Measurements of IP3 and P1dB for Spectrum Monitoring with Software Defined Radios

This paper discusses methodology for characterizing linearity summary parameters of software-defined radio receivers. First, we introduce a highly automated testbed for 1-dB compression point (P1dB) and third-order input-intercept point (IIP3). With this system, we have observed some surprising deviations from the expected slope of 3 dB/dB in the power response of the third-order intermodulation distortion (IMD3). In response to this, we developed a measurement technique based on linear regression that accounts for the actual slope. A methodology study considers this technique in comparison with an existing technique for IIP3 measurements of spectrum monitoring receivers. In consideration for the increase in IMD3 model complexity, we also propose an alternative parameter to simplify the quantitative evaluation of spectral regrowth near the receiver noise floor. We argue that this equivalence point provides a designer with more valuable information than an IIP3 point for spectrum monitoring applications.