Self-Oscillating Spread Spectrum Loop for RF Emission Control
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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 when using fixed frequency carriers that may experience resonance amplification.
Innovation Solution
A self-oscillating spread spectrum frequency control loop with a gated voltage-controlled oscillator (VCO) generates a spread spectrum carrier by receiving a triangle wave signal, increasing the bandwidth of the transmitted signal, thereby 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 communication rate can be maintained at high levels, but radiated RF emissions increase due to resonance amplification from wiring and cables
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 sequence, causing the RF signal to hop across multiple frequency channels. This dynamic frequency variation prevents resonance amplification at any single frequency, thereby reducing radiated emissions while maintaining high communication rates through the bandwidth-efficient spread spectrum modulation technique
Solution Approach 2:
The patent implements parameter changes by systematically varying the frequency parameter of the RF carrier. Instead of using a constant frequency, the system changes the carrier frequency dynamically based on a pseudorandom code sequence, spreading the spectral energy across a wide bandwidth. This parameter transformation converts a narrowband high-power signal into a wideband low-power signal, achieving both high data rates and reduced peak emissions
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 the need for frequency modulation circuits
Solution Approach 1:
The patent applies self-service through the use of a voltage-controlled oscillator (VCO) that automatically generates the frequency-modulated spread spectrum carrier signal. The VCO inherently performs the frequency modulation function by varying its output frequency in response to control voltage changes derived from the spread spectrum code sequence. This self-contained approach eliminates the need for separate complex modulation circuits, as the frequency modulation is achieved through the natural characteristics of the VCO component
Solution Approach 2:
The patent utilizes parameter changes to simplify the modulation architecture by transforming the modulation requirement into a voltage-to-frequency conversion problem. Instead of implementing complex phase or frequency modulation circuits, the system changes the operating parameter (frequency) of a simple VCO component based on digital control signals. This parameter transformation approach reduces circuit complexity while achieving the desired spread spectrum frequency variation
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 effectively reduces radiated RF emissions by spreading the spectrum across a wide frequency band, compliance with FCC limits is ensured, and the design is suitable for use in RF digital isolators to minimize interference and resonance issues.
Implementation Method 1
a gated voltage-controlled oscillator (VCO) which, when gated on, 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
Implementation Method 2
The loop includes a delaying ramp generator that generates a triangle wave that is received into the V control input of the gated VCO. The triangle wave causes the VCO to excursions above, then below, the nominal frequency
Data Source
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AI summary
A self-oscillating spread spectrum frequency control loop (100) contains a gated voltage-controlled oscillator, VCO, (104) which receives a digital signal (102) that can start or stop its oscillation. The VCO generates a spread spectrum carrier by receiving a triangle wave signal (114) from a delaying ramp generator (112) in a loop, its ramp direction controlled by a frequency comparator (108). The loop generates a spectrum spread as wide as possible above a minimum frequency.