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Ever wondered how long it takes for your high-speed digital signal or precise RF pulse to travel down a tiny micro coaxial cable? That travel time is called propagation delay, and it’s a critical factor in timing-sensitive applications like high-speed digital circuits, radar systems, telecommunications, and high-frequency test equipment. Knowing how to calculate it is essential for designing reliable systems.
Why Propagation Delay Matters
When signals travel along any cable, including micro coaxial cables (like popular types RG178, RG316, or tiny micro-miniature cables), they don’t move instantaneously. They travel at a significant fraction of the speed of light, but less than the speed of light in a vacuum.
The Key Ingredient: Velocity Factor (VF)
The fundamental principle driving propagation delay is the Velocity Factor (VF), often expressed as a percentage. This number tells you how fast the signal travels relative to the speed of light in a vacuum (c).
c): Approximately 3 x 10^8 meters per second (300,000,000 m/s) or 11.8 inches per nanosecond (ns).εr or Dk) of this material slows down the wave. A higher dielectric constant means a slower signal speed and higher propagation delay.Signal Speed in Cable (v) = Speed of Light in Vacuum (c) * Velocity Factor (VF)VF = 1 / √εrVelocity Factor: The Critical Specification
This is the single most important piece of information you need from the micro coaxial cable’s manufacturer datasheet. Typical VF values for common micro coax dielectrics are:
Always double-check the datasheet for the specific cable you are using! VF can vary slightly between manufacturers and specific cable constructions.
Calculating Propagation Delay
Once you have the Velocity Factor (VF), calculating propagation delay is straightforward. It’s simply the length of the cable divided by the speed of the signal within that cable.
v):v = c * VFc = Speed of light (3 x 10^8 m/s or 11.8 in/ns)VF = Velocity Factor (as a decimal from the datasheet)t_pd):t_pd = Cable Length (d) / Signal Speed (v)Bringing it All Together (Simpler Formulas)
You often see propagation delay expressed in terms of delay per unit length. This avoids directly calculating v each time.
The propagation delay per unit length is directly related to the Velocity Factor:
Delay per Meter = 1 / v = 1 / (c * VF) = (3.333 ns/m) / VFc is ~3 x 10^8 m/s, so 1/c = ~3.333 nanoseconds per meter)Delay per Foot = (1 ns/ft) / VF * 1.0167 ≈ (1.0167 ns/ft) / VF1 foot / (11.8 inches/ns * 1/12 ft/inch) = 1.0167 ns/ft delay for light. This approximation is excellent for practical calculations).Practical Calculation Steps
Here’s the simple process using the common “delay per foot” formula:
Delay per Foot ≈ 1.0167 / VF1 / VF for a quick estimate good to within ~1.7%)Total Propagation Delay = Delay per Foot * Length in FeetTotal Propagation Delay (ns) = (Length in Feet * 1.0167) / VFTotal Propagation Delay (ns) = (Length in Meters * 3.333) / VFExample Calculation
Let’s say you are using a 10-foot length of RG316 micro coaxial cable. Its datasheet lists a Velocity Factor of 0.695 (69.5%).
This means a signal entering one end of the 10-foot RG316 cable will emerge from the other end approximately 14.63 nanoseconds later.
Key Takeaways
Delay per Foot ≈ 1.0167 / VF (ns/ft) [~1.7% accurate]Total Delay (ns) = (Length in Feet * 1.0167) / VF [Accurate]Total Delay (ns) = (Length in Meters * 3.333) / VF [Accurate]By using the cable’s Velocity Factor and these simple formulas, you can reliably determine the propagation delay of any micro coaxial cable for your critical designs and measurements.
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