Coaxial Cable Driver Layout for Common-Mode EMI Cancellation
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Solution Overview
Problem
Single-ended coaxial cables face challenges in achieving low electromagnetic interference (EMI) and immunity to bulk current injection, especially at high speeds, due to their unbalanced nature and susceptibility to electromagnetic radiation, which is not adequately addressed by existing technologies.
Innovation Solution
A differential to single-ended coupling device is used, with a differential driver that includes outputs to the core and shield of the coaxial cable, utilizing a termination resistor close to the cable's characteristic impedance and high-frequency low-impedance paths to connect a local transmit ground supply, effectively canceling common mode currents and reducing EMI emissions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single-ended driver is used to drive a coaxial cable, then the system is simpler and less expensive, but it generates high electromagnetic radiation and shows poor EMI properties
Solution Approach 1:
The invention segments the single-ended signaling function into two separate differential signal paths (one for each polarity). By using a differential driver with two independent output circuits, each driving one polarity of the signal, the system achieves balanced transmission that cancels electromagnetic radiation while maintaining single-ended cable simplicity.
Solution Approach 2:
The invention introduces asymmetry in the grounding arrangement by using separate local ground connections for each differential output rather than a common ground. This asymmetric grounding strategy, combined with the differential signaling, creates balanced current paths that reduce common mode radiation and improve EMI properties.
2Object-generated harmful factors
If a differential cable is used for high speed communication, then EMI properties are improved, but intra-pair skew generates wave coupling and signal destruction
Solution Approach 1:
The invention extracts the skew problem from the transmission medium by using a single-ended coaxial cable instead of a differential pair. The coaxial cable's single-ended nature eliminates intra-pair skew entirely, while the differential driver architecture maintains balanced signaling to achieve low radiation without suffering from skew-induced signal coupling.
3Manufacturing precision
If source termination is added to a single-ended driver, then signal quality is improved, but the system becomes more complex and EMI is not sufficiently reduced
Solution Approach 1:
The invention merges the termination function into the differential driver architecture itself rather than using separate termination components. By integrating series termination resistors directly in the differential output paths, the system achieves signal quality improvement while maintaining the balanced differential structure that reduces EMI, avoiding the need for additional external termination components.
4Manufacturing precision
If equalization is used to compensate for frequency dependent loss, then high frequency signal quality is improved, but the received signal becomes more susceptible to bulk current injection
Solution Approach 1:
The invention creates equipotential reference paths by providing separate local ground connections for each differential output. This equipotential arrangement ensures that both signal paths reference the same ground potential, reducing common mode voltage that would otherwise make the equalized high-frequency signals susceptible to bulk current injection and electromagnetic interference.
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 solution significantly reduces EMI emissions by canceling common mode currents and achieving a balanced PCB layout, improving EMI emission by up to 20 dB compared to prior art solutions, while maintaining low radiation and immunity to bulk current injection.
Implementation Method 1
By using a differential driver, we generate two complementary signals, one for each polarity. This allows us to cancel the common mode component at the output, thereby reducing electromagnetic radiation.
Data Source
AI summary
A cable driver (301) for driving a single ended transmission medium such as a coaxial cable (115) comprising a core (120) and a shield (121) comprises a differential driver (104, 377) comprising a first output (151) for putting a first signal to the core (120) of the single ended transmission medium (115), a second output (152) for putting a second signal to the shield (121) of the single ended transmission medium (115) through a termination resistor (118) having an impedance close to the characteristic impedance (Z0) of the single ended transmission medium (115), and a third output (153) for putting a transmit ground supply signal (GNDT), local to the differential driver, to the shield (121) of the single ended transmission medium (115) through a first high frequency low impedance path (112). In use, the current through the third output (153) will be substantially the inverse of the common mode current through the first and second outputs (151, 152). Hence both the common mode current and its inverse are provided to the single ended transmission medium (115). They cancel each other out and thus advantageously low EMI emissions occur.