Enhanced predistortion for slewing correction
The present invention relates to a circuit arrangement and method of applying predistortion to a baseband signal used for modulating a pulse-shaped signal, wherein an envelope information of the baseband signal is detected and slewing distortions of the pulse-shaped signal are reduced by applying at least one of a phase modulation and a duty cycle 5 modulation to the baseband signal as additional predistortion in response to the detected envelope information. Thereby, slewing distortions in the pulse-shaped signal are removed or at least reduced.
1. A circuit arrangement for applying predistortion to a baseband signal applied as a modulation signal to a pulse modulation circuit which generates a pulse-shaped signal, said circuit arrangement comprising:
a. a detection unit configured to determine envelope information of said baseband signal, said envelope information comprising baseband components sin(θ 1 (t)), cos(θ 1 (t)), sin(θ 2 (t)), and cos(θ 2 (t)) with θ 1 (t)=Φ P (t)+Φ M (t)+Ψ(t) and θ 2 (t)=Φ P (t)−Φ M (t)+Ψ(t), wherein Φ P (t) is a phase of the baseband signal, Φ M (t) corresponds to a duty cycle d(t), and Ψ(t) is an extra phase predistortion to compensate for symmetric slewing; and
b. a predistortion unit configured to supply the baseband components to the pulse modulation circuit which comprises four multipliers in order to multiply the baseband components by two sine- and cosine-shaped RF carrier signals, wherein at least one of the duty cycle and the phase of the baseband signal is modulated to generate additional predistortion for reducing slewing distortions in response to said determined envelope information.
2. The circuit arrangement according to claim 1 , wherein said detection unit is arranged to determine said envelope information based on a normalized envelope of said baseband signal.
3. The circuit arrangement according to claim 2 , wherein said detection unit is arranged to determine said envelope information by calculating a level of a first harmonic of said normalized envelope of said baseband signal.
4. The circuit arrangement according to claim 1 , wherein said predistortion unit is arranged to calculate said modulation of said duty cycle or said phase based on said envelope information, and to change said calculation when said envelope information exceeds a predetermined threshold value.
5. The circuit arrangement according to claim 4 , further comprising a threshold determination unit configured to determine said predetermined threshold value based on an amplitude and at least one slew rate value of said pulse-shaped signal.
6. The circuit arrangement according to claim 1 , further comprising a control loop configured to control at least one of a rising edge and a falling edge of said pulse-shaped signal to obtain a symmetrical slew of said pulse-shaped signal.
7. The circuit arrangement according to claim 6 , wherein said control loop comprises a slope detector configured to measure slopes of at least one of said rising edge and said falling edge of said pulse-shaped signal, and a slew rate correction unit configured to control the slew rate of at least one of said rising edge and said falling edge of said pulse-shaped signal to obtain a substantially symmetrical slewing of said pulse-shaped signal.
8. A method of applying predistortion to a baseband signal used for modulating a pulse-shaped signal, said method comprising:
a. determining envelope information of said baseband signal, said envelope information comprising baseband components sin(θ 1 (t)), cos(θ 1 (t)), sin(θ 2 (t)), and cos(θ 2 (t)) with θ 1 (t)=Φ P (t)+Φ M (t)+Ψ(t) and θ 2 (t)=Φ P (t)−Φ M (t)+Ψ(t), wherein Φ P (t) is a phase of the baseband signal, Φ M (t) corresponds to a duty cycle d(t), and Ψ(t) is an extra phase predistortion to compensate for symmetric stewing; and
b. supplying the baseband components to a pulse modulation circuit which comprises four multipliers in order to multiply the baseband components by two sine- and cosine-shaped RF carrier signals, wherein at least one of the duty cycle and the phase of the baseband signal is modulated to generate additional predistortion for reducing slewing distortions in response to said determined envelope information.
9. The method according to claim 8 , further comprising controlling at least one of a rising edge and a falling edge of said pulse-shaped signal to obtain a symmetrical slew of said pulse-shaped signal.