Partially Coherent RF Noise Addition and its Mitigation in Power-combined Solid-state Amplifiers

The cascaded noise behavior of RF components arranged in series is well known but that of parallel arrangements such as arrays and beamformers has received far less coverage. In such arrangements it is not always the case that signals add coherently while noise adds incoherently. High power solid-state amplifiers require large-scale RF power combining of many identical devices at their output. Wideband output at high efficiency imposes true-time-delay matching on the signal paths being summed and saturation of the final-stage amplifiers. Immunity from radio frequency interference (RFI) and oscillation require significant gain upstream of the parallel output stages which are to be combined. These requirements were observed to act in concert in a 30 kW peak power wideband (410 to 450 MHz) solid-state UHF transmitter to produce coherent addition of RF thermal noise from the various paths in these combiners, even when they were isolated and terminated with absorbing loads. In a monostatic radar, where a high-gain aperture is shared by the transmitter and receiver, inter-pulse noise power spectral density (PSD) must be suppressed to below −100 dBm/MHz through techniques such as bias-switching of the devices and/or RF switching of the common path in order to eliminate desensitization of the receiver via T/R leakage. Implementation of these methods, and their tradeoffs, are discussed in this paper.