Digital phase feedback for determining phase distortion
A feedback loop is used to determine phase distortion created in a signal by directly extracting the phase distortion information from a feedback signal using original frequency modulation information.
1. A circuit for determining a phase distortion comprising:
a phase modulator that receives a reference signal and a frequency modulation signal for generating a first signal;
a delay block that receives and delays the frequency modulation signal;
a Sigma-Delta modulator that receives the delayed frequency modulation signal for determining a corresponding divider ratio;
a multi-modulus divider that receives a feedback signal derived from the first signal and divides the feedback signal according to the divider ratio received from the Sigma-Delta modulator for removing phase modulation applied by the phase modulator; and
a time-to-digital converter for receiving the feedback signal output from the multi-modulus divider and the reference signal as inputs, and for outputting a derived signal representing phase distortion of the first signal.
2. The circuit according to claim 1 , further comprising:
a multiplier for multiplying the first signal with an amplitude modulation signal prior to deriving the feedback signal from the first signal; and
a phase restoration block that receives the feedback signal prior to the multi-modulus divider for restoring phase to the feedback signal.
3. The circuit according to claim 1 , further comprising a lowpass filter for receiving the derived signal representing phase distortion of the first signal and for removing noise added by the Sigma-Delta modulator.
4. The circuit according to claim 1 , wherein the delay block is an all-pass filter that delays the frequency modulation signal to match a delay in the feedback signal.
5. The circuit according to claim 1 , further comprising a directional coupler for receiving the first signal following modulation and from which the feedback signal is derived as a trace signal.
6. The circuit according to claim 1 , further comprising a reference clock for providing the reference signal to the phase modulator and for providing the reference signal to the time-to-digital converter.
7. The circuit according to claim 1 , wherein the derived signal that is output by the time-to-digital converter is a derived time difference representing phase distortion added to the first signal in a transmit path including the phase modulator.
8. The circuit according to claim 1 ,
wherein the phase modulator and a multiplier or a power amplifier make up a transmit path of the first signal, and
wherein phase distortion added to the feedback signal by a feedback path is small compared to phase distortion added to the first signal by the transmit path.
9. The circuit according to claim 2 , wherein the phase restoration block includes a signal limiter for removing amplitude information from the feedback signal.
10. The circuit according to claim 1 , wherein the multi-modulus divider removes the phase modulation applied by the phase modulator by dividing a frequency of the feedback signal according to the divider ratio received from the Sigma-Delta modulator to reduce the frequency of the feedback signal, whereby the phase modulation is removed.
11. The circuit according to claim 1 , wherein the derived signal is used for adaptive predistortion compensation for compensating for phase distortion in the first signal caused by a transmit path including the phase modulator.
12. A circuit for determining a phase distortion in a transmit path of a polar transmitter, comprising:
a reference clock for outputting a reference signal;
a phase modulator that receives the reference signal and a frequency modulation signal for generating a modulated first signal;
a multiplier that multiplies the modulated first signal with an amplitude modulation signal for producing a transmit signal, wherein the transmit path includes the phase modulator and the multiplier;
a coupler for deriving a feedback signal from the transmit signal;
a phase restoration block that receives the feedback signal and removes amplitude information therefrom;
a delay block that receives and delays the frequency modulation signal to match a delay in the feedback signal;
a Sigma-Delta modulator for receiving the delayed frequency modulation signal for determining a corresponding divider ratio;
a multi-modulus divider for receiving the feedback signal derived from the phase restoration block and dividing the feedback signal according to the divider ratio received from the Sigma-Delta modulator for removing phase modulation applied by the phase modulator; and
a time-to-digital converter for receiving the feedback signal from the multi-modulus divider and the reference signal as inputs and for outputting a derived representation of the phase distortion in form of a time delta.
13. The circuit according to claim 12 , wherein the time-to-digital converter outputs the time delta as a digital signal representing phase distortion caused by the transmit path of the first signal.
14. The circuit according to claim 12 , wherein the measured phase distortion signal is used for reducing transmit distortion and therefore improving spectral quality or other transmit parameter of said transmit path.