BACKGROUND

As new electronic devices are developed and integrated circuit (“IC”) technology advances, new IC products are commercialized. One example IC product for electronic devices is an oscillator. A conventional oscillator circuit generates signal(s) with different resonant frequencies. In addition to including one or more inductors and capacitors, conventional oscillators include a power supply and active circuit to replenish energy due to losses in the oscillator circuit. Different oscillator circuits vary with regard to topology, power consumption, resonant frequencies, quality factor, phase noise, and other parameters. One type of oscillator includes a bulk-acoustic wave (BAW) resonator or microelectromechanical system (MEMS) resonator coupled to an active circuit. Flicker noise (such as, 1/f noise and colored noise) in active devices used in voltage controller oscillators gets up-converted around the oscillation frequency thereby degrading close-in phase noise. This is particularly an issue for close-in frequency offsets (e.g., 1 Hz, 10 Hz, 100 Hz).

SUMMARY

In at least one example, an oscillator comprises: a bulk-acoustic wave (BAW) resonator having a first BAW resonator terminal and a second BAW resonator terminal; and an active circuit coupled to the first and second BAW resonator terminals and having a series resonance topology. The series resonance topology includes: a first transistor having a first current terminal, a second current terminal and a first control terminal, the first current terminal coupled to a voltage supply terminal, and the control terminal coupled to the first BAW resonator terminal; a second transistor having a third current terminal, a fourth current terminal and a second control terminal, the third current terminal coupled to the voltage supply terminal, and the second control terminal coupled to the second BAW resonator terminal; a first resistor having a first resistor terminal and a second resistor terminal, the first resistor terminal coupled to the second current terminal, and the second resistor terminal coupled to a ground terminal; a second resistor a third resistor terminal and a fourth resistor terminal, the third resistor terminal coupled to the fourth current terminal, and the fourth resistor terminal coupled to the ground terminal; a capacitive network coupled to the first and second BAW resonator terminals, the second current terminal, and the fourth current terminal; and an inductor having a first inductor terminal and a second inductor terminal, the first inductor terminal coupled to the capacitive network, and the second inductor terminal coupled to ground terminal.

In another example, a system comprises circuitry having a clock input; and an oscillator having a clock output coupled to the clock input of the circuitry. The oscillator has: a BAW resonator having a first BAW resonator terminal and a second BAW resonator terminal; and an active circuit coupled to the first and second BAW resonator terminals and having a series resonance topology. The series resonance topology includes: a first transistor having a first current terminal, a second current terminal and a first control terminal, the first current terminal coupled to a voltage supply terminal, and the control terminal coupled to the first BAW resonator terminal; a second transistor having a third current terminal, a fourth current terminal and a second control terminal, the third current terminal coupled to the voltage supply terminal, and the second control terminal coupled to the second BAW resonator terminal; a first resistor having a first resistor terminal and a second resistor terminal, the first resistor terminal coupled to the second current terminal, and the second resistor terminal coupled to a ground terminal; a second resistor a third resistor terminal and a fourth resistor terminal, the third resistor terminal coupled to the fourth current terminal, and the fourth resistor terminal coupled to the ground terminal; a capacitive network coupled to the first and second BAW resonator terminals, the second current terminal, and the fourth current terminal; and an inductor having a first inductor terminal and a second inductor terminal, the first inductor terminal coupled to the capacitive network, and the second inductor terminal coupled to ground terminal.

