RF Receiver Spur Suppression via Chirped LO Signals
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Solution Overview
Problem
Superheterodyne receivers face challenges in reducing spectral energy of spurs and unwanted images within the baseband signal frequency range, which often require complex circuitry and degrade the signal-to-noise ratio, and existing techniques for mitigating spurs in time-interleaved ADCs are not practical for real-time applications.
Innovation Solution
The implementation of a method using coherent angle modulated local oscillator signals for stable frequency downconversion, spreading the spectral energy of spurs and unwanted images across a frequency band through two-step frequency conversion, which includes mixing and filtering with a first and second angle modulated LO signal, and utilizing linear chirping of the LO signals to maintain stability and reduce spurious energy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If narrowband filtering is used to attenuate spurs and unwanted images, then the spectral energy of spurs is reduced, but the bandwidth of the receiver is reduced
Solution Approach 1:
The patent applies dynamic frequency translation by chirping the local oscillator signals, which causes spurious components to migrate across frequency bins over time while the desired signal remains stationary. This dynamic approach replaces static narrowband filtering, allowing spur suppression without bandwidth limitation.
Solution Approach 2:
The patent employs periodic chirp sequences applied to the local oscillator signals during ADC calibration and operation. This periodic frequency modulation creates time-varying spectral separation between desired signals and spurious components, enabling selective attenuation without fixed bandwidth constraints.
2Object-generated harmful factors
If image rejection mixers are used to reduce spurs and images, then the spectral purity is improved, but the temperature stability deteriorates
Solution Approach 1:
The patent replaces hardware-based image rejection mixers with a software-defined approach using digital signal processing and frequency-domain filtering. This substitution eliminates temperature-sensitive analog components while maintaining spur suppression through algorithmic methods that are inherently more stable across temperature variations.
Solution Approach 2:
The patent changes the operational parameters of the local oscillator by applying time-varying frequency modulation (chirps) rather than using fixed-frequency mixers. This parameter change enables dynamic separation of spurious components in the frequency domain, achieving image rejection without relying on temperature-sensitive mixer architectures.
3Object-generated harmful factors
If digital programmable FIR filters are used to compensate for time and frequency errors, then the spurious energy is reduced, but the device complexity and gate counts increase
Solution Approach 1:
The patent performs preliminary calibration by applying known chirp sequences to the local oscillator and measuring the resulting spectral responses before actual signal processing. This preliminary characterization of the system's spurious components enables subsequent filtering operations to be more targeted and efficient, reducing the computational complexity required during real-time operation.
Solution Approach 2:
The patent creates a digital model or copy of the spurious signal characteristics through calibration measurements, then uses this model to design optimized filter coefficients. This copying approach allows the complex filtering task to be performed offline during calibration, simplifying the real-time processing requirements.
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 mitigates spurious energy and improves the spurious-free dynamic range without degrading the signal-to-noise ratio, providing a stable and efficient solution for real-time applications by spreading spurious signals across multiple frequency bins, thus reducing their impact on the receiver's performance.
Implementation Method 1
mixing and filtering an RF signal with a first angle modulated local oscillator (LO) signal to provide a receive (Rx) intermediate frequency (IF) signal
Implementation Method 2
mixing and filtering the Rx IF signal with a second angle modulated LO signal that has a linear relationship with the first LO signal
Implementation Method 3
A first stage mixer is used to step down the RF signal to an intermediate frequency (IF) signal, and a demodulator is used to recover the baseband signal from the IF signal
Data Source
AI summary
Radio frequency (RF) receivers and methods to spread spectral energy of spurious responses of mixers over a frequency band are disclosed. For example, a receiver includes first and second mixers, and first and second variable frequency oscillators (VFOs). The first mixer is configured to receive an RF signal and provide an intermediate frequency (IF) signal. The second mixer is coupled with the first mixer and configured to receive the IF signal and provide a baseband signal. The first VFO is coupled with the first mixer and configured to provide a first angle modulated LO signal. The second VFO is coupled with the second mixer and configured to provide a second angle modulated LO signal. The first and second mixers provide a stable frequency downconversion.