Nonlinear Characterization by Harmonic Generation: An Alternative Technique for Measuring Intermodulation Distortions

The same intrinsic nonlinear property that produces intermodulation distortion from two or more excitation tones, also produces harmonic generation from a single excitation tone. The measurement of one of these distortions, therefore, can be used to derive the results of the other. We have configured, tested and qualified an experimental setup to measure the nonlinear properties by harmonic generation using a broadband 50 Ω transmission line as the test device. This setup has a 50 dB dynamic range enhancement over the intermodulation measurement setup. The harmonic generation technique was used to characterize a high temperature superconducting (HTS) coplanar transmission line. The input power at 1.5 GHz versus the output power for a 30 μm by 5mm TI2Ba2CaCu2O8 coplanar transmission line was measured at 50, 70,77 and 85 K. At 77 K, the 2nd order intercept (SOI) was +136 dBm, the 3rd order intercept (TOI) was +75 dBm. At the maximum input power that this HTS device can handle, +43 dBm, the 3rd harmonic was -65 dBc, just within the 75dB dynamic range for the two tone intermodulation measurement. The unexpected nonlinear performance of this HTS device would have been missed by an intermodulation measurement.