Wide-Area AM Signal Sensing with Pixel-Level Down-Conversion
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Solution Overview
Problem
Digital focal plane arrays (DFPAs) face challenges in sampling amplitude-modulated (AM) optical signals with high carrier frequencies, as the Nyquist sampling rate often exceeds the frame rate, making direct detection and demodulation inefficient.
Innovation Solution
The DFPA employs in-phase and quadrature counters to down-convert AM signals by mixing the photocurrent with square waves, allowing for Nyquist sampling at a lower rate than the carrier frequency, enabling efficient detection and demodulation of AM signals with center frequencies ranging from 100 kHz to 10 GHz and bandwidths from 10 kHz to 100 kHz.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the DFPA samples AM signals at the Nyquist rate of the carrier frequency, then the signal can be accurately detected, but the frame rate becomes excessively high and inefficient
Solution Approach 1:
The patent segments the AM signal processing into two distinct parts: the high-frequency carrier component and the low-frequency information-bearing component. By using in-phase and quadrature counters to separately track these components, the system applies Nyquist sampling only to the lower bandwidth information signal rather than the entire high-frequency carrier, thereby resolving the contradiction between detection accuracy and frame rate efficiency
Solution Approach 2:
The patent introduces in-phase and quadrature counters as intermediary elements that mediate between the high-frequency AM carrier and the lower-rate sampling system. These counters act as frequency translators, converting the high-frequency signal information into lower-frequency count representations that can be efficiently sampled at reduced frame rates while preserving signal integrity
2Measurement precision
If the DFPA uses a high frame rate to detect AM signals, then sampling accuracy is maintained, but the system complexity and processing burden increase
Solution Approach 1:
The patent extracts the high-frequency carrier information from the AM signal using in-phase and quadrature demodulation, separating it from the lower-frequency information-bearing content. This extraction allows the system to process only the essential information at lower sampling rates, reducing computational complexity and processing burden while maintaining sampling accuracy for the relevant signal components
3Ease of operation
If the DFPA directly detects high-frequency AM signals without down-conversion, then the detection process is simpler, but the sampling rate must exceed the frame rate capability
Solution Approach 1:
The patent performs preliminary down-conversion of the high-frequency AM carrier to lower frequencies using in-phase and quadrature counters before the main sampling operation. This preliminary frequency reduction enables the subsequent sampling process to operate at achievable frame rates while maintaining the simplicity of the overall detection through a systematic two-stage approach
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 allows for efficient detection and demodulation of AM signals at a significantly lower sampling rate than the Nyquist rate, reducing the frame rate by a factor of five, while maintaining signal integrity, and enabling the detection of AM signals with unknown phase and frequency.
Implementation Method 1
A photodetector in the pixel transduces the AM optical signal to an AM analog signal
Implementation Method 2
A current-to-frequency converter in the pixel and operably coupled to the photodetector converts the AM analog signal into an electronic pulse train
Implementation Method 3
The circuitry mixes a first copy of the electronic pulse train with an in-phase square wave modulated at a mixing frequency fLO to produce an in-phase representation of the AM optical signal
Data Source
AI summary
Amplitude-modulated (AM) signals spanning a spatial wide area can be efficiently detected using a slowly scanning optical system. The system decouples the AM carrier from the AM signal bandwidth (or carrier uncertainty), enabling Nyquist sampling of only the information-bearing AM signal (or the known frequency bandwidth). The system includes a staring sensor with N pixels (e.g., N>106) that searches for a sinusoidal frequency of unknown phase and frequency, perhaps constrained to a particular band by a priori information about the signal. Counters in the sensor pixels mix the detected signals with local oscillators to down-convert the signal of interest, e.g., to a baseband frequency. The counters store the down-converted signal for read out at a rate lower than the Nyquist rate of AM signal. The counts can be shifted among pixels synchronously with the optical line-of-sight for scanning operation.