Calibration of a Weather Radar Receiver with a Noise Diode

The receiver of a weather radar is an extremely sensitive device. Although modern receivers are designed and tested for immunity against variations of the temperature of the environment, there is still the possibility of thermal drifts, e.g. if a heat exchanger or air conditioning fails. In order to maintain the calibration of a weather radar during a 24/7 mode of operation, a regular calibration of the receiver is necessary.

There are two methods for the online calibration of a weather radar receiver. With "online calibration" we refer to a calibration which is performed during the normal operation of the radar, either during a dedicated time slot of the scanning schedule or by injecting signals into dedicated range gates.

The first method uses a test signal generator (TSG) which injects a sinusoidal signal with a known amplitude and frequency into the radar receiver. This method has several drawbacks. The TSG itself is a complicated device which is also prone to thermal variations and also to electromagnetic interference (EMI). Due to its complexity it has a much lower MTBF (mean time between failures) than a noise diode. A radar with a magnetron transmitter may use a numerical AFC (NAFC) in order to allow the tracking of the transmitter frequency drift in real-time. Since the NAFC switches the frequency of the numerical oscillator (NCO) the TSG frequency must be locked to the radar frequency if the receiver response varies over the drift range.

These disadvantages are avoided if a noise source is used for the receiver calibration which is the second method. A noise source is is a simple microwave device based on an avalanche diode. When biased properly it generates a powerful, wideband, and well defined noise signal. Due to its simplicity it is relatively stable with respect to thermal variations, at least compared to a TSG. The wide frequency spectrum which is injected into the receiver allows the investigation of the receiver response over the whole bandwidth. Using the Y-method the noise figure of the receiver can also be monitored.

However a noise source is specified and calibrated in terms of the equivalent noise ratio (ENR) and not in dBm. It will be shown how ENR can be converted to dBm. The crucial role of the Matched Filter of the digital radar receiver will be explained together with the transformation of its 3 dB bandwidth to its noise bandwidth. Finally a validation of the noise calibration of a radar receiver will be shown.