Suppressing transmitter intermodulations with Channelized Active Noise Elimination (CANE)

The linearity of a RF transmitter must often be traded off with its efficiency. High efficiency operation often requires the transmitter to operate in a deep saturation or a switching-mode state. The non-linear distortion components of the signal such as the Third-Order Intermodulation (IM3) terms are the primary causes of the increased Error Vector Magnitude (EVM) and Adjacent Channel Power Ratio (ACPR) level and poorer transmitted signal integrity. In this paper, we proposed a digitally enhanced technique to suppress the intermodulation terms in the output of the power amplifiers (PAs), which is called Channelized Active Noise Elimination (CANE) technique. CANE utilizes the combining of multiple PA channels driven with digitally delayed baseband signal to form an active FIR filter. The rejection band of the FIR filter can be placed where the IM3 appears to realize intermodulation suppression. Similar idea can also be extended to a multi-carrier RF system, with multiple software defined filters formed for different carriers on the same set of hardware. Measurements have shown about 17dB IM3 suppression for a single carrier and 10dB IM3 suppression for a two-carrier signal.