Digital FM radio transmitter

1. A digital FM transmitter, comprising:

a digital controlled oscillator clocked by a master clock at a clock rate f CLK and generating an overflow output signal at an average frequency f DCO , where overflow transitions are aligned with the master clock and contain an instantaneous phase error determined by a remainder value of the digital controlled oscillator at the time of an overflow condition;

a processor for receiving digital input samples and generating therefrom a modulating signal for input to said digital controlled oscillator whereby the overflow output signal of the digital controlled oscillator is modulated in accordance with said modulating signal;

a phase noise attenuator for attenuating said instantaneous phase error from the overflow output signal; and

a bandpass filter for filtering frequency components of the overflow output signal with the phase error attenuated outside a predetermined frequency band and supplying said filtered output signal as a modulated RF carrier to an antenna.

2. A digital FM transmitter as claimed in claim 1 , wherein said processor is a digital signal processor.

3. A digital FM transmitter as claimed in claim 1 , further comprising an attenuator for attenuating said RF carrier supplied to said antenna.

4. A digital FM transmitter as claimed in claim 1 , wherein said digital signal processor comprises a pre-emphasis filter for boosting higher frequency components of the modulating signal.

5. A digital FM transmitter as claimed in claim 4 , further comprising a frequency deviation multiplier for weighting the modulating signal by a weighting factor.

6. A digital FM transmitter as claimed in claim 5 , further comprising an adder for adding said modulating signal to a center frequency of said digital controlled oscillator.

7. A digital FM transmitter as claimed in claim 5 , wherein said weighting factor satisfies following equation:

f c /D FM =N fc /(max_ampl.N D FM )

where f c is the carrier frequency, D FM is maximum frequency deviation, max_ampl is the maximum amplitude of the modulating signal, N fc is a number specifying the carrier frequency generated by the digital controlled oscillator, and N DFM is the number representing the maximum frequency deviation.

8. A digital FM transmitter as claimed in claim 1 , wherein said phase noise attenuator comprises an analog phase locked loop to filter phase noise outside a loop bandwidth thereof.

9. A digital FM transmitter as claimed in claim 1 , wherein said phase noise attenuator comprises a tapped delay line controlled by the remainder of the digital controlled oscillator.

10. A digital FM transmitter as claimed in claim 9 , wherein said tapped delay line is followed by an analog phase locked loop.

11. A digital FM transmitter as claimed in claim 1 , wherein said phase noise attenuator comprises a digital sigma-delta jitter shaper.

12. A digital FM transmitter as claimed in claim 11 , wherein the output of said jitter shaper controls the division factor of the feedback loop of an analog phase locked loop.

13. A method of generating a frequency modulated RF signal comprising:

generating an overflow output signal with a digital controlled oscillator clocked by a master clock at a clock rate f CLK and having an average frequency f DCO , where overflow transitions are aligned with the master clock and contain an instantaneous phase error determined by a remainder value of the digital controlled oscillator at the time of an overflow condition;

receiving input samples at a digital signal processor;

generating a modulating signal in said digital signal processor;

supplying said modulating signal to said digital controlled oscillator to modulate the overflow output signal of the digital controlled oscillator;

attenuating the instantaneous phase error from the overflow output signal; and

filtering the overflow output signal with the instantaneous phase error attenuated to remove frequency components of the overflow output signal outside a predetermined frequency band to supply to an antenna with a modulated RF carrier.

14. A method as claimed in claim 13 , wherein the overflow output signal of said digital controlled oscillator has a fundamental frequency outside the FM band, and said RF signal is obtained by filtering a harmonic of said output within said FM band.

15. A method as claimed in claim 13 , wherein the gain of said RF signal is controlled by selecting which harmonic of the overflow output signal is located in the FM Radio Band.

16. A method as claimed in claim 13 , wherein the digital samples are weighted prior to adding them to the center frequency of the digital controlled oscillator to generate said modulated carrier.

17. A method as claimed in claim 13 , wherein said samples are pre-emphasized to boost higher frequency components in software prior to modulating said carrier.

18. A method as claimed in claim 13 , wherein the phase error is attenuated by increasing the clock frequency of the digital controlled oscillator.