The invention provides a transmitter comprising two (or more) phase locked loops controlling respective oscillators, and implementing different phase modulation. Multiple phases are derived from the respective oscillators, and an edge rotator forms an output signal from a combination of the phases. The oscillators can operate at different frequencies, neither of which is an integer multiple of the other, whereas the output signals of the multiplexers of the first and second phase locked loops are closer in frequency and can be the same. This reduces the problem of pulling, with a circuit that can be implemented with low power and area and with the versatility of being digitally intensive.
1. A transmitter comprising:
an all digital first phase locked loop controlling a first oscillator and receiving a first phase modulated signal;
an all digital second phase locked loop controlling a second oscillator and receiving a second phase modulated signal;
a power amplifier driven by the outputs of the first and second phase locked loops;
a divider deriving multiple phases from the first and second phase locked loops; and
an edge rotator associated with each phase locked loop to form a respective output signal from a combination of the multiple phases,
wherein the first and second oscillators operate at different frequencies, the frequency of the first oscillator is not an integer multiple of the frequency of the second oscillator, and the output signals are closer in frequency than the first and second oscillator frequencies,
wherein an average of the frequencies of the output signals of each edge rotator of the first and second phase locked loops are the same.
2. A transmitter as claimed in claim 1 wherein the deriving includes deriving 4, 8 or more phases.
3. A transmitter as claimed in claim 1 , wherein each edge rotator of the first and second phase locked loops comprises a respective multiplexer.
4. A transmitter as claimed in claim 3 , wherein the divider comprises a divide by 4 divider for deriving 8 phases.
5. A transmitter as claimed in claim 3 , wherein each edge rotator of the first and second phase locked loops comprises a ring counter.
6. A transmitter as claimed in claim 5 , wherein the ring counter of the edge rotator of the first phase locked loop rotates in the opposite sense to the ring counter of the edge rotator of the second phase locked loop.
7. A transmitter as claimed in claim 1 , wherein the first phase locked loop and the second phase locked loop are all-digital, and the first and second oscillators are digitally controlled.
8. A transmitter as claimed in claim 1 , wherein the first and second phase locked loops each comprise a retiming circuit for generating multiplexer control signals.
9. A transmitter as claimed in claim 8 , wherein the retiming circuit comprises a set of edge triggered flip flops.
10. A transmitter as claimed in claim 1 , wherein the first and second oscillators are each tunable, with a different tuning range.
11. A transmitter as claimed in claim 1 , wherein the first oscillator is tunable with a tuning range 6.45 GHz to 8.5 GHz and the second oscillator is tunable with a tuning range from 7.15 GHz to 9.2 GHz.
12. A transmitter as claimed in claim 1 , wherein the period of the output of the first phase locked loop is one of 3.5, 4.5, 4.75, 5.25, 6, 7, 8, 9, and 10 times the period of the first oscillator output signal, and the period of the output of the second phase locked loop is a different one of 3.5, 4.5, 4.75, 5.25, 6, 7, 8, 9, and 10 times the period of the second oscillator output signal.
13. A transmitter as claimed in claim 1 , wherein the period of the output of the first phase locked loop is 3.5 times the period of the first oscillator output signal, and the period of the output of the second phase locked loop is 4.5 times the period of the second oscillator output signal.
14. A mobile telephony base station comprising a transmitter as claimed in claim 1 .