Harmonic suppression in switching amplifiers
Configurable suppression of harmonics in a radio frequency (RF) transmitter circuit having two class-D switching amplifiers that produce a differential output signal having introduced harmonics is contemplated. A selected harmonic is used to determine a time duration. A harmonic suppression circuit modifies a radio frequency polar modulated data signal that is encoded using an amplitude component and a phase component. The modification is responsive to the determined time duration. The switching power amplifiers amplify the modified polar modulated data signal to produce an amplified signal. The amplified signal includes three signal levels, a high signal level, a middle signal level and a low signal level. The timing of transitions between the high and low signal levels represents the phase component, and the transitions include the middle signal level for the time duration, thereby suppressing the selected harmonic.
1. A method for providing configurable suppression of harmonics in a radio frequency (RF) transmitter circuit that includes two class-D switching power amplifiers that produce a differential output signal having introduced harmonics, the method comprising:
determining a time duration from a selected harmonic of the introduced harmonics;
adjusting, based upon the determined time duration, a delay of a delay circuit;
modifying, using a harmonic suppression circuit of the radio frequency transmitter circuit, a RF polar modulated data signal that is encoded using an amplitude component and a phase component that is defined relative to a carrier wave, the modification including adding the delay of the delay circuit to the RF polar modulated data signal; and
amplifying the modified RF polar modulated data signal using a first of the switching power amplifiers and amplifying the unmodified RF polar modulated data signal using a second of the switching power amplifiers to produce an amplified signal of the RF polar modulated data signal, the amplified signal being a differential output signal that includes three signal levels, a high signal level, a middle signal level and a low signal level, wherein timing of transitions between the high and low signal levels represents the phase component and wherein the transitions include the middle signal level for the determined time duration, thereby suppressing the selected harmonic.
2. The method of claim 1 , wherein step of modifying the RF polar modulated data signal includes introducing the middle signal level during a transition from the high to low signal levels and from the low to high signal levels.
3. The method of claim 1 , wherein the step of determining the time duration is carried out as a function of a frequency of the selected harmonic.
4. The method of claim 1 , wherein the step of determining the time duration is carried out by calculating a half a period of a frequency of the selected harmonic.
5. The method of claim 1 , further including a step of implementing the time duration by delaying one of two clock signals driving flip flops to respective inputs to the two class-D switching amplifiers.
6. The method of claim 1 , further including a step of implementing the time duration using a delay-locked loop (DLL) positioned before an input of one of the two class-D switching amplifiers.
7. The method of claim 1 , further including a step of adjusting an amplitude of the amplified signal by modifying a supply voltage provided to each of the two class-D switching amplifiers in response to the amplitude component.
8. A device comprising:
a transmitter circuit that includes
two class-D switching amplifiers configured and arranged to produce a differential output signal having introduced harmonics in response to a radio frequency polar modulated data signal that is encoded using an amplitude component and a phase component that is defined, relative to a carrier, by timings of transitions between a high signal level and a low signal level; and
a harmonic suppression circuit that:
includes a delay circuit that is configured to introduce a delay that is tunable in response to a time duration corresponding to a selected harmonic; and
that
is configured and arranged to
modify the polar modulated data signal using the delay circuit to introduce the delay to the polar modulated data signal;
provide the modified polar modulated data signal to a first of the class-D switching power amplifiers;
provide the unmodified polar modulated data signal to a second of the class-D switching power amplifiers; and
produce, from outputs of the class-D switching power amplifiers, an amplified signal of the polar modulated data signal that includes a middle signal level that is between the high and low signal levels and for the time duration, thereby suppressing the selected harmonic.
9. The device of claim 8 , wherein the harmonic suppression circuit is further configured and arranged to set the time duration to half of a period of a frequency of the selected harmonic.
10. The device of claim 8 , wherein the delay circuit includes a DLL that is configured and arranged to delay the polar modulated data signal for the time duration before reaching one of the two class-D switching amplifiers.
11. The device of claim 8 , wherein the delay circuit includes two flip-flops clocked by two different clock signals, the clock signals having a phase difference corresponding to the time duration.
12. A transmitter device comprising:
a harmonic suppression circuit configured and arranged to introduce, to a polar modulated data signal that includes an amplitude component and a phase component that is defined by transitions between high and low signal levels, a third signal level having a voltage that is between voltages of the high and low signal levels and that has a time duration that is adjustable to different duration values based upon a frequency corresponding to different selected harmonics of harmonics introduced by class-D amplification;
a supply generator circuit configured to provide a first voltage supply output corresponding to the high signal level and a second voltage supply output corresponding to the third signal level and to adjust the high signal level and the third signal level based upon the amplitude component, and
a three-level class-D switching amplifier configured and arranged to
receive power from the first voltage supply output and second voltage supply output; and
amplify the polar modulated data signal to produce a single-ended output signal that includes the low, high, and third signal levels, wherein the third signal level corresponds to the time duration from the harmonic suppression circuit, thereby suppressing a particular selected harmonic.
13. The device of claim 12 , wherein the harmonic suppression circuit is further configured and arranged to introduce the third signal level during the transitions, and wherein transitions include transitions from the high to low signal levels and from the low to high signal levels.
14. The device of claim 12 , wherein the harmonic suppression circuit is further configured and arranged to introduce the third signal level during the transitions for the time duration by calculating the time duration as a function of a frequency of the particular selected harmonic.
15. The device of claim 12 , further including a supply voltage circuit configured and arranged to provide supply voltage to the three-level switching amplifier and to vary a voltage level of the supply voltage in response to the amplitude component.
16. The device of claim 15 , wherein the three-level switching amplifier is configured and arranged to generate the low signal level at zero voltage, the high signal level at the supply voltage as modified in response to the amplitude component and the third signal level at one-half of the supply voltage as modified in response to the amplitude component.
17. The device of claim 12 , wherein the time duration corresponds to half of a period for a frequency of the particular selected harmonic.
18. The device of claim 12 , wherein the harmonic suppression circuit configured and arranged to provide attenuation of at least 20 dB for a seventh harmonic of the polar modulated data signal, wherein the polar modulated data signal has a carrier wave frequency of 13.56 MHz.