Bootstrap Capacitor Mixer Linearity
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current LTE transmitter designs face challenges in meeting stringent out-of-band emission limits due to odd-order non-linearity in the IQ modulator, particularly for narrow transmit signals, which results in unwanted frequency products and requires complex filtering and power reduction to comply with regulatory specifications.
Innovation Solution
A mixer design utilizing a CMOS transmission gate with a bootstrap capacitor configuration to maintain constant on-resistance independent of input signals, achieved by applying a virtual drive voltage that is independent of the input signal, thereby reducing non-linearity and enhancing linearity and dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional CMOS switches are used in the mixer, then the device complexity is low and manufacturing is easy, but the on-resistance varies with input signal voltage causing odd-order non-linearity and unwanted out-of-band emissions
Solution Approach 1:
The patent changes the voltage parameter applied to the FET gate by using a bootstrap capacitor to generate a drive voltage that is the sum of the input signal voltage and a fixed bias voltage. This parameter change ensures the FET operates in the saturation region with constant on-resistance, eliminating odd-order non-linearity and improving linearity without significantly increasing device complexity
Solution Approach 2:
The bootstrap capacitor acts as an intermediary element that couples the input signal to the FET gate in a controlled manner. By introducing this intermediate energy storage element, the circuit achieves linear operation through capacitive bootstrapping without requiring complex active compensation circuits or multiple feedback stages
2Object-generated harmful factors
If power back-off is applied to meet emission limits, then out-of-band emissions are reduced, but the transmit power and coverage are decreased
Solution Approach 1:
The patent applies preliminary linearization action by ensuring the mixer operates in a linear region from the outset through proper biasing and bootstrap capacitive coupling. By preventing non-linearity before it generates harmful emissions, the system can operate at full power without requiring subsequent power back-off to meet emission specifications
3Object-generated harmful factors
If complex filtering is implemented to remove unwanted frequency products, then out-of-band emissions are suppressed, but the device complexity and cost increase
Solution Approach 1:
The patent extracts the source of non-linearity by using the bootstrap capacitor to isolate the FET's non-linear characteristics from the signal path. By taking out the voltage variation that causes non-linearity and replacing it with a stable biased operation, unwanted frequency products are eliminated at their source rather than requiring complex filtering to remove them later
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively reduces unwanted out-of-band emissions by more than 30 dB, meeting the -66 dBc specification without requiring power back-off, and ensures compliance with LTE linearity requirements while maintaining low noise and high conversion gain.
Implementation Method 1
the gate being arranged to be driven by a sampling clock signal, the gate comprising at least one FET having a respective control node, the mixer having circuitry for applying a voltage between the gate and source nodes of the at least one FET in the on-state, which voltage is independent of input signals at the input nodes
Data Source
AI summary
A switching circuit is linearized by using a capacitor to apply a drive voltage to an FET, wherein the drive voltage is independent of the signal switched by the switching circuit.