Active Mixer Switching Circuit With Phase-Shifted Distortion Suppression
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Solution Overview
Problem
Active mixers in RF transmitters face distortion issues due to static overlap distortion (SOD) and dynamic switching distortion (DSD), which corrupt the fundamental signal and introduce unwanted spectral components, making it challenging to minimize interference.
Innovation Solution
A switching circuit design with two pairs of switching devices, where the first pair is controlled by a differential input frequency signal and the second pair by a phase-shifted counterpart, with a common node coupling between pairs to reduce distortion effects, resulting in four transitions per cycle instead of two, shifting distortion harmonics away from the fundamental output frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional switching stage with one pair of switching devices is used, then the circuit structure is simple, but static overlap distortion and dynamic switching distortion occur causing signal corruption
Solution Approach 1:
The switching stage is segmented into two pairs of switching devices (first pair M1a, M1b and second pair M2a, M2b) arranged in a stacked configuration. Each pair handles different aspects of the switching function, with the first pair controlled by the second input frequency signal and the second pair controlled by a phase-shifted counterpart. This segmentation allows the circuit to reduce distortion effects while maintaining functional separation.
Solution Approach 2:
The patent introduces a temporal dimension by using phase-shifted control signals for the second pair of switching devices. The phase shift creates four transitions per cycle instead of two, effectively adding a time-based layer to the switching operation. This dimensional change shifts distortion harmonics away from the fundamental output frequency, improving signal purity without excessive structural complexity.
2Manufacturing precision
If switching devices switch instantaneously, then no overlap distortion occurs, but in practice finite switching time causes both switching devices to be on simultaneously dividing input current
Solution Approach 1:
The switching function is divided between two pairs of devices. The first pair (M1a, M1b) handles the primary switching controlled by the second input frequency signal, while the second pair (M2a, M2b) handles additional switching controlled by a phase-shifted version of the same signal. This segmentation distributes the switching action across multiple devices, reducing the current division effect during overlap periods and improving transition linearity.
Solution Approach 2:
The patent employs periodic switching action with a specific phase relationship between the two pairs. The phase shift creates a periodic pattern where the switching events are distributed throughout the cycle, ensuring that not all switching transitions occur simultaneously. This periodic distribution of switching events reduces the severity of overlap distortion and improves the linearity of current transitions.
3Reliability
If parasitic capacitance is present at the input node, then voltage swing is generated causing distortion current flow, but eliminating parasitic capacitance is not feasible
Solution Approach 1:
The patent converts the harmful effect of parasitic capacitance into a beneficial outcome by using the stacked switching configuration with phase-shifted control. The multiple transitions per cycle (four instead of two) caused by the phase shift create distortion currents at harmonics that are then down-converted to frequencies away from the fundamental output signal. This transforms the harmful parasitic capacitance effect into a mechanism that pushes distortion away from the desired signal frequency.
Solution Approach 2:
The patent changes the temporal parameters of the switching operation by introducing a phase shift between the control signals for the two pairs of switching devices. This parameter change results in four transitions per cycle instead of two, fundamentally altering how parasitic capacitance interacts with the switching waveform. The modified switching parameters cause distortion harmonics to appear at different frequencies, specifically away from the fundamental output frequency, thereby suppressing their harmful effects.
Data Source
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AI summary
There is provided a switching circuit for an active mixer. The switching circuit comprises a first pair of parallel switching devices and a second pair of parallel switching devices. The first and second pairs of parallel switching devices are arranged in a stacked configuration between an input node at which an input current comprising a first input frequency signal is received and a pair of differential output nodes. The first pair of switching devices are controlled by a second input frequency signal. The second pair of switching devices are controlled by a phase-shifted counterpart of the second input frequency signal. A common node between the first switching devices of the first and second pairs of switching devices is electrically coupled to a common node between the second switching devices of the first and second pairs of switching devices.