Noise in lossless microwave multiports

Abstract

This article addresses the noise behaviour (noise temperature and noise figure) of some passive microwave multiport circuits. The analysis method is based on the noise-wave formulation. With the exception of the attenuator case, which is used as a reference, the circuit elements considered are lossless devices, in the sense that neither conductive nor dielectric losses are accounted for. The analysis shows that, when connected to matched loads in some of their ports, these multiports circuits lose their lossless nature and their scattering matrix is not unitary; therefore, they generate thermal noise. The article addresses and formalizes mathematically the noise properties of a number of lossless microwave devices such as N-port power splitters, circulators, and hybrid couplers. While the noise-wave mathematical formulation may be cumbersome in some cases, all the devices and configurations analyzed in this work have been characterized in terms of noise figure and noise temperature, which is a much more practical approach in most situations. Some implications of the use of these devices and configurations in antenna arrays for antenna noise temperature evaluations have been also addressed.

Continuous SWIFT

“…The connection of the linear coil using a hybrid with the unused output terminated by a 50 ohm load at room temperature decreases SNR by the fraction of the terminator’s noise power that reaches the preamplifier through the hybrid [33], and is used here only for “proof of principle”. A more optimal connection scheme for the linear coil could be achieved with a circulator [33].…”

Noise Cancellation Effects in Integrated Photonics with Wilkinson Power Dividers

“…A crucial aspect in the implementation of lossy devices is accurately accounting for their noise performance. − Following the fluctuation–dissipation theorem, − all lossy devices necessarily support fluctuating currents that introduce thermal noise into the system. This effect is particularly relevant in networks containing elements with gain, where independent sources must be introduced for the active and lossy elements , and where thermal noise signals might be amplified.…”

Evaluating Noise in Passive Beamforming Networks for Multibeam Applications

“…In the remainder of this work, the physical temperature will be assumed the reference temperature (T0=290K). At this point, any two-port network can be described by two main parameters: the noise factor, F, and the signal gain, G. For instance, it is well-known that the noise factor of a matched attenuator at reference temperature T0 corresponds to its loss (F=L=1/G) [6]- [9]. This is a direct consequence of the application of Bosma's Theorem [10], which allows us to compute the noise power generated intrinsically in any passive multiport network with known scattering matrix S. The theorem establishes that:…”