Dynamic characterisation of amplifier AM-PM distortion

1. A method of determining an AM-PM distortion measurement for an amplifier, the method comprising:

generating a test waveform to be provided to the input of the amplifier;

periodically puncturing the test waveform with a fixed-level reference signal to generate a modified test waveform which alternates between test periods in which a portion of the test waveform is present and reference periods in which the fixed-level reference signal is present;

measuring the amplifier AM-PM distortion in a test period;

measuring the phase difference between the input and the output of the amplifier in reference periods either side of the test period;

estimating a phase error in the test period in dependence on phase differences measured in the reference periods; and

estimating the true amplifier AM-PM distortion by removing the estimated phase error from the measured amplifier AM-PM distortion.

2. The method of claim 1 wherein the step of estimating the phase error comprises measuring the amplifier phase shift during the reference period either side of the test period, and using interpolation techniques to estimate the phase error during the test period.

3. The method of claim 1 wherein the step of measuring the phase difference between the input and the output of the amplifier in reference periods either side of the test period comprises, in each reference period either side of the test period, measuring a complex signal at the input to and output from the amplifier using measurement receivers, and comparing a captured phase of the measured signals in each reference period.

4. The method of claim 1 wherein the step of measuring the amplifier AM-PM distortion in a test period comprises measuring a complex signal at the input to and output from the amplifier using measurement receivers connected to the input and output of the amplifier, and comparing a captured phase of the measured signals.

5. The method of claim 1 wherein the reference periods either side of the test period are adjacent to the test period.

6. The method of claim 1 wherein the test waveform is representative of a signal which is provided as an input to the amplifier in normal use.

7. A non-transitory computer program including non-transitory computer program code stored on a non-transitory computer program product which, when run on a computer system, performs the method of claim 1 .

8. A non-transitory computer program product for storing non-transitory computer program code which, when run on a computer system, performs the method of claim 1 .

9. A measurement system for determining an AM-PM distortion measurement for an amplifier, the measurement system being adapted to:

generate a test waveform to be provided to the input of the amplifier;

periodically puncture the test waveform with a fixed-level reference signal to generate a modified test waveform which alternates between test periods in which a portion of the test waveform is present and reference periods in which the fixed-level reference signal is present;

measure the amplifier AM-PM distortion in a test period;

measure the phase difference between the input and the output of the amplifier in reference periods either side of the test period;

estimate a phase error in the test period in dependence on phase differences measured in the reference periods; and

estimate the true amplifier AM-PM distortion by removing the estimated phase error from the measured amplifier AM-PM distortion.

10. The measurement system of claim 9 further adapted to estimate the phase error by measuring the amplifier phase shift during the reference period either side of the test period, and use interpolation techniques to estimate the phase error during the test period.

11. The measurement system of claim 9 further adapted to measure the phase difference between the input and the output of the amplifier in reference periods either side of the test period by, in each reference period either side of the test period, measuring a complex signal at the input to and output from the amplifier using measurement receivers.

12. The measurement system of claim 9 further adapted to measure the amplifier AM-PM distortion in a test period by measuring a complex signal at the input to and output from the amplifier using measurement receivers connected to the input and output of the amplifier, and comparing a captured phase of the measured signals.

13. The measurement system of claim 9 wherein the test waveform is a representation of a signal which is provided as an input to the amplifier in normal use.

14. The measurement system of claim 9 wherein the reference periods either side of the test period are adjacent the test period.

15. A measurement system for determining an AM-PM distortion measurement for an amplifier, the measurement system comprising:

means for generating a test waveform to be provided to the input of the amplifier;

means for periodically puncturing the test waveform with a fixed-level reference signal to generate a modified test waveform which alternates between test periods in which a portion of the test waveform is present and reference periods in which the fixed-level reference signal is present;

means for measuring the amplifier AM-PM distortion in a test period;

means for measuring the phase difference between the input and the output of the amplifier in reference periods either side of the test period;

means for estimating a phase error in the test period in dependence on phase differences measured in the reference periods; and

means for estimating the true amplifier AM-PM distortion by removing the estimated phase error from the measured amplifier AM-PM distortion.