Theory of three-level paramagnetic masers. Part 4: Noise figure

The input noise temperature of a maser amplifier is equal to |<Tm>| + X(|<Tm>| + <Te>), where <Tm> is the mean effective temperature of the paramagnetic ions, <Te> is the mean temperature of the parts of the amplifier in thermal equilibrium (excluding the signal source) and X is the noise parameter of the amplifier. The noise parameter of an amplifier is defined as the ratio of the power dissipated in the parts of the amplifier in thermal equilibrium (excluding the signal source) to the net power generated in the amplifier when the susceptibility tensors of its various parts are transposed and power is sent in through the output port. The noise parameter of a one-port cavity maser is equal to the ratio of the modulus of the cavity Q-factor in the presence of the paramagnetic ions to the cavity Q-factor in the absence of paramagnetic ions. It tends to zero as the ohmic losses in the cavity tend to zero. The noise parameter of a two-port cavity maser is always greater than that of a one-port cavity maser having the same magnetic and ohmic Q-factors. If the two-port device is operated at maximum bandwidth, its noise parameter tends to unity as the ohmic losses in the cavity tend to zero, and is always greater than twice that of the one-port device. The noise parameter of a travelling-wave maser is equal to that of a one-port cavity maser having the same Q-factors in the presence and in the absence of the paramagnetic ions.