Method and apparatus for up-conversion of radio frequency (RF) signals
This patent describes a method and system which overcomes the LO-leakage problem of direct conversion and similar RF transmitters. To solve this problem a virtual LO™ signal is generated within the baseband which is tuned to the incoming RF signal. The virtual local oscillator (VLO) signal is constructed using signals that do not contain a significant amount of power (or no power at all) at the wanted output RF frequency. Any errors is generating the virtual LO signal are minimized using a closed loop correction scheme.
1. A radio frequency (RF) up-convertor with reduced local oscillator leakage, for emulating the modulation of an input signal x(t) with a local oscillator signal having frequency ƒ, said up-convertor comprising:
a synthesizer for generating mixing signals φ 1 and φ 2 which vary irregularly over time, where:
φ 1 *φ 2 has significant power at the frequency ƒ of said local oscillator signal being emulated;
neither φ 1 nor φ 2 has significant power at the frequency ƒ of said local oscillator signal being emulated, and
said mixing signals φ 1 and φ 2 are designed to emulate said local oscillator signal having frequency ƒ in a time domain analysis;
a first mixer coupled to said synthesizer for mixing said input signal x(t) with said mixing signal φ 1 to generate an output signal x(t) φ 1 ; and
a second mixer coupled to said synthesizer and to the output of said first mixer for mixing said signal x(t) φ 1 with said mixing signal φ 2 to generate an output signal x(t) φ 1 φ 2 , said output signal x(t) φ 1 φ 2 emulating the modulation of said input signal x(t) with said local oscillator signal having frequency f.
2. The radio frequency (RF) up-converter of claim 1 wherein said synthesizer further comprises:
a synthesizer for generating mixing signals φ 1 and φ 2 , where φ 1 *φ 1 *φ 2 does not have a significant amount of power within the bandwidth of said output signal x(t) φ 1 φ 2 .
3. The radio frequency (RF) up-convertor of claim 1 wherein said synthesizer further comprises:
a synthesizer for generating mixing signals φ 1 and φ 2 , where φ 1 * φ 2 does not have a significant amount of power within the bandwidth of said output signal x(t) φ 1 φ 2 .
4. The convertor of claim 3 , further comprising:
a closed loop error correction circuit.
5. The radio frequency (RF) up-convertor of claim 4 , wherein said closed loop error correction circuit further comprises:
an error level measurement circuit for measuring an error in said output signal x(t) φ 1 φ 2 ; and
a time-varying signal modification circuit for modifying a parameter of one of said mixing signals φ 1 φ 2 to minimize said error level.
6. The radio frequency (RF) up-convertor of claim 5 , wherein said error level measurement circuit comprises a power measurement.
7. The radio frequency (RF) up-convertor of claim 5 , wherein said error level measurement circuit comprises a voltage measurement.
8. The radio frequency (RF) up-convertor of claim 5 , wherein said error level measurement circuit comprises a current measurement.
9. The radio frequency (RF) up-convertor of claim 5 , wherein said modified parameter is the phase delay of one of said mixing signals φ 1 and φ 2 .
10. The radio frequency (RF) up-convertor of claim 5 , wherein said modified parameter is the fall or rise time of one of said mixing signals φ 1 and φ 2 .
11. The radio frequency (RF) up-convertor of claim 5 , wherein said modified parameter includes both the phase delay and the fall or rise time of one of said mixing signals φ 1 and φ 2 .
12. The radio frequency (RF) up-convertor of claim 4 , wherein said closed loop error correction circuit comprises a digital signal processor (DSP).
13. The radio frequency (RF) up-convertor of claim 4 , wherein said closed loop error correction circuit comprises analogue components.
14. The radio frequency (RF) up-convertor of claim 4 , wherein said closed loop error correction circuit further comprises:
an error level measurement circuit for measuring an error in said output signal x(t) φ 1 ; and
a time-varying signal modification circuit for modifying a parameter of one of said mixing signals φ 1 and φ 2 to minimize said error level.
15. The radio frequency (RF) up-convertor of claim 3 wherein said synthesizer further comprises:
a synthesizer for generating mixing signals φ 1 and φ 2 , where said mixing signals φ 1 and φ 2 can change with time in order to reduce errors.
16. The radio frequency (RF) up-convertor of claim 3 , further comprising:
a DC offset correction circuit.
17. The radio frequency (RF) up-convertor of claim 16 , wherein said DC offset correction circuit comprises:
a DC offset generating circuit for generating a DC offset voltage;
a summer for adding said DC offset voltage to an output of one of said mixers; and
a DC error level measurement circuit for modifying the level of said DC offset voltage to minimize error level.
18. The radio frequency (RF) up-convertor of claim 17 , wherein said DC error level measurement circuit comprises a power measurement circuit.
19. The radio frequency (RF) up-convertor of claim 17 , wherein said DC error level measurement circuit comprises a voltage measurement circuit.
20. The radio frequency (RF) up-convertor of claim 17 , wherein said DC error level measurement circuit comprises a current measurement circuit.
21. The radio frequency (RF) up-convertor of claim 3 , further comprising:
a local oscillator coupled to said synthesizer for providing a periodic signal having a frequency that is an integral multiple of the frequency of said local oscillator signal being emulated.
22. The radio frequency (RF) up-convertor of claim 1 , further comprising: a filter for removing unwanted signal components.
23. The radio frequency (RF) up-convertor of claim 22 , where said filter comprises:
a filter for removing unwanted signal components from said x(t) φ 1 signal.
24. The radio frequency RF up-converter of claim 1 , wherein said mixing signal is a square wave.
25. The radio frequency (RF) up-convertor of claim 1 , wherein said mixing signals φ 1 and φ 2 effect the modulation of an in-phase component of said input signal x(t), and a complementary up-convertor with mixing signals 90 degrees out of phase, is used to effect the modulation of a quadrature component of said input signal x(t).
26. The radio frequency (RF) up-convertor of claim 1 , wherein said synthesizer uses a single time base to generate both mixing signals φ 1 and φ 2 .
27. The radio frequency (RF) up-convertor of claim 1 , wherein said mixing signals φ 1 and φ 2 are digital waveforms.
28. The radio frequency (RF) up-convertor of claim 1 , wherein said mixing signals φ 1 and φ 2 are square waveforms.
29. The radio frequency (RF) up-convertor of claim 1 , where said synthesizer uses different patterns to generate signals φ 1 and φ 2 .
30. An integrated circuit comprising the radio frequency (RF) up-convertor of claim 1 .
31. A method of modulating a baseband signal x(t) comprising the steps of:
generating mixing signals φ 1 and φ 2 which vary irregularly over time, where:
φ 1 *φ 2 has significant power at the frequency ƒ of a local oscillator signal being emulated;
neither φ 1 nor φ 2 has significant power at the frequency ƒ of said local oscillator signal being emulated, and
said mixing signals φ 1 and φ 2 are designed to emulate said local oscillator signal having frequency f, in a time domain analysis;
mixing said input signal x(t) with said mixing signal φ 1 to generate an output signal x(t) φ 1 ; and
mixing said signal x(t) φ 1 with said mixing signal φ 2 to generate an output signal x(t) φ 1 φ 2 .