Modern Doppler weather radars are using digital receivers. The received signals are digitized at an intermediate (IF) frequency which avoids analog downconversion to baseband. Because the dynamic range of digital receivers surpasses the dynamic range of logarithmic receivers only a linear receiver channel is required for the reception of reflectivity and Doppler signals.

The fact that the receiver has a linear characteristic makes accurate calibration easy. Only one signal of know power must be injected in order to get an amplitude calibration of the complete receiver dynamic range. However it must be ensured that the receiver exhibits indeed a linear behaviour.

This paper presents two methods for the check of the linearity of linear receivers, the comparison method and the two-tone method. The comparison method compares the receiver under test with a known test instrument of equal or better linearity, usually a modern spectrum analyzer. The method provides an accuracy of the linearity test of 0.2 dB or even better. The calibration of the receiver can be performed during the measurement.

A disadvantage of the comparison method is the quite large number of data points required. The measurement can be easily performed with an automated environment, however, this requires some programming and the accuracy may be reduced. These problems are avoided with the two-tone method. Here two signals are injected simultaneously, which are both within the receiver bandwidth. The amplitude and the location of the in-band intermodulation products provide a means for the linearity of the receiver. With a single measurement the complete dynamic range of the receiver is scanned.

Examples, results and an accuracy budget for the comparison method are provided with the paper.