector;
generating a first control signal if the reference signal lags the drive signal; and
determining whether the duty cycle of the first control signal exceeds a predetermined percent; and
increasing the frequency of the drive signal if the duty cycle of the first control signal exceeds the predetermined percent.
2. The method of claim 1 , wherein the predetermined percent is at least 25 percent.
3. The method of claim 1 , wherein the predetermined percent is at least 50 percent.
4. The method of claim 1 , wherein the predetermined percent is at least 75 percent.
5. The method of claim 1 wherein determining whether the duty cycle of the first control signal exceeds a predetermined percent comprises determining whether the duty cycle of the first control signal exceeds a predetermined percent for at least a predetermined number of cycles of the drive signal, the predetermined number of cycles of the drive signal being at least two.
6. The method of claim 5 wherein the predetermined number of cycles of the drive signal is at least three cycles.
7. A circuit for driving an oscillating member, comprising:
a variable-frequency oscillator having an oscillator output;
an arrival-time detector having a first arrival-time detector input and a second arrival-time detector input, an up output and a down output, and a reset input, the first arrival-time detector input coupled to the oscillator output;
a phase shifter having an phase shifter input and a phase shifter output, the phase shifter input coupled to the oscillator output;
an oscillating member having a member input and a member output, the member input coupled to the phase shifter output, and the member output coupled to the second arrival-time detector input;
an error detector configured to determine whether the up output or down output is high at a falling edge of the oscillator output, the error detector having a first error detector input coupled to the up output of the arrival-time detector, and a second error detector input coupled to the down output of the arrival-time detector, and an error detector output coupled to the reset input of the arrival-time detector.
8. A method of driving an oscillating member using a phase locked loop, the method comprising:
applying a drive signal having a drive frequency to the oscillating member;
providing a first reference signal to a phase detector, the first reference signal indicative of the phase shift imparted on the drive signal by the oscillating member;
providing a second reference signal to the phase detector, the second signal having the same frequency as the drive signal, but leading the drive signal by ninety degrees;
generating a first control signal if a phase detector determines that the first reference signal leads the second reference signal, and generating a second control signal if a phase detector determines that the first reference signal lags the second reference signal; and
transposing at least one of the first control signal and the second control signal is active at a falling edge of the drive signal.
9. A method of driving an oscillating member using a phase locked loop according to claim 8 , wherein applying a drive signal comprises:
generating the second reference signal; and
producing the drive signal by imposing a phase shift of negative ninety degrees on the second reference signal,
such that the second reference signal leads the drive signal by 90 degrees.
10. A method of driving an oscillating member using a phase locked loop according to claim 8 , wherein providing a second reference signal comprises:
generating the drive signal; and
producing the second reference signal by imposing a phase shift of 270 degrees on the reference signal,
such that the second reference signal leads the drive signal by 90 degrees.