PCB Signal Integrity & EMC 1 — Questions and Answers
Question 1: What is the industry-standard characteristic impedance target for most controlled-impedance PCB traces?
- 25 ohms
- 50 ohms (Correct answer)
- 75 ohms
- 100 ohms
Correct answer: 50 ohms
50 ohms is the industry-standard characteristic impedance used to match source and load impedances, minimizing reflections on high-speed traces.
Question 2: What phenomenon occurs when a high-speed signal travels down a transmission line and encounters an impedance discontinuity?
- Resonance
- Signal reflection (Correct answer)
- Diffraction
- Signal absorption
Correct answer: Signal reflection
Reflections occur at impedance discontinuities because signal energy that cannot pass through the mismatch is sent back toward the source.
Question 3: Which termination method places a resistor matching the trace's characteristic impedance between the signal line and ground at the far end of the trace?
- Series termination
- Thevenin termination
- AC termination
- Parallel (shunt) termination (Correct answer)
Correct answer: Parallel (shunt) termination
Parallel termination places a resistor equal to the characteristic impedance to ground at the load end, absorbing reflected energy and preventing signal bounce.
Question 4: At what fraction of signal wavelength does a PCB trace typically need to be treated as a transmission line requiring controlled impedance?
- λ/100
- λ/20
- λ/10 (Correct answer)
- λ/2
Correct answer: λ/10
Traces longer than 1/10 of the signal wavelength (or equivalent rise-time length) must be treated as transmission lines to avoid signal integrity problems.
Question 5: In a stripline PCB layer configuration, where is the signal trace located within the stackup?
- On the top surface of the PCB
- On the bottom surface of the PCB
- Between two reference planes (Correct answer)
- Adjacent to only one reference plane
Correct answer: Between two reference planes
Stripline traces are embedded between two ground or power reference planes, providing shielding on both sides and superior EMI performance compared to microstrip.
Question 6: What is the primary signal integrity advantage of differential signaling over single-ended signaling in high-speed PCB design?
- Requires fewer PCB traces
- Higher power efficiency
- Superior common-mode noise rejection (Correct answer)
- Simpler termination requirements
Correct answer: Superior common-mode noise rejection
Differential signaling rejects common-mode noise because interference coupled equally onto both signal lines cancels out at the differential receiver.
Question 7: Which instrument is used to measure the impedance profile of a PCB trace as a function of distance along its length?
- Vector network analyzer
- Time Domain Reflectometer (TDR) (Correct answer)
- Spectrum analyzer
- Digital oscilloscope
Correct answer: Time Domain Reflectometer (TDR)
A TDR sends a fast-rise-time pulse down the trace and measures reflections over time, revealing impedance variations and discontinuities along the trace.
PCB Signal Integrity & EMC 1
What is the industry-standard characteristic impedance target for most controlled-impedance PCB traces?