High pass modulation of a phase locked loop
View Patent ↗A radio frequency modulator based on direct frequency/phase modulation of output signal of a controllable oscillator ( 724 ) that is a part of a phase locked loop (PLL) provides a direct modulator that is able to operate over a wide frequency range with a flat frequency response. A modulation signal is digitally processed ( 721, 730 ) before injection to a high-pass path of a direct modulator. Applicability of digital signal processing is based on the fact that the modulation signal is a base band signal. Therefore, the modulation signal ( 702 ) occupies such a band in the frequency domain so that a sufficient ratio of a sampling rate to an upper edge frequency of the modulation signal can be achieved. Digital processing is used for compensating an effect of non-flat high-pass PLL transfer function and/or to perform pre-distortion of the input signal of a controlled oscillator to compensate an effect of non-linearity of a controlled oscillator.
1. An apparatus comprising:
a phase locked loop having a controlled oscillator,
a modulation signal input for receiving a modulation signal,
a digital processing circuitry for processing a modulation signal in a digital form for performing at least one of the following: compensating an effect of a non-flat frequency response of a high-pass phase locked loop transfer function, and compensating an effect of non-linearity of the controlled oscillator, and
a connection circuitry disposed to convert a digital output signal of said digital processing circuitry into a form supported by a high-pass injection point of the phase locked loop and to operate the direct modulator as a high-pass direct modulator by feeding a resulting converted signal into said high-pass injection point of the phase locked loop.
2. The apparatus according to claim 1 , wherein said digital processing circuitry comprises a digital pre-compensation filter disposed to filter the modulation signal for compensating the effect of the non-flat frequency response of the high-pass phase locked loop transfer function.
3. The apparatus according to claim 2 , wherein said digital pre-compensation filter is a finite impulse response filter.
4. The apparatus according to claim 2 , wherein said digital pre-compensation filter is an adaptive finite impulse response filter.
5. The apparatus according to claim 2 , wherein said digital pre-compensation filter is an infinite impulse response filter.
6. The apparatus according to claim 1 , wherein said digital processing circuitry comprises a digital pre-distortion device disposed to compensate the effect of non-linearity of the controlled oscillator.
7. The apparatus according to claim 6 , wherein said digital pre-distortion device comprises a memory element disposed to store at least one pre-distortion value, the pre-distortion value being disposed to modify at least one input signal of the controlled oscillator for compensating the effect of non-linearity of the controlled oscillator.
8. The apparatus according to claim 1 , wherein said connection circutry comprises a digital-to-analogue converter and an analogue low-pass filter.
9. The apparatus according to claim 1 , wherein bandwidth of a closed-loop transfer function of said phase locked loop is disposed to be selectable from at least two different values.
10. The apparatus according to claim 9 , wherein said bandwidth of a closed-loop transfer function is disposed to be determined with an adjustable gain in series with a loop filter of said phase locked loop.
11. The apparatus according to claim 9 , wherein said bandwidth of a closed-loop transfer function is disposed to be determined with at least one adjustable component in the loop filter of said phase locked loop.
12. The apparatus according to claim 1 , comprising a first switch element for determining if the direct modulator is operated as a high-pass direct modulator, a circuitry for operating the direct modulator as a low-pass direct modulator, and a second switch element for determining if the direct modulator is operated as a low-pass direct modulator.
13. The apparatus according to claim 12 , wherein the circuitry for operating the direct modulator as a low-pass direct modulator comprise a digital pre-compensation filter disposed to filter the modulation signal for compensating the effect of the non-flat frequency response of a low-pass phase locked loop transfer function.
14. The apparatus according to claim 1 , comprising an amplitude modulator disposed to modulate amplitude of output signal of the controlled oscillator.
15. A mobile communication device comprising:
a direct modulator having a modulation signal input for receiving a modulation signal and being based on a phase locked loop, the phase locked loop having a controlled oscillator,
digital processing circuitry for processing a modulation signal in a digital form for performing at least one of the following: compensating an effect of a non-flat frequency response of a high-pass phase locked loop transfer function, and compensating an effect of non-linearity of the controlled oscillator, and
a connection circuitry disposed to convert a digital output signal of said digital processing circuitry into a form supported by a high-pass injection point of the phase locked loop and to operate the direct modulator as a high-pass direct modulator by feeding a resulting converted signal into said high-pass injection point of the phase locked loop.
16. The mobile communication device according to claim 15 , comprising a control circuitry for selecting a bandwidth of a closed-loop transfer function of said phase locked loop from at least two different values.
17. The mobile communication device according to claim 15 , comprising a circuitry for operating the direct modulator as a low-pass direct modulator and control circuitry for determining if the direct modulator is operated as a high-pass direct modulator and for determining if the direct modulator is operated as a low-pass direct modulator.
18. The mobile communication device according to claim 17 , wherein the circuitry for operating the direct modulator as a low-pass direct modulator comprise a digital pre-compensation filter disposed to filter the modulation signal for compensating the effect of the non-flat frequency response of a low-pass phase locked loop transfer function.
19. The mobile communication device according to claim 15 , wherein the mobile communication device is a mobile phone.
20. A communication module comprising:
a direct modulator having a modulation signal input for receiving a modulation signal and being based on a phase locked loop, the phase locked loop having a controlled oscillator,
a digital processing circuitry for processing a modulation signal in a digital form for performing at least one of the following: compensating an effect of a non-flat frequency response of a high-pass phase locked loop transfer function, and compensating an effect of non-linearity of the controlled oscillator, and
a connection circuitry disposed to convert a digital output signal of said digital processing circuitry into a form supported by a high-pass injection point of the phase locked loop and to operate the direct modulator as a high-pass direct modulator by feeding a resulting converted signal into said high-pass injection point of the phase locked loop.
21. The communication module according to claim 20 , comprising control circuitry for selecting a bandwidth of a closed-loop transfer function of said phase locked loop from at least two different values.
22. The communication module according to claim 20 , comprising a circuitry for operating the direct modulator as a low-pass direct modulator and a control circuitry for determining if the direct modulator is operated as a high-pass direct modulator and for determining if the direct modulator is operated as a low-pass direct modulator.
23. The communication module according to claim 22 , wherein the circuitry for operating the direct modulator as a low-pass direct modulator comprise a digital pre-compensation filter disposed to filter the modulation signal for compensating the effect of the non-flat frequency response of a low-pass phase locked loop transfer function.
24. A method comprising:
processing a modulation signal in a digital form for performing at least one of the following list: compensating an effect of a non-flat frequency response of a high-pass transfer function of a phase locked loop of a direct modulator, and compensating an effect of non-linearity of a controlled oscillator of the phase locked loop of the direct modulator,
converting a digital output signal of said processing into a form supported by a high-pass injection point of the phase locked loop, and
operating the direct modulator as a high-pass direct modulator by feeding a resulting converted signal into said high-pass injection point of the phase locked-loop.
25. The method according to claim 24 , wherein said processing the modulation signal in a digital form comprises filtering the modulation signal with a digital filter for compensating the effect of the non-flat frequency response of the high-pass phase locked loop transfer function.
26. The method according to claim 25 , wherein said filtering the modulation signal is performed with a digital finite impulse response filter.
27. The method according to claim 25 , wherein said filtering the modulation signal is performed with a digital adaptive finite impulse response filter.
28. The method according to claim 25 , wherein said filtering the modulation signal is performed with a digital infinite impulse response filter.
29. The method according to claim 24 , wherein said processing modulation signal in a digital form comprises compensating the effect of non-linearity of the controlled oscillator with a digital pre-distortion device.
30. The method according to claim 29 , wherein said compensating the effect of non-linearity of the controlled oscillator comprises reading at least one pre-distortion value from a memory element and modifying at least one input signal of the controlled oscillator with said pre-distortion value for compensating the effect of non-linearity of the controlled oscillator.
31. The method according to claim 24 , wherein said converting the digital output signal comprises analogue-to-digital converting and low-pass filtering of an output of said analogue-to-digital converting.
32. The method according to claim 24 , comprising modulating an amplitude of output signal of the controlled oscillator.
33. An apparatus comprising:
means for processing modulation signal in a digital form for performing at least one of the following list: compensating an effect of a non-flat frequency response of a high-pass transfer function of a phase locked loop of a direct modulator, and compensating an effect of non-linearity of a controlled oscillator of the phase locked loop of the direct modulator,
means for converting an digital output signal of said processing into a form supported by a high-pass injection point of the phase locked loop, and means for operating the direct modulator as a high-pass direct modulator by feeding a resulting converted signal into said high-pass injection point of the phase locked-loop.