Yfactor

Y-factor is a metric used in RF engineering and radiometry to assess a receiver’s noise performance by comparing its output under two known input loads. It is defined as the ratio of the receiver output power with a hot load to that with a cold load: Y = P_hot / P_cold. If the loads are at temperatures T_hot and T_cold and the receiver adds an effective noise temperature T_rx, the relationship is Y = (T_hot + T_rx) / (T_cold + T_rx).

From this, the receiver noise temperature can be derived: T_rx = (T_hot − Y T_cold) / (Y − 1). A larger Y generally indicates better noise performance. The noise figure NF, often expressed in decibels, is NF = 10 log10(1 + T_rx / T_0), where T_0 is a reference temperature (commonly 290 K).

In practice, Y-factor measurements require stable, known-temperature loads and a calibrated power detector. A typical setup uses a hot load at ambient temperature (around 290 K) and a cold load achieved by cooling or a calibrated noise source with known excess noise. Impedance matching and linear operation are important to avoid biasing the result.

Y-factor measurements are widely applied to characterize RF amplifiers, low-noise receivers, microwave front ends, and radio astronomy receivers. They enable direct comparison of devices and calibration of their noise performance without relying on an absolute noise source. Variants of the method exist for different receiver architectures, such as balanced or correlation-based systems, and for different reference temperatures.