System and method of coherent averaging of repetitive signals for measurement

1. A method implemented using a signal analyzer comprising a processor and memory, the method comprising:

accessing repeated copies of an orthogonal frequency division multiplexed (OFDM) output signal produced by a device under test (DUT) in response to corresponding repeated copies of an OFDM input signal, wherein the OFDM input signal comprises a series of OFDM input symbols and the OFDM output signal comprises a series of OFDM output symbols;

for each copy of the OFDM output signal, time aligning the OFDM output symbols to the OFDM input symbols of the corresponding OFDM input signal, and de-rotating a phase of the OFDM output signal with respect to the OFDM input signal;

summing coherently and averaging the copies of the OFDM output signal;

determining a variance of each copy of the OFDM output signal, and an ensemble variance of all of the copies of the OFDM output signal from the coherently summed and averaged copies of the OFDM output signal;

discarding any copies of the OFDM output signal whose variance differs from the ensemble variance by more than a threshold amount to produce a qualified set of copies of the OFDM output signal;

determining a mean value of the qualified set of copies of the OFDM output signal; and

determining a total noise power of the qualified set of copies of the OFDM output signal from the mean value.

2. The method of claim 1 , further comprising determining an error vector magnitude (EVM) of the qualified set of copies of the OFDM output signal.

3. The method of claim 2 , further comprising determining the EVM in the time domain.

4. The method of claim 2 , further comprising determining a first constituent of the EVM due to the analyzer.

5. The method of claim 4 , further comprising determining a second constituent of the EVM due to a source which supplies the copies of the OFDM input signal to the DUT.

6. The method of claim 5 , further comprising determining a third constituent of the EVM due to the DUT by removing from the EVM the first constituent due to the analyzer and the second constituent due to the source.

7. The method of claim 6 , further comprising determining a first component of the EVM due to the DUT produced by distortion, and determining a second component of the EVM due to the EVM produced by random noise.

8. The method of claim 1 , further comprising nulling out unused tones in the copies of the OFDM output signal prior to coherently summing and averaging the copies of the OFDM output signal.

9. A signal analyzer, comprising:

an input terminal configured to receive repeated copies of an orthogonal frequency division multiplexed (OFDM) output signal produced by a device under test (DUT) in response to corresponding repeated copies of an OFDM input signal, wherein the OFDM input signal comprises a series of OFDM input symbols and the OFDM output signal comprises a series of OFDM output symbols;

a processor configured to:

access the copies of the OFDM output signal of the DUT;

for each copy of the OFDM output signal, time align the OFDM output symbols to the OFDM input symbols of the corresponding OFDM input signal, and de-rotate a phase of the OFDM output signal with respect to the OFDM input signal;

coherently sum and average the copies of the OFDM output signal;

determine a variance of each copy of the OFDM output signal, and an ensemble variance of all of the copies of the OFDM output signal from the coherently summed and averaged copies of the OFDM output signal;

discard any copies of the OFDM output signal whose variance differs from the ensemble variance by more than a threshold amount, to produce a qualified set of copies of the OFDM output signal;

determine a mean value of the qualified set of copies of the OFDM output signal; and

determine a total noise power of the qualified set of copies of the OFDM output signal from the mean value.

10. The signal analyzer of claim 9 , wherein the processor is further configured to determine an error vector magnitude (EVM) of the qualified set of copies of the OFDM output signal.

11. The signal analyzer of claim 10 , wherein the processor is further configured to determine the EVM in the time domain.

12. The signal analyzer of claim 10 , wherein the processor is further configured to determine a first constituent of the EVM due to the signal analyzer.

13. The signal analyzer of claim 12 , wherein the processor is further configured to determine a second constituent of the EVM due to a source which supplies the copies of the OFDM input signal to the DUT.

14. The signal analyzer of claim 13 , wherein the processor is further configured to determine a third constituent of the EVM due to the DUT by removing from the EVM the first constituent due to the analyzer and the second constituent due to the source.

15. The signal analyzer of claim 14 , wherein the processor is further configured to determine a first component of the EVM due to the DUT produced by distortion, and determine a second component of the EVM due to the EVM produced by random noise.

16. The signal analyzer of claim 9 , wherein the processor is further configured to null out unused tones in the copies of the OFDM output signals prior to or after coherently summing and averaging the copies of the OFDM output signal.

17. A tangible non-volatile storage medium having stored thereon instructions which when executed by a processor cause the processor to:

access copies of an orthogonal frequency division multiplexed (OFDM) output signal of a device under test (DUT), wherein the DUT produces the copies of the OFDM output signal from corresponding copies of an OFDM input signal which is supplied to the DUT, wherein the OFDM input signal comprises a series of OFDM input symbols and the OFDM output signal comprises a series of OFDM output symbols;

for each copy of the OFDM output signal, time align the OFDM output symbols to the OFDM input symbols of the corresponding OFDM input signal, and de-rotate a phase of the OFDM output signal with respect to the OFDM input signal;

coherently sum and average the copies of the OFDM output signal;

determine a variance of each copy of the OFDM output signal, and an ensemble variance of all of the copies of the OFDM output signal from the coherently summed and averaged copies of the OFDM output signal;

discard any copies of the OFDM output signal whose variance differs from the ensemble variance by more than a threshold amount to produce a qualified set of copies of the OFDM output signal;

determine a mean value of the qualified set of copies of the OFDM output signal; and

determine a total noise power of the qualified set of copies of the OFDM output signal from the mean value.

18. The tangible non-volatile storage medium of claim 17 , wherein the instructions further cause the processor to determine an error vector magnitude (EVM) of the qualified set of copies of the OFDM output signal.

19. The tangible non-volatile storage medium of claim 18 , wherein the instructions further cause the processor to determine the EVM in the time domain.

20. The tangible non-volatile storage medium of claim 18 , wherein the instructions further cause the processor to determine a first constituent of the EVM due to an analyzer which receives the copies of the OFDM output signal.