Mixer Transconductor Feedback for IM2 Distortion Cancellation
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Solution Overview
Problem
Direct conversion receivers face significant challenges with second-order inter-modulation distortion (IM2) due to feedthrough and non-linear characteristics of transconductor circuits in mixers, leading to interference with baseband signals.
Innovation Solution
A negative feedback circuit is introduced in the transconductor circuit of the mixer, using operational amplifiers to adjust the input signal by comparing a reference voltage with the detected voltage between the emitter ends of transistors, thereby canceling out the IM2 distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a direct conversion receiver uses a mixer with transconductor circuit, then the receiver achieves higher integration and lower complexity, but second-order inter-modulation distortion (IM2) occurs due to feedthrough and non-linear characteristics
Solution Approach 1:
The patent introduces a negative feedback circuit that detects the IM2 distortion signal at the output of the transconductor circuit and feeds back a compensating signal to the input. The feedback circuit includes an operational amplifier configured to sense the distortion components and generate correction signals that are subtracted from the original input signal, thereby canceling the IM2 distortion in real-time while maintaining the simple direct conversion architecture
2Ease of manufacture
If the mixer uses an actual implementation with feedthrough instead of an ideal mixer, then the circuit becomes realizable, but interference signals around DC are generated that interfere with baseband signals
Solution Approach 1:
The patent converts the harmful feedthrough effect into a beneficial cancellation mechanism. By deliberately detecting the feedthrough-induced IM2 distortion and using it to generate a compensating signal through the negative feedback circuit, the originally harmful leakage path becomes the source of information needed to cancel the distortion, transforming the manufacturing reality into an opportunity for distortion rejection
3Productivity
If the transconductor circuit operates with non-linear characteristics, then the circuit achieves frequency conversion function, but IM2 distortion is strengthened
Solution Approach 1:
The negative feedback circuit continuously monitors the output of the non-linear transconductor circuit, extracts the IM2 distortion components, and generates compensating signals that are fed back to the input. This feedback mechanism allows the transconductor to maintain its essential non-linear frequency conversion function while actively canceling the harmful IM2 distortion products in real-time
Data Source
AI summary
A transconductor circuit used in a mixer for canceling second-order inter-modulation distortion includes a first transistor and a second transistor, of which the base (gate) ends coupled to a first input end and a second input end, for receiving a differential input signal; and a negative feedback circuit, of which the input end coupled to the emitter (source) ends of the first transistor and the second transistor, of which the out end coupled to the base (gate) ends of the first transistor and the second transistor, for adjusting the voltage of the base (gate) of the first transistor and the second transistor according to the difference between a reference voltage and the detected voltage of the emitter (source) of the first transistor and the second transistor.