This paper proposes, for the first time, a compact model for avalanche noise diodes that can easily be inserted in advanced CAD tools. The model is based on the theory of the Read type microwave avalanche diodes. It fully accounts for the electronic tuning of the avalanche frequency with the square root of the diode biasing current and is scalable with the device area. The equivalent circuit has been written for ADS of Keysight Technologies and is verified against published experiments. The modeled input impedance is within 20% of the measured one in the whole frequency band from DC to 40 GHz. Futhermore, the Excess Noise Ratio (ENR) of the diode can be predicted as a funtion of the avalanche current and is in good agreement with experiments. The developed model is useful for the simulation and the design of calibration circuits adopted by microwave radiometers, radio astronomy receivers and high reliability wireless apparatuses.