Local-Oscillator Third-Harmonic Injection for Improved Broadband Mixer Linearity

We have studied dependencies of the mixer linearity upon injecting a third local oscillator (LO) harmonic of various magnitudes and phases relative to the LO fundamental. Two packaged double-balanced-mixer samples from independent manufacturers were tested for two-tone third-order IM (IM3) products. The two-tone RF input signal covered a 13 GHz to 18 GHz frequency range with a 4 MHz tone spacing and with the LO 52 MHz above the lower RF input tone. For both samples and at select third LO harmonic phases, we observed around 4 dB average third-order input intercept point (IIP3) improvement across the entire frequency range, with about 5 dB increase at frequencies where IIP3 was below 20 dBm with purely sinusoidal LO. In contrast, certain harmonic phases resulted in more than 10 dB increase, compared to no injection, of the IM3 products that were also unstable over time. The results in this paper form the basis for further research toward optimal LO waveforms for improved broadband mixer linearity.