Computing I/Q impairments at system output based on I/Q impairments at system input
View Patent ↗Systems and methods for measuring transmitter and/or receiver I/Q impairments are disclosed, including iterative methods for measuring transmitter I/Q impairments using shared local oscillators, iterative methods for measuring transmitter I/Q impairments using intentionally-offset local oscillators, and methods for measuring receiver I/Q impairments. Also disclosed are methods for computing I/Q impairments from a sampled complex signal, methods for computing DC properties of a signal path between the transmitter and receiver, and methods for transforming I/Q impairments through a linear system.
1. A computer-implemented method for computing I/Q impairments at a complex output of an electrical system based on I/Q impairments at a complex input of the electrical system, the method comprising:
computing a first spectrum based on:
a model spectrum H(f) that models the electrical system,
an input gain imbalance spectrum g(f) that characterizes gain imbalance at the complex input, and
an input phase skew spectrum φ(f) that characterizes phase skew at the complex input;
computing a second spectrum based on:
a frequency-reflected version H(−f) of the model spectrum H(f),
the input gain imbalance spectrum g(f), and
the input phase skew spectrum φ(f);
computing a sum of the first spectrum and the second spectrum, and a difference of the first spectrum and the second spectrum;
computing an output gain imbalance spectrum and an output phase skew spectrum based on real and imaginary parts of the sum, and real and imaginary parts of the difference, wherein the output gain imbalance spectrum and the output phase skew spectrum respectively characterize gain imbalance and phase skew at the complex output; and
storing the output gain imbalance spectrum and the output phase skew spectrum in a memory.
2. The method of claim 1 , wherein the model spectrum H(f) represents an inverse of a signal path, wherein the signal path is a path from an I/Q modulator of a transmitter to a demodulator of a receiver, wherein the input gain imbalance spectrum and the input phase skew spectrum represent gain imbalance and phase skew at an output of the demodulator, wherein the output gain imbalance spectrum and the output phase skew spectrum represent gain imbalance and phase skew at an output of the I/Q modulator.
3. The method of claim 2 , further comprising:
computing an inverse of a spectrum of the signal path to determine the model spectrum H(f).
4. The method of claim 2 , wherein the model spectrum H(f) is based on a DC scaling and a DC rotation of the signal path.
5. The method of claim 4 , wherein the DC scaling is determined by:
capturing a first response signal from the demodulator in response to a zero signal being supplied as input to the I/Q modulator;
capturing a second response signal from the demodulator in response to a constant signal being supplied as input to the I/Q modulator, wherein the constant signal is equal to a non-zero complex constant;
averaging the first response signal to obtain a first average and averaging the second response signal to obtain a second average;
computing a difference between the second average and the first average; and
computing the DC scaling based on the difference and the non-zero complex constant.
6. The method of claim 1 , wherein the model spectrum H(f) represents an inverse of a signal path, wherein the signal path is a path from an I/Q modulator of a transmitter to a demodulator of a receiver, wherein the input gain imbalance spectrum and the input phase skew spectrum represent gain imbalance and phase skew at an input of the demodulator, wherein the output gain imbalance spectrum and the output phase skew spectrum represent gain imbalance and phase skew at an output of the I/Q modulator.
7. The method of claim 1 , wherein the method is performed in a programmable hardware element.
8. The method of claim 1 , wherein the method is performed by a processor in response to execution of program instructions.
9. The method of claim 1 , further comprising:
measuring the input gain imbalance spectrum g(f) and the input phase skew spectrum φ(f) of an electronic device.
10. A non-transitory computer-accessible memory medium for computing I/Q impairments at a complex output of an electrical system based on I/Q impairments at a complex input of the electrical system, wherein the memory medium stores program instructions, wherein the program instructions, when executed by a computer system, cause the computer system to:
compute a first spectrum based on:
a model spectrum H(f) that models the electrical system,
an input gain imbalance spectrum g(f) that characterizes gain imbalance at the complex input, and
an input phase skew spectrum φ(f) that characterizes phase skew at the complex input;
compute a second spectrum based on:
a frequency-reflected version H(−f) of the model spectrum H(f),
the input gain imbalance spectrum g(f), and
the input phase skew spectrum φ(f);
compute a sum of the first spectrum and the second spectrum, and a difference of the first spectrum and the second spectrum;
compute an output gain imbalance spectrum and an output phase skew spectrum based on real and imaginary parts of the sum, and real and imaginary parts of the difference, wherein the output gain imbalance spectrum and the output phase skew spectrum respectively characterize gain imbalance and phase skew at the complex output; and
store the output gain imbalance spectrum and the output phase skew spectrum in a memory.
11. The non-transitory computer-accessible memory medium of claim 10 , wherein the model spectrum H(f) represents an inverse of a signal path, wherein the signal path is a path from an I/Q modulator of a transmitter to a demodulator of a receiver, wherein the input gain imbalance spectrum and the input phase skew spectrum represent gain imbalance and phase skew at an output of the demodulator, wherein the output gain imbalance spectrum and the output phase skew spectrum represent gain imbalance and phase skew at an output of the I/Q modulator.
12. The non-transitory computer-accessible memory medium of claim 11 , wherein the program instructions, when executed by the computer system, further cause the computer system to:
compute an inverse of a spectrum of the signal path to determine the model spectrum H(f).
13. The non-transitory computer-accessible memory medium of claim 11 , wherein the model spectrum H(f) is based on a DC scaling and a DC rotation of the signal path.
14. The non-transitory computer-accessible memory medium of claim 11 , wherein the model spectrum H(f) represents an inverse of a signal path, wherein the signal path is a path from an I/Q modulator of a transmitter to a demodulator of a receiver, wherein the input gain imbalance spectrum and the input phase skew spectrum represent gain imbalance and phase skew at an input of the demodulator, wherein the output gain imbalance spectrum and the output phase skew spectrum represent gain imbalance and phase skew at an output of the I/Q modulator.
15. The non-transitory computer-accessible memory medium of claim 10 , wherein the program instructions, when executed by the computer system, further cause the computer system to:
measure the input gain imbalance spectrum g(f) and the input phase skew spectrum φ(f) of an electronic device.
16. A computer system for computing I/Q impairments at a complex output of an electrical system based on I/Q impairments at a complex input of the electrical system, the computer system comprising:
a processor; and
memory storing program instructions, wherein the program instructions, when executed by the processor, cause the processor to:
compute a first spectrum based on:
a model spectrum H(f) that models the electrical system,
an input gain imbalance spectrum g(f) that characterizes gain imbalance at the complex input, and
an input phase skew spectrum φ(f) that characterizes phase skew at the complex input;
compute a second spectrum based on:
a frequency-reflected version H(−f) of the model spectrum H(f),
the input gain imbalance spectrum g(f), and
the input phase skew spectrum φ(f);
compute a sum of the first spectrum and the second spectrum, and a difference of the first spectrum and the second spectrum;
compute an output gain imbalance spectrum and an output phase skew spectrum based on real and imaginary parts of the sum, and real and imaginary parts of the difference, wherein the output gain imbalance spectrum and the output phase skew spectrum respectively characterize gain imbalance and phase skew at the complex output; and
store the output gain imbalance spectrum and the output phase skew spectrum in said memory.
17. The computer system of claim 16 , wherein the model spectrum H(f) represents an inverse of a signal path, wherein the signal path is a path from an I/Q modulator of a transmitter to a demodulator of a receiver, wherein the input gain imbalance spectrum and the input phase skew spectrum represent gain imbalance and phase skew at an output of the demodulator, wherein the output gain imbalance spectrum and the output phase skew spectrum represent gain imbalance and phase skew at an output of the I/Q modulator.
18. The computer system of claim 17 , wherein the program instructions, when executed by the processor, further cause the processor to:
compute an inverse of a spectrum of the signal path to determine the model spectrum H(f).
19. The computer system of claim 17 , wherein the model spectrum H(f) is based on a DC scaling and a DC rotation of the signal path.
20. The computer system of claim 16 , wherein the model spectrum H(f) represents an inverse of a signal path, wherein the signal path is a path from an I/Q modulator of a transmitter to a demodulator of a receiver, wherein the input gain imbalance spectrum and the input phase skew spectrum represent a gain imbalance and a phase skew at an input of the demodulator, wherein the output gain imbalance spectrum and the output phase skew spectrum represent a gain imbalance and a phase skew at an output of the I/Q modulator.