transmission line | PartQuest™ Explore

This example demonstrates the use of the TDFS method (Time Domain Frequency Sweep) to measure impedance stability ratio. The system is a switching DC/DC step-down power converter used for LED lighting, supplied by a battery through a long power cable.The TDFS impedance stability measurement model applies a 0.2A sinusoidal stimulus current at the point where the cable connects to the converter. The voltage at that point, as well as the stimulus current splits (toward the source and the load) are measured, and the associated source and load impedances vs. frequency are computed.The ratio of the source to load impedance is the impedance stability ratio (Tm). Similar to the open-loop frequency response requirement for closed-loop stability, Tm must not have magnitude = 1.0 at phase = 180 degrees, at any frequency.For this system, with a cable length of 400 meters and 8 AWG wire, the results show that the impedance ratio magnitude (green waveform) reaches unity (0 dB) at close to 2 kHz, where the phase (light blue waveform) is approximately 165 degress. This implies a "phase margin" of only 15 degrees, For a longer cable, this margin is further reduced. As shown in the companion example, "Transmission Line Fed LED Driver - Switching", for the cable length = 800 meters the system is unstable.This instability is the result of the "source" impedance (which includes the cable) variation over frequency, interacting with the varying load impedance. Note that at low frequency, the load effectively has a negative impedance, due to the constant power nature of the DC to DC converter.

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This 4-node CAN (FD) network can be used to verify signal quality for various network configurations specified by the user. Many of the component parameters can be modified to represent network variants. This includes transmission line lengths and electrical properties, termination impedances, transceiver drive strengths and edge-rates, etc.* Just run a new simulation to see the impact on signal quality due to that change.It is particularly interesting to increase the length of one of the stub lines, for example the transmission line going to node2 (lower left), from its nominal 1 meter to perhaps 10 meters. This will cause a complete collapse in data communication! (Look for bit errors in the bottom wave viewer)If the design is copied and saved, then the user can free-form edit the network topology, adding new nodes, lines and other passive or active components. New simulation runs will help the user assess the viability of those network configurations.*Note: The user can also specify the "slow" and "fast" data rates and fixed test patterns, to be transmitted at the beginning of the simulation, on the CAN FD data source (u1, bottom middle), After these fixed patterns are sent, the data source then sends a continuous sequence of random data at the fast data rate, until the simulation ends.

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A Verification Platform for CAN and CAN FD Signal Quality

Mike Donnelly
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Mike Donnelly

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Member of the PartQuest Explore Development Team. Focused on modeling and simulation of analog, mixed-signal and multi-discipline systems covering a broad range of applications, including power electronics, controls and mechatronic systems.

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