Linearity and Dynamic Range of a Digital Receiver

The digital receiver is state-of-the-art of modern Doppler weather radars A digital receiver samples the intermediate frequency (IF) signal of a radar receiver. Matched filtering, baseband conversion and phase referencing for coherent-on-receive operation are realized by digital signal processing hardware and software. Logarithmic amplifiers, analog I/Q demodulators and a phase-locked coherent oscillator (COHO) are obsolete. For the first time it is possible to realize dynamic range figures with linear signal reception which were reserved to logarithmic receivers until then. A key advantage of a linear receiver is its capability the be calibrated with a stable power reference (single-point calibration). This paper presents some considerations in regard to the specification and performance testing of digital receivers. The key parameters of a linear receiver are its linearity and its dynamic range, which is defined by the minimum detectable signal (MDS) and the compression of the receiver. The standard measurement procedure for a receiver is the injection of test signals at its input port and the interpretation of the receiver output response. If a digital receiver is used the control software has to provide appropriate interfaces which allow access of the digital baseband raw data without any corrections like 1/r2 applied. The linearity is usually measured by injecting a cw microwave signal with stable frequency and known power, which is varied in equidistant steps in order to cover the complete dynamic range. Deviations can be easily revealed by plotting the receiver gain. For signals greater than 10·MDS this method is adequate but for signals close to the MDS corrections have to be applied which consider the statistical nature of a sinusoidal signal embedded in thermal noise. The situation gets more complex if the statistical nature of weather signals is also taken into consideration. We will present measurements of the linearity of a GEMATRONIK Aspen® DRX digital receiver. The theory of the correction of weak signals will be discussed and the impact of the correction on the receiver response will be shown.