Capacitance measurement circuitry for a resonant tank
Wireless circuitry is provided that includes a transformer having a primary coil and a secondary coil, a first group of switchable capacitors coupled to a first terminal of the primary coil, a second group of switchable capacitors coupled to a second terminal of the primary coil, and capacitance measurement circuitry selectively coupled to a center tap of the primary coil during a measurement mode. The capacitance measurement circuitry can include a current source, a current sink, a comparator, a voltage generator for providing one or more threshold/reference voltages to the comparator, a frequency measurement circuit, and a capacitance calculation circuit configured to compute a capacitance of the first and second groups of switchable capacitors.
1 . Circuitry comprising:
a transformer having a primary coil and a secondary coil;
first switchable capacitors coupled to a first terminal of the primary coil;
second switchable capacitors coupled to a second terminal of the primary coil; and
capacitance measurement circuitry coupled to a center tap of the primary coil and configured to measure a capacitance of the first and second switchable capacitors.
2 . The circuitry of claim 1 , further comprising:
a first switch having a first terminal coupled to the center tap of the primary coil and having a second terminal configured to receive a bias voltage.
3 . The circuitry of claim 2 , further comprising:
a second switch having a first terminal coupled to the center tap of the primary coil and having a second terminal coupled to the capacitance measurement circuitry.
4 . The circuitry of claim 3 , wherein the capacitance measurement circuitry comprises:
a current source selectively coupled to the second terminal of the second switch;
a third switch coupled between the current source and the second terminal of the second switch;
a current sink selectively coupled to the second terminal of the second switch; and
a fourth switch coupled between the current sink and the second terminal of the second switch.
5 . The circuitry of claim 4 , wherein the capacitance measurement circuitry further comprises:
a fifth switch coupled between the current source and a ground power supply line; and
a sixth switch coupled between the current sink and a positive power supply line.
6 . The circuitry of claim 4 , wherein the capacitance measurement circuitry further comprises:
a comparator having a first input coupled to the second terminal of the second switch and configured to generate a comparator output signal, wherein the fourth switch is configured to receive the comparator output signal and wherein the third switch is configured to receive an inverted version of the comparator output signal.
7 . The circuitry of claim 6 , wherein the capacitance measurement circuitry further comprises:
a voltage generation circuit configured to output at least a first threshold voltage and a second threshold voltage to a second input of the comparator.
8 . The circuitry of claim 7 , wherein the capacitance measurement circuitry further comprises:
a counter configured to receive the comparator output signal and to output a corresponding frequency of the comparator output signal.
9 . The circuitry of claim 8 , wherein the capacitance measurement circuitry further comprises:
a capacitance calculation circuit configured to compute the capacitance of the first and second switchable capacitors based on a current level of the current source, a current level of the current sink, the frequency of the comparator output signal, and a voltage difference between the first threshold voltage and the second threshold voltage.
10 . The circuitry of claim 8 , wherein the capacitance measurement circuitry further comprises:
a capacitance calculation circuit configured to compute the capacitance of the first and second switchable capacitors based on a current level of the current source, a current level of the current sink, a first frequency value measured when a voltage difference between the first threshold voltage and the second threshold voltage is at a first value, and a second frequency value measured when the voltage difference is at a second value different than the first value.
11 . Wireless circuitry comprising:
a transformer having a primary coil and a secondary coil, the secondary coil having a first terminal coupled to a radio-frequency circuit and having a second terminal coupled to a ground line;
a first plurality of switchable capacitors coupled to a first terminal of the primary coil;
a second plurality of switchable capacitors coupled to a second terminal of the primary coil; and
capacitance measurement circuitry coupled to a center tap of the primary coil, wherein the capacitance measurement circuitry includes
a current source selectively coupled to the center tap of the primary coil during a charging phase, and
a current sink selectively coupled to the center tap of the primary coil during a discharging phase.
12 . The wireless circuitry of claim 11 , further comprising:
a comparator having a first input selectively coupled to the center tap of the primary coil and having an output on which a comparator output signal is generated;
a voltage generator configured to selectively provide a plurality of reference voltages to a second input of the comparator; and
a frequency measurement unit coupled to the output of the comparator and configured to output a frequency of the comparator output signal.
13 . An electronic device comprising:
an antenna; and
a radio-frequency circuit having an output coupled to the antenna, wherein the radio-frequency circuit includes
a transformer having a first coil and a second coil that is coupled to the output,
first switchable capacitors coupled to a first terminal of the first coil,
second switchable capacitors coupled to a second terminal of the first coil, and
measurement circuitry coupled to a center tap of the first coil and configured to measure a capacitance of the first and second switchable capacitors.
14 . The electronic device of claim 13 , wherein the radio-frequency circuit further comprises:
a first switch having a first terminal coupled to the center tap of the first coil and a second terminal configured to receive a bias voltage; and
a second switch having a first terminal coupled to the center tap of the first coil and a second terminal coupled to the measurement circuitry.
15 . The electronic device of claim 14 , wherein the measurement circuitry comprises:
a current source selectively coupled to the second terminal of the second switch;
a third switch coupled between the current source and the second terminal of the second switch;
a current sink selectively coupled to the second terminal of the second switch; and
a fourth switch coupled between the current sink and the second terminal of the second switch.
16 . The electronic device of claim 15 , wherein the measurement circuitry further comprises:
a fifth switch coupled between the current source and a ground power supply line; and
a sixth switch coupled between the current sink and a positive power supply line.
17 . The electronic device of claim 15 , wherein the measurement circuitry further comprises:
a comparator having a first input coupled to the second terminal of the second switch and configured to generate a comparator output signal, wherein the fourth switch is configured to receive the comparator output signal and wherein the third switch is configured to receive an inverted version of the comparator output signal.
18 . The electronic device of claim 17 , wherein the measurement circuitry further comprises:
a voltage generation circuit configured to output at least a first threshold voltage and a second threshold voltage to a second input of the comparator; and
a counter configured to receive the comparator output signal and to output a corresponding frequency of the comparator output signal.
19 . The electronic device of claim 18 , wherein the measurement circuitry further comprises:
a calculation circuit configured to compute the capacitance of the first and second switchable capacitors based on a current level of the current source, a current level of the current sink, the frequency of the comparator output signal, and a voltage difference between the first threshold voltage and the second threshold voltage.
20 . The electronic device of claim 18 , wherein the measurement circuitry further comprises:
a calculation circuit configured to compute the capacitance of the first and second switchable capacitors based on one or more of: a current level of the current source, a current level of the current sink, a first frequency value measured when a voltage difference between the first threshold voltage and the second threshold voltage is at a first value, and a second frequency value measured when the voltage difference is at a second value different than the first value.