English: Stanford University High Frequency Electronics Laboratory

Frequency Spectrum of intermodulation distortion in a radio-frequency signal passed through the linear broad-band amplifier I built. The process of intermodulation is due to third-order harmonics of closely spaced signals. I tested this phenomenon with two signals: a Local Oscillator (LO) at 270 MHz, +0dBm, and an RF signal at 275 MHz. (Closer spacing would push the limitations of the RF spectrum analyzer resolution). Clear intermodulation products were seen at 265 and 280 MHz. As seen on the attached graph, the intermodulation power is 55.66 dB lower than the signal power.

This graph nicely shows our desired signal peaks, as well as the side-band intermodulation. These closely spaced distortions would likely interfere with our signal, since it would be difficult to build a high-quality filter to cancel them out in our application. However, their overall power is more than 50 dB below the desired signal, which was sufficiently low for our purposes. We used this measurement to estimate the IP3 (third order intercept point) parameter.