Oscillator circuits and methods to compensate frequency pulling
An oscillator circuit may include a local oscillator to generate a carrier signal having a tunable frequency, a first modulator and a power amplifier coupled in cascade to the local oscillator to generate an output signal. The first modulator may be activated from a first modulating signal having a first frequency alternatively defining ON and OFF states of the first modulator. An estimator unit may receive the carrier signal during a time window and detect an estimated frequency variation of the carrier signal during the ON and OFF states. A compensation unit may include a second modulator to generate a compensation signal proportional to the estimated frequency variation and modulated with a second modulating frequency. The second modulating frequency may be substantially the same as the first modulating frequency, and the compensation signal may be added to a bias signal of the local oscillator to tune the tunable frequency.
1. An oscillator circuit comprising:
a local oscillator configured to generate a carrier signal having a tunable frequency;
a first modulator and a power amplifier coupled to the local oscillator and configured to generate an output signal, the first modulator configured to be activated from a first modulating signal alternatively defining ON and OFF states of the first modulator;
an estimator unit configured to receive the carrier signal during a time window and to detect an estimated frequency variation of the carrier signal during the ON and OFF states of the first modulator; and
a compensation unit coupled to the local oscillator and comprising a second modulator configured to generate a compensation signal for tuning the tunable frequency, the compensation signal being related to the estimated frequency variation based upon a second modulating signal different than the first modulating signal.
2. The oscillator circuit of claim 1 wherein the compensation unit further comprises a logic unit coupled to said second modulator; and wherein the logic unit is configured to receive the estimated frequency variation and to define a pre-compensation signal.
3. The oscillator circuit of claim 2 wherein the logic unit is configured to generate an output digital signal; and wherein the compensation unit further comprises a digital-to-analog converter configured to receive the output digital signal from the logic unit and to generate the pre-compensation signal.
4. The oscillator circuit of claim 1 wherein the local oscillator comprises an input terminal; and further comprising a first adder coupling the input terminal of the local oscillator to a voltage source, the first adder being configured to generate a voltage compensation signal by summing a bias voltage signal generated by the voltage source and the compensation signal generated by the compensation unit.
5. The oscillator circuit of claim 4 wherein the local oscillator comprises an input bias terminal; and further comprising a second adder coupling the input bias terminal to a current source, the second adder being configured to generate a current compensation signal by summing a bias current signal generated by the current source and the compensation signal generated by the compensation unit.
6. The oscillator circuit of claim 1 wherein the compensation unit comprises a finite state machine.
7. The oscillator circuit of claim 1 wherein the first and second modulating signals have a same frequency and a different duty cycle.
8. A transceiver comprising:
a transmitter comprising
a local oscillator configured to generate a carrier signal having a tunable frequency,
a first modulator and a power amplifier coupled to the local oscillator and configured to generate an output signal, the first modulator configured to be activated from a first modulating signal alternatively defining ON and OFF states of the first modulator,
an estimator unit configured to receive the carrier signal during a time window and to detect an estimated frequency variation of the carrier signal during the ON and OFF states of the first modulator, and
a compensation unit coupled to the local oscillator and comprising a second modulator configured to generate a compensation signal for tuning the tunable frequency, the compensation signal being related to the estimated frequency variation based upon a second modulating signal different than the first modulating signal; and
a receiver comprising
a low-noise amplifier (LNA) having an input for a received signal and an output,
an adder having a first input coupled to output of the LNA, a second input coupled to the carrier signal, and an output, and
a demodulator coupled to the output of the adder for demodulating the signal output by the adder at a same frequency associated with the first modulation signal.
9. The transceiver of claim 8 wherein said transmitter transmits radar signals; wherein the first and second modulating signals have a modulating frequency of 24 GHz; and wherein the estimated frequency variation is less than 1 MHz.
10. The transceiver of claim 8 wherein the compensation unit further comprises a logic unit coupled to the second modulator; and wherein the logic unit is configured to receive the estimated frequency variation and to define a pre-compensation signal.
11. The transceiver of claim 10 wherein the logic unit is configured to generate an output digital signal; and wherein the compensation unit further comprises a digital-to-analog converter configured to receive the output digital signal from the logic unit and to generate the pre-compensation signal.
12. The transceiver of claim 8 wherein the compensation unit comprises a finite state machine.
13. The transceiver of claim 8 wherein the first and second modulating signals have a same frequency and a different duty cycle.
14. A method to compensate frequency pulling in an oscillator circuit comprising:
generating a carrier signal using a local oscillator operating at a tunable frequency;
modulating the carrier signal based upon a first modulating signal by obtaining a modulated carrier signal and amplifying the modulated carrier signal to generate an output signal;
processing the carrier signal in a time window and based upon a clock signal to define, during the time window, first time intervals and second time intervals;
estimating a first estimated frequency during the first time intervals and a second estimated frequency during the second time intervals;
determining an estimating frequency variation of the carrier signal as a difference between the first estimated frequency and the second estimated frequency;
generating a pre-compensation signal from the estimated frequency variation value;
modulating the pre-compensation signal with a second modulating signal different than the first modulating signal to obtain a compensation signal; and
tuning the local oscillator based upon the compensation signal.
15. The method of claim 14 wherein processing the carrier signal comprises:
providing the first modulating signal as the clock signal;
defining a logic state N of the local oscillator by using a previous logic state N−1 of the local oscillator; and
generating an output digital signal using the defined logic state (N) of the local oscillator;
converting the output digital signal into an analog pre-compensation signal.
16. The method of claim 14 wherein the first and second modulating signals have a same frequency and a different duty cycle.
17. The method of claim 14 wherein the oscillator is implemented in a transceiver comprising a transmitter and receiver; and further comprising:
transmitting an output signal from the transmitter and receiving an input signal at the receiver;
amplifying the input signal and combining the input signal with the carrier signal to produce a receiving signal; and
demodulating the receiving signal using a demodulator switch at a same frequency associated with the first modulation signal.