# How do you measure PCB, cable, and interconnect impedance, dielectric constant, and length?

**URL:** <https://sierraconnect.protoexpress.com/t/how-do-you-measure-pcb-cable-and-interconnect-impedance-dielectric-constant-and-length/4553>\
**Category:** Today's Discussion\
**Tags:** controlled-impedance\
**Created:** [October 7, 2026, 1:36pm UTC](https://sierraconnect.protoexpress.com/t/how-do-you-measure-pcb-cable-and-interconnect-impedance-dielectric-constant-and-length/4553 "2026-10-07T13:36:24Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Sushmitha](https://sea2.discourse-cdn.com/sierraconnect/user_avatar/sierraconnect.protoexpress.com/sushmitha/32/464_2.png) [@Sushmitha](https://sierraconnect.protoexpress.com/u/Sushmitha)\
**Post date:** [October 7, 2026, 1:36pm UTC](https://sierraconnect.protoexpress.com/t/how-do-you-measure-pcb-cable-and-interconnect-impedance-dielectric-constant-and-length/4553/1 "2026-10-07T13:36:24Z")

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Controlled impedance designs depend on accurate transmission-line characteristics, but verifying them requires the right measurement approach. Test coupons, PCB traces, cables, and interconnects can be measured using Time Domain Reflectometry (TDR) to determine impedance, dielectric constant (Dk), velocity factor (Vf), and length.

What measurement approaches do you use to verify these parameters during design validation and testing? I wanted to start a discussion on the practical approaches designers and engineers use to make these measurements.

 ![Measurement setup for the TDR measurements in this application note. The setup uses the PerfectPulse USB connected TDR with a splitter (included) that has one output connected to the DUT and the other connec](https://global.discourse-cdn.com/sierraconnect/original/1X/ed0533b337be5c33b03e404236fbe6c8d056c9bb.jpeg)

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<div class="post-metadata">

**Author:** ![Sushmitha](https://sea2.discourse-cdn.com/sierraconnect/user_avatar/sierraconnect.protoexpress.com/sushmitha/32/464_2.png) [@Sushmitha](https://sierraconnect.protoexpress.com/u/Sushmitha)\
**Post date:** [October 7, 2026, 1:36pm UTC](https://sierraconnect.protoexpress.com/t/how-do-you-measure-pcb-cable-and-interconnect-impedance-dielectric-constant-and-length/4553/2 "2026-10-07T13:36:41Z")

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A TDR launches a fast step edge into the transmission line and measures the reflected signal. The reflection coefficient can be used to calculate impedance, while the round-trip propagation time can be used to determine either the length or dielectric constant.

If the physical length is known, the propagation time can be used to calculate Dk. If the velocity factor is known, the same measurement can be used to determine the cable or trace length.

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<div class="post-metadata">

**Author:** ![Sushmitha](https://sea2.discourse-cdn.com/sierraconnect/user_avatar/sierraconnect.protoexpress.com/sushmitha/32/464_2.png) [@Sushmitha](https://sierraconnect.protoexpress.com/u/Sushmitha)\
**Post date:** [October 7, 2026, 1:36pm UTC](https://sierraconnect.protoexpress.com/t/how-do-you-measure-pcb-cable-and-interconnect-impedance-dielectric-constant-and-length/4553/3 "2026-10-07T13:36:55Z")

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In practice, TDR measurements can help with:

- Verifying PCB test coupon impedance
- Checking cable quality and detecting degraded crimps
- Measuring trace or cable length
- Determining dielectric constant and velocity factor

Measurement quality also depends on the test setup. Use high-quality cables, probes, and adapters, and measure the flattest portion of the impedance waveform, typically between 50% and 75% of the trace or cable length.

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<div class="post-metadata">

**Author:** ![Sushmitha](https://sea2.discourse-cdn.com/sierraconnect/user_avatar/sierraconnect.protoexpress.com/sushmitha/32/464_2.png) [@Sushmitha](https://sierraconnect.protoexpress.com/u/Sushmitha)\
**Post date:** [October 7, 2026, 1:37pm UTC](https://sierraconnect.protoexpress.com/t/how-do-you-measure-pcb-cable-and-interconnect-impedance-dielectric-constant-and-length/4553/4 "2026-10-07T13:37:45Z")

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The measurement method should match the interconnect type, available test equipment, and required accuracy. For controlled impedance designs, it is also important to define the measurement approach and acceptance criteria early so the test results can be compared against the design requirements.

For a practical explanation of how to measure PCB, cable, and interconnect impedance, dielectric constant, velocity factor, and length, check out this article: [Measuring PCB, Cable and Interconnect Impedance, Dielectric Constant, Velocity Factor, and Lengths](https://www.protoexpress.com/blog/measuring-pcb-cable-and-interconnect-impedance-dielectric-constant-velocity-factor-and-lengths/).