Coefficient estimation for digital IQ calibration
An RF reception system and method uses IF quadrature mixing, in which there is further mixing and channel filtering in the digital domain, to isolate a frequency of interest. A coefficient estimator is used for generating a phase correction coefficient and an amplitude correction coefficient from filtered in-phase and quadrature desired signals and from filtered in-phase and quadrature image signals.
1. An RF receiver, comprising:
a quadrature mixer which receives the RF input signal and which outputs in-phase and quadrature channels;
analogue to digital converters for the in-phase and quadrature channels;
an amplitude compensator for adjusting the amplitude of one or both of the channels to correct for amplitude mismatch, based on an amplitude correction coefficient;
a phase compensator for adjusting the relative phase between the two channels to correct for phase mismatch, based on a phase correction coefficient;
a first mixer generating in-phase and quadrature desired signals from the amplitude adjusted and phase adjusted channels by mixing with a first frequency, and a first digital channel filter at the output of the first mixer generating a filtered in-phase desired signal and a filtered quadrature desired signal;
a second mixer generating in-phase and quadrature image signals from the amplitude adjusted and phase adjusted channels by mixing with a second frequency, and a second digital channel filter at the output of the second mixer generating a filtered in-phase image signal and a filtered quadrature image signal;
a coefficient estimator for generating the phase correction coefficient and the amplitude correction coefficient from the filtered in-phase and quadrature desired signals and the filtered in-phase and quadrature image signals.
2. A receiver as claimed in claim 1 , wherein the coefficient estimator comprises:
a third mixer for mixing the filtered in-phase and quadrature desired signals with a third frequency to generate shifted in-phase and quadrature desired signals;
a fourth mixer for mixing the filtered in-phase and quadrature image signals with a fourth frequency which is the negative of the first frequency to generate shifted in-phase and quadrature image signals; and
an adder for combining the shifted in-phase and quadrature signals to generate a combined in-phase signal and a combined quadrature signal.
3. A receiver as claimed in claim 2 , wherein the fourth frequency is the analogue to digital converter sampling frequency divided by 4.
4. A receiver as claimed in claim 2 , wherein the coefficient estimator further comprises:
an amplitude coefficient circuit for obtaining an amplitude difference between the combined in-phase signal and the combined quadrature signal.
5. A receiver as claimed in claim 4 , wherein the amplitude coefficient circuit comprises an integrator for integrating the difference between samples of the combined in-phase signal and the combined quadrature signal.
6. A receiver as claimed in claim 2 , wherein the coefficient estimator further comprises:
a phase coefficient circuit for obtaining a phase difference between the combined in-phase signal and the combined quadrature signal.
7. A receiver as claimed in claim 6 , wherein the phase coefficient circuit comprises an integrator for integrating the correlation between samples of the combined in-phase signal and the combined quadrature signal.
8. A receiver as claimed in claim 1 , wherein the amplitude compensator comprises a variable gain amplifier in series with one of the channels, controlled by the amplitude correction coefficient.
9. A receiver as claimed in claim 1 , wherein the phase compensator comprises a variable gain amplifier and an adder, for adding an amplified component of one channel to the other channel, the variable gain amplifier of the phase compensator being controlled by the phase correction coefficient.
10. A receiver as claimed in claim 1 , wherein the first and second digital channel filters have an output oversampled at least by a factor of 2.
11. A method for RF reception in an RF receiver, comprising:
a set of circuits within the RF receiver configured to,
perform quadrature mixing of a received RF input signal to generate in-phase and quadrature channels;
convert the in-phase and quadrature channels to digital;
adjust the amplitude of one or both of the channels to correct for amplitude mismatch, based on an amplitude correction coefficient;
adjust the relative phase between the two channels to correct for phase mismatch, based on a phase correction coefficient;
mix and channel filter the amplitude and phase adjusted channels to generate in-phase and quadrature desired signals, by mixing with a first frequency;
mix and channel filter the amplitude and phase adjusted channels to generate in-phase and quadrature image signals, by mixing with a second frequency; and
generate the phase correction coefficient and the amplitude correction coefficient from the filtered in-phase and quadrature desired signals and the filtered in-phase and quadrature image signals.
12. The method as claimed in claim 11 , wherein the circuit for generating the phase correction coefficient and the amplitude correction coefficient further comprises a set of circuits configured to:
mix the filtered in-phase and quadrature desired signals with a third frequency to generate shifted in-phase and quadrature desired signals;
mix the filtered in-phase and quadrature image signals with a fourth frequency which is the negative of the first frequency to generate shifted in-phase and quadrature image signals;
combine the shifted in-phase and quadrature signals to generate a combined in-phase signal and a combined quadrature signal.
13. The method as claimed in claim 12 , wherein the fourth frequency is the analogue to digital converter sampling frequency divided by 4.
14. The method as claimed in claim 12 , wherein:
the circuit configured to generate the amplitude correction coefficient further comprises obtaining an amplitude difference between the combined in-phase signal and the combined quadrature signal by integrating the difference between samples of the combined in-phase signal and the combined quadrature signal; and
the circuit configured to generate the phase correction coefficient further comprises obtaining a phase difference between the combined in-phase signal and the combined quadrature signal by integrating the correlation between samples of the combined in-phase signal and the combined quadrature signal.