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EMC Question of the Week: October 15, 2018
To send a 10 Mbps CMOS digital signal a distance of 20 cm on a microstrip circuit board trace, you must
- use a controlled-impedance trace and matched termination
- use a controlled-impedance trace and matched source
- either a or b
- control the transition time
Answer
The best answer is "d". The propagation delay for a signal traveling 20 cm on a typical microstrip trace is about a nanosecond. Most CMOS devices will switch faster than this, so to control ringing, we must either match the transmission line or slow the transition time. Matching should always be the option of last resort. Matched terminations increase the power dissipated in CMOS digital circuits and they make it difficult to control the harmonic content of the signals. Using a series resistor to slow the transition time to about 10 nanoseconds (10% of the bit width) results in a good digital signal that doesn't require a matched termination.
Yes, all four of the answers listed are valid options if signal integrity is the only goal. Nevertheless, controlling the transition time is clearly the best option when EMC, power efficiency, and low-cost are also goals.
One additional note: Must you control the transition time if it is already 10 nanoseconds or more? The best answer is still yes. It is dangerous to use components that happen to switch slowly, when the design is counting on this to meet EMC and signal integrity requirements. Digital device switching speeds tend to get faster as manufacturing processes improve. If product performance relies on a particular device to have slow transition times, the digital device's transition times should be explicitly controlled, either by specification or by external components.
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