RF Mixer Transformer Center-Tap Termination for Harmonic Rejection
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Solution Overview
Problem
Designing satisfactory mixers and local oscillator circuitry for electronic devices with wireless communications capabilities is challenging due to issues with intermodulation distortion and harmonic rejection, which affect the linearity and performance of wireless communications.
Innovation Solution
The implementation of transformer-based mixer circuitry with a local oscillator harmonic rejection circuit, which includes a transformer with a primary and secondary coil and passive components like inductors and capacitors, coupled to a center tap, to mitigate harmonic signals and improve non-linearity performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional mixer circuitry is used, then device complexity is reduced, but intermodulation distortion increases and linearity performance deteriorates
Solution Approach 1:
A transformer is introduced as an intermediary component between the mixer transistors and the output. The transformer provides impedance transformation and signal coupling while isolating the mixer core from direct loading effects, thereby improving linearity performance without requiring fundamental changes to the mixer transistor configuration.
Solution Approach 2:
The patent modifies the impedance parameters at the mixer output by using the transformer's turns ratio to achieve impedance matching. This parameter change optimizes the load seen by the mixer transistors, reducing intermodulation distortion and improving third-order intercept point (IP3) performance.
2Object-generated harmful factors
If passive termination is added to the transformer center tap, then harmonic rejection improves, but device complexity increases
Solution Approach 1:
The harmful second-order harmonic signals generated at the transformer center tap are extracted and diverted to a dedicated passive termination network. This separate termination path removes the harmful harmonics from the main signal path without affecting the fundamental mixing operation, thereby improving harmonic rejection.
Solution Approach 2:
The patent converts the potentially harmful second-order harmonic signals generated at the center tap into a beneficial filtering opportunity. By providing a dedicated termination path for these harmonics, the circuit transforms what would be interference into a controlled dissipation path, improving overall signal quality.
3Manufacturing precision
If transformer-based mixer circuitry is implemented, then intermodulation distortion is reduced, but manufacturing complexity increases
Solution Approach 1:
The transformer is designed to perform multiple functions simultaneously: signal coupling between stages, impedance transformation for optimal power transfer, and harmonic filtering through its inherent parasitic capacitances and the added center tap termination. This multi-functionality reduces the need for additional separate components.
Solution Approach 2:
The patent combines the impedance matching function and the harmonic rejection function into a single transformer structure with center tap termination. This merging of functions into one component reduces the overall component count and simplifies the manufacturing process compared to using separate impedance matching networks and harmonic filters.
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
This configuration enhances the third-order and fifth-order non-linearity performance, reducing intermodulation distortion and improving the intercept points, thereby enhancing the signal difference-to-noise ratio and error vector magnitude in wireless communications.
Implementation Method 1
a transformer having a primary coil and a secondary coil
Implementation Method 2
The passive component can include an inductor having a first terminal coupled to the center tap of the secondary coil
Implementation Method 3
A capacitor can be coupled in parallel or in series with the inductor
Data Source
AI summary
An electronic device may include wireless circuitry having a radio-frequency mixer. The mixer may include an input stage, an output stage, and a transformer coupled between the input and output stages. The input stage may include input transistors forming a transconductance cell. The output stage may include two pairs of mixer transistors configured to receive an oscillating signal. The transformer may include a primary coil coupled between the input transistors and a secondary coil coupled between the two pairs of mixer transistors. A harmonic rejection circuit may be coupled to a center tap of the secondary coil. The harmonic rejection circuit may be configured to mitigate harmonic signals that are associated with the oscillating signal and that are generated at the inputs of the output stage. The harmonic rejection circuit may include an inductor, an optional capacitor, an optional current source transistor, or other passive component.