Receiver System LO Frequency Determination for RF Image Interference
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Solution Overview
Problem
Conventional mixer-based receivers face challenges in accurately measuring broadband periodically modulated RF signals due to limited intermediate frequency (IF) bandwidth, leading to interference from unwanted mixing products such as harmonics and alias components, which are not effectively filtered by existing systems like VNAs and PNAs.
Innovation Solution
A method and system that determine tone frequencies and LO frequencies to minimize interference by using discrete Fourier transform (DFT) record size and ADC sampling rate, allowing for phase-coherent multichannel measurements and spectral stitching to capture the full spectrum of periodically modulated RF signals without image filters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional mixer-based receivers are used to measure broadband periodically modulated RF signals, then the measurement process can be performed with standard equipment, but the limited IF bandwidth causes interference from unwanted mixing products such as harmonics and alias components
Solution Approach 1:
The patent applies preliminary action by determining the optimal relationship between LO frequency, ADC sampling rate, and DFT record size before performing the frequency conversion and measurement. By pre-calculating and setting these parameters to satisfy specific mathematical relationships, the system ensures that images fall exactly on DFT bins that can be identified and removed, preventing interference before it affects the measurement accuracy
Solution Approach 2:
The patent converts the harmful effect of images into a beneficial situation by deliberately designing the frequency conversion process so that images fall precisely on DFT bin centers. This allows the images to be easily identified and removed through digital signal processing, transforming what would normally be harmful interference into a controllable and eliminable artifact
2Adaptability or versatility
If multiple LO frequency settings are used to capture the full bandwidth of broadband RF signals, then the total bandwidth can be measured, but the complexity of the measurement process increases with multiple ADC data record acquisitions
Solution Approach 1:
The patent applies parameter changes by systematically varying the LO frequency across multiple measurement segments while maintaining specific mathematical relationships with the ADC sampling rate and DFT record size. This allows the system to cover broad bandwidth by changing frequency parameters in a controlled manner, with each segment contributing to the overall spectrum through spectral stitching
3Object-affected harmful factors
If image rejection filters are added to conventional super-heterodyne receivers, then interference from images can be reduced, but the device complexity and cost increase
Solution Approach 1:
The patent replaces the mechanical/analog image rejection filters with a digital signal processing approach. By using precise frequency conversion with specifically determined LO frequencies and applying DFT-based image identification and removal algorithms, the system eliminates the need for complex analog filtering hardware while achieving effective image rejection through software-based processing
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
Enables accurate and interference-free measurement of broadband RF signals by eliminating unwanted mixing products, allowing for precise characterization of devices like power amplifiers across aerospace and wireless communication systems.
Implementation Method 1
when the periodically modulated RF signals are mixed with local oscillator (LO) frequencies in a conventional receiver to frequency convert to corresponding IF frequencies
Implementation Method 2
determining a discrete Fourier transform (DFT) record size of samples provided by the ADC
Data Source
AI summary
A system and method prevent interference caused by images resulting from mixing an incoming periodically modulated RF signal with multiple LO signals generated by a LO in a receiver system. The method includes determining tone frequencies of multiple tones and determining tone spacing between adjacent tones in the periodically modulated RF signal using a known period of modulation of the periodically modulated RF signal; identifying a sampling rate of an ADC of the receiver system; determining a DFT record size of samples provided by the ADC based on at least the tone spacing and the ADC sampling rate; and determining LO frequencies of the multiple LO signals based on at least the DFT record size and the ADC sampling rate, such that images created by respectively mixing the determined LO frequencies with portions of the periodically modulated RF signal avoid interfering with direct mixing components of the plurality of tones.