Method and apparatus for down-conversion of radio frequency (RF) signals with reduced local oscillator leakage
This patent describes a method of removing the LO-leakage and 1/f noise problems associated with direct conversion RF receivers and other demodulators. In order to solve this problem a virtual LO™ signal is generated within the RF signal path 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 LO frequency. Any errors in generating the virtual LO signal are minimized using a closed loop correction scheme.
1. A radio frequency (RE) down-convertor with reduced local oscillator leakage, for emulating the demodulation of an input signal x(t) with a local oscillator signal having frequency f, said down-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 f of said local oscillator signal being emulated;
neither φ 1 nor φ 2 has significant power at the frequency f 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;
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) down-convertor of claim 1 wherein said synthesizer is further operable to generate mixing signals φ 1 and φ 2 , such that the product φ 1 *φ 1 *φ 2 will not result in a significant amount of power within the bandwitdh of an input signal that the down-converter is designed to down-convert to baseband.
3. The radio frequency (RF) down-convertor of claim 2 , further comprising:
a DC offset correction circuit.
4. The radio frequency (RF) down-convertor of claim 3 , wherein said DC offset correction circuit comprises:
a DC source having a DC output; and
a summer for adding said DC output to an output of one of said mixers.
5. The radio frequency (RF) down-convertor of claim 2 , further comprising:
a closed loop error correction circuit.
6. The radio frequency (RF) down-convertor of claim 5 , wherein said closed loop error correction circuit further comprises:
an error level measurement circuit 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.
7. The radio frequency (RF) down-convertor of claim 6 , wherein said error level measurement circuit comprises a power measurement.
8. The radio frequency (RF) down-convertor of claim 6 , wherein said error level measurement circuit comprises a voltage measurement.
9. The radio frequency (RF) down-convertor of claim 6 , wherein said error level measurement circuit comprises a current measurement.
10. The radio frequency (RF) down-convertor of claim 6 , wherein said modified parameter is the phase delay of one of said mixing signals φ 1 and φ 2 .
11. The radio frequency (RF) down-convertor of claim 6 , wherein said modified parameter is the fall or rise time of one of said mixing signals φ 1 and φ 2 .
12. The radio frequency (RF) down-convertor of claim 6 , 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 .
13. The radio frequency (RF) down-convertor of claim 2 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.
14. The radio frequency (RF) down-convertor of claim 1 , further comprising:
a filter for removing unwanted signal components from said x(t) φ 1 signal.
15. The radio frequency (RF) down-convertor of claim 1 , wherein said mixing signal φ 2 is a square wave.
16. The radio frequency (RF) down-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 down-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).
17. The radio frequency (RF) down-convertor of claim 1 , wherein said mixing signals φ 1 and φ 2 are irregular.
18. The radio frequency (RF) down-convertor of claim 1 , wherein said signals φ 1 and φ 2 are digital waveforms.
19. The radio frequency (RF) down-convertor of claim 1 , wherein said mixing signals φ 1 and φ 2 are square waveforms.
20. The radio frequency (RF) down-convertor of claim 1 , further comprising:
a local oscillator coupled to said synthesizer for providing a signal having a frequency that is an integral multiple of the desired mixing frequency.
21. An integrated circuit comprising the radio frequency (RF) down-convertor of claim 1 .
22. The radio frequency (RF) down-convertor of claim 1 , where said synthesizer uses different patterns to generate signals φ 1 and φ 2 .
23. The radio frequency (RF) down-convertor of claim 1 , wherein said synthesizer uses a single time base to generate both mixing signals φ 1 and φ 2 .
24. A method of demodulating a radio frequency (RF) signal x(t) with reduced local oscillator leakage 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 f of a local oscillator signal being emulated, neither φ 1 nor φ 2 has significant power at the frequency 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 .