Differential MOSFET RF Peak Detector for Weak Interferer Sensing
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Solution Overview
Problem
Conventional peak detectors (PDETs) struggle to detect weak interferer RF signals relative to desired RF signals, leading to saturation of low-noise amplifiers (LNAs) and degradation of signal-to-noise ratio (SNR) in wireless communication systems, as they attenuate desired RF signals and fail to maintain linear operation.
Innovation Solution
A peak detector arranged in a common-source topology using a differential pair of transistors, which detects interferer RF signals by combining second-order non-linear drain currents at the drain terminals, providing a higher small-signal gain to detect weaker interferer signals without degrading LNA performance, and includes an automatic gain-control (AGC) circuit to adjust LNA gain and prevent saturation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional peak detectors are used to detect RF signal power, then the circuit can detect desired RF signals, but weak interferer RF signals cannot be detected and the LNA saturates
Solution Approach 1:
The peak detector is segmented into two independent detection paths: one path detects desired RF signals using the LNA output, while another path detects interferer RF signals using a separate amplifier path. This segmentation allows each path to operate independently without interfering with the other, enabling detection of weak interferers without causing LNA saturation.
Solution Approach 2:
An automatic gain control (AGC) circuit is introduced as an intermediary between the peak detector and the LNA. The AGC circuit receives detection results from the peak detector and dynamically adjusts the LNA gain to prevent saturation while maintaining linear operation. This intermediary mechanism resolves the contradiction by providing real-time feedback control.
2Measurement precision
If peak detectors attenuate desired RF signals to detect interferers, then interferer detection is possible, but signal-to-noise ratio degrades
Solution Approach 1:
The detection system is divided into separate functional segments: one segment processes desired RF signals through the LNA with high gain to maintain SNR, while another segment processes interferer detection through a separate path. This prevents the need to attenuate desired signals for interferer detection, as each segment handles its specific task independently.
Solution Approach 2:
Different quality requirements are applied to different parts of the system: the LNA path maintains high gain and low noise for desired signal processing, while the interferer detection path uses appropriate amplification for weak interferer signals. This local optimization allows each path to be tuned for its specific function without compromising overall system performance.
3Device complexity
If conventional peak detectors are used, then the circuit structure is simple, but detection sensitivity for weak interferer signals is insufficient
Solution Approach 1:
The peak detector circuit is designed with multi-functionality to perform both desired signal detection and interferer detection using the same basic circuit architecture. The differential pair structure can process multiple signal types simultaneously, reducing the need for separate dedicated circuits and maintaining relative simplicity while enhancing detection capabilities.
Solution Approach 2:
An automatic gain control (AGC) feedback loop is implemented where the peak detector output feeds back to control the LNA gain. This feedback mechanism enables the system to automatically adjust to maintain optimal detection sensitivity without requiring complex manual calibration or multiple fixed-gain stages.
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 enables the detection of weaker interferer RF signals with improved sensitivity, maintaining LNA performance and reducing signal attenuation, thereby enhancing the detection sensitivity and preventing LNA saturation.
Implementation Method 1
Transistors have a non-linear voltage and current relationship. A small-signal voltage is an alternating current (AC) voltage signal where the amplitude of the small-signal is small relative to direct current (DC) bias voltages that are applied to bias the transistors in the PDET.
Implementation Method 2
A non-linear voltage and current relationship during small-signal operation can be represented with first-order and higher-order drain currents as a function of the small-signal gate-to-source voltage (Vgs). For instance, a small-signal drain current id is modeled as a sum of a linear current source gmVgs, and non-linear current sources, for example, gm2Vgs2, gm3Vgs3, and higher-order non-linear current sources.
Data Source
AI summary
An apparatus comprises a transistor pair including a first metal oxide semiconductor field effect transistor (MOSFET) coupled to a second MOSFET. The first MOSFET includes a first gate terminal and a first drain terminal. The second MOSFET comprises a second gate terminal and a second drain terminal. The first gate terminal is configured to receive a first signal. The second gate terminal is configured to receive a second signal that is phase shifted with respect to the first signal. An output node is coupled to the first drain terminal and the second drain terminal and configured to output a third signal that is proportional to a power of the first signal and the second signal.