Self-Oscillating Spread Spectrum Loop for RF Isolator Emissions
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Solution Overview
Problem
RF digital isolators face challenges in complying with FCC radiated RF emission limits due to unintended antenna formation from wiring, which can cause excess radiated RF, especially with fixed frequency carriers that may experience resonance amplification.
Innovation Solution
A self-oscillating spread spectrum frequency control loop using a gated voltage-controlled oscillator (VCO) generates a spread spectrum carrier by varying frequency through a triangle wave signal, effectively increasing bandwidth and reducing emissions across a wide frequency range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a fixed frequency RF carrier is used, then the RF isolator can transmit digital data efficiently, but radiated RF emissions increase due to resonance amplification from wiring and unintended antennas
Solution Approach 1:
The patent applies dynamics by transitioning from a fixed frequency carrier to a dynamically varying frequency carrier. The VCO continuously modulates the carrier frequency according to a spread spectrum code, making the frequency time-variant. This dynamic frequency variation prevents resonance amplification at any single frequency while maintaining efficient data transmission through the isolated barrier.
Solution Approach 2:
The patent implements parameter changes by modifying the frequency parameter of the RF carrier. Instead of using a constant frequency, the system varies the frequency across a wide range according to a pseudorandom code. This parameter transformation spreads the energy spectrum, reducing peak emissions at any particular frequency while preserving the integrity of digital data transmission.
2Object-generated harmful factors
If spread spectrum RF is used to reduce radiated emissions, then compliance with FCC limits is achieved, but the device complexity increases due to additional frequency control circuitry
Solution Approach 1:
The patent merges the frequency control function directly into the RF carrier generation stage by using a VCO. The spread spectrum modulation is integrated with the carrier generation process, eliminating the need for separate frequency modulation stages. This consolidation reduces overall system complexity while achieving spread spectrum emission reduction.
Solution Approach 2:
The system employs a self-generated pseudorandom code sequence that is known to both transmitter and receiver. The receiver uses the same code to despread the signal, eliminating the need for complex synchronization protocols or external reference signals. This self-service approach simplifies the frequency control mechanism while maintaining spread spectrum benefits.
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 significantly reduces radiated RF emissions by spreading the spectrum across a wide frequency band, compliance with FCC limits is achieved, and the system is resistant to jamming and interception.
Implementation Method 1
a gated voltage-controlled oscillator (VCO) receives a digital signal that can start or stop its oscillation. When gated on, the VCO is able to generate a spread spectrum carrier by receiving a triangle wave signal that is self-generated
Implementation Method 2
The triangle wave is self-generated by a delaying ramp generator when enclosed in a loop, where its ramp direction is controlled by a frequency comparator
Data Source
AI summary
A self-oscillating spread spectrum frequency control loop contains a gated voltage-controlled oscillator (VCO) which receives a digital signal that can start or stop its oscillation. The VCO generates a spread spectrum carrier by receiving a triangle wave signal from a delaying ramp generator in a loop, its ramp direction controlled by a frequency comparator. The loop generates a spectrum spread as wide as possible above a minimum frequency. RF isolators that utilize low-pass filters in the transmitter and high-pass filters in the receiver, where the F-3 dB cutoff frequencies of both filters vary in a correlated manner, are used to not produce spread spectrum frequencies below the minimum frequency. Die from a given wafer lot, when designed such that the low- and high-pass cutoff frequencies track, can be used to form RF digital isolators whose minimum spread spectrum frequency does not go below the minimum frequency required by that wafer lot.