Fast Settling Peak Detector Using Fundamental Frequency Cancellation
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Solution Overview
Problem
Conventional peak detectors in AGC circuits for wireless transceivers are slow to respond to changes in RF signal amplitude due to the need to filter out fundamental frequencies, which increases settling time, or require additional power and complexity with differential circuits.
Innovation Solution
A fast settling peak detector circuit that reduces the fundamental frequency component using a cancellation branch with signal inversion or phase-shifting, allowing for reduced filtering capacitance and faster settling times without the need for a tail current source.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a filter capacitor is used to reject fundamental frequency and harmonics, then the amplitude indication accuracy is improved, but the settling time increases
Solution Approach 1:
The patent extracts and removes the fundamental frequency component from the output signal using a notch filter, separating it from the amplitude indication signal. This allows the filter capacitor to be reduced in size while still achieving accurate amplitude measurement, as the fundamental frequency is removed before the capacitor processes the signal.
Solution Approach 2:
The patent changes the capacitance parameter of the filter capacitor from a large value (needed when fundamental frequency is present) to a smaller value (sufficient when fundamental frequency is removed). This parameter change directly reduces the settling time while maintaining amplitude indication accuracy through the combined effect of notch filtering and capacitive filtering.
2Measurement precision
If a fully differential circuit is used to reject fundamental frequency, then the amplitude indication accuracy is improved, but the circuit complexity and power consumption increase
Solution Approach 1:
The patent segments the fundamental frequency rejection function into a separate notch filter stage, distinct from the main differential amplification path. This allows the use of a simpler single-ended or pseudo-differential peak detector circuit while achieving fundamental frequency rejection through the dedicated notch filter, thereby reducing overall circuit complexity.
Solution Approach 2:
The notch filter serves multiple functions: it rejects the fundamental frequency component, prevents it from saturating the filter capacitor, and enables the use of smaller capacitance values. This multi-functional element replaces the need for complex fully differential circuitry while achieving the same amplitude indication accuracy.
3Measurement precision
If a fully differential circuit is used to reject fundamental frequency, then the amplitude indication accuracy is improved, but the voltage overhead increases
Solution Approach 1:
The patent extracts the fundamental frequency component before it can consume excessive voltage headroom in the amplification stages. By removing this large-amplitude component through notch filtering, the remaining signal requires less voltage overhead for processing, enabling operation with lower supply voltages.
Solution Approach 2:
The patent changes the capacitance parameter to a smaller value, which directly reduces the voltage overhead required for the filter stage. Smaller capacitors require less charge storage and thus operate effectively with reduced voltage headroom, making the circuit suitable for low-voltage RF applications.
Data Source
AI summary
The present disclosure describes aspects of a fast settling peak detector. In some aspects, a peak detector circuit includes a first transistor having a gate coupled to an input of the circuit at which a signal is received and a drain coupled to a source of a second transistor. Current may flow in the first and second transistors responsive to the signal. The circuit also includes a third transistor having a gate coupled, via a signal-inverting component, to the input of the circuit and a drain coupled to a source of a fourth transistor. Through an inversion of the signal, other current flowing in the third and fourth transistor can reduce or cancel a frequency component of the current in the first and second transistors. In some cases, this precludes a need to filter the frequency component from an output of the circuit.