Second Order Intermodulation Cancellation Circuit
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Solution Overview
Problem
Direct conversion RF receivers face challenges in accurately filtering out second-order intermodulation distortion (IM2) products due to their proximity to the baseband signal, requiring complex and silicon-intensive solutions.
Innovation Solution
A simpler IM2 canceller is implemented using a differential mixer with matched load resistors and a differential amplifier to detect the difference between varying common-mode and reference signals, allowing for dynamic compensation of IM2 distortion in the baseband signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If complex IM2 cancellation circuits with multiple feedback loops are used, then IM2 distortion is effectively cancelled, but device complexity and silicon area increase significantly
Solution Approach 1:
The patent extracts only the essential IM2 cancellation functionality by using a single feedback loop that samples the common-mode signal and feeds it back through a differential amplifier, eliminating the need for complex multi-loop architectures while maintaining effective IM2 distortion cancellation
Solution Approach 2:
The common-mode signal, which would normally be discarded, is repurposed as the feedback signal for IM2 cancellation. This single signal serves multiple functions: it carries IM2 distortion information, provides the reference for cancellation, and eliminates the need for separate cancellation circuitry
2Measurement precision
If multiple reference voltages are generated to offset IM2 distortion, then cancellation accuracy improves, but device complexity and power consumption increase
Solution Approach 1:
The patent uses a single common-mode signal that serves multiple purposes: it provides the IM2 distortion information, acts as the reference for the differential amplifier, and eliminates the need for generating multiple separate reference voltages, thereby reducing device complexity while maintaining cancellation accuracy
Solution Approach 2:
The circuit uses its own common-mode signal output to provide the feedback reference, eliminating the need for external reference voltage generation. The system is self-sufficient, using the distortion-containing signal itself as the basis for its own cancellation
3Object-generated harmful factors
If filtering circuits are used to remove IM2 products, then distortion is reduced, but baseband signal bandwidth is limited due to IM2 frequency proximity
Solution Approach 1:
The patent employs feedback by sampling the common-mode signal containing IM2 distortion and feeding it back through a differential amplifier to the differential baseband signal, dynamically canceling IM2 products without requiring bandwidth-limiting filtering circuits
Solution Approach 2:
The differential amplifier acts as an intermediary that subtracts the common-mode IM2 signal from the differential baseband signal, enabling IM2 removal without the need for filtering that would restrict baseband bandwidth
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 approach effectively cancels residual differential IM2 signals, improving the receiver's performance by dynamically offsetting distortion caused by inherent mixer distortion and RF interference, with a 20dB improvement in IM2 intercept point.
Implementation Method 1
The mixer mixes the RF signal and the LO signal to generate a differential baseband signal
Implementation Method 2
A third resistor and matched fourth resistor are also connected across the differential mixer output, and their common node is connected to Vcc via a capacitor, effectively creating an AC ground above a certain frequency
Data Source
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AI summary
In a technique for cancelling out target IM2 components in the mixer output of a wireless receiver (10), a differential RF signal and a differential local oscillator (LO) signal are mixed by a mixer (28) to demodulate the RF signal. A first common node signal is generated between a first resistor (R3a) and a second resistor (R3b) coupled across the mixer's differential output terminals. A second common node signal is generated between a third resistor (R2a) and a fourth resistor (R2b) coupled across the differential output terminals, where a capacitor (C1) is coupled between the second common node and a power supply terminal. The second common node signal provides a stable reference signal for IM2 components above a certain frequency. The two common node signals are subtracted at a differential amplifier (3k) to create a difference signal. The difference signal is scaled at a multiplier (38) by a scaling factor obtained during calibration. The scaled difference signal is coupled to the mixer output to offset IM2 distortion.