engineering

Propagation delay

Propagation delay is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Propagation delay rather than just read about it. In short: Propagation delay is the time duration taken for a signal to reach its destination, for example in the electromagnetic field, a wire, gas, fluid or solid body. Physics An electromagnetic wave travelling through a medium has a propagation delay determined by the speed of light in that particular medium, or ca.(circa/approx) 1 nanosecond per 29.98 centimetres (11.80 in) in a vacuum.

Key takeaways

Propagation delay belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Propagation delay to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Propagation delay from memory before moving on to harder problems.

Reference excerpt

Propagation delay is the time duration taken for a signal to reach its destination, for example in the electromagnetic field, a wire, gas, fluid or solid body.

Physics An electromagnetic wave travelling through a medium has a propagation delay determined by the speed of light in that particular medium, or ca.(circa/approx) 1 nanosecond per 29.98 centimetres (11.80 in) in a vacuum. An electric signal travelling through a wire has a propagation delay of ca. 1 nanosecond per 15 centimetres (5.9 in). See also radio propagation, velocity factor, signal velocity and mechanical wave.

Electronics

Logic gates can have a gate delay ranging from picoseconds to more than 10 nanoseconds, depending on the technology being used. It is the time between the gate input becoming stable and the gate output becoming stable. Manufacturers often refer to the time from the input changing to 50% of its final input level, to the output reaching 50% of its final output level; this may depend on the direction of the level change, in which case separate fall and rise delays tPHL and tPLH or tf and tr are given. Reducing gate delays allows digital circuits to process data at a faster rate and improve overall performance. Determining the propagation delay of a combined circuit requires identifying the longest path of propagation delays from input to output, and adding each propagation delay along this path. The principle of logical effort utilizes propagation delays to compare designs implementing the same logical statement. The difference in propagation delays of logic elements is the major contributor to glitches in asynchronous circuits as a result of race conditions.

Propagation delay may increase or decrease with operating temperature depending on the device type. As the temperature increases the gate delay decreases for FinFET transistors due to Inverse Temperature Dependence; for metal wires and other conductive materials the propagation delay increases due to rising electrical resistance. Marginal increases in supply voltage can increase propagation delay, since the upper switching threshold voltage VIH (often expressed as a percentage of the high-voltage supply rail) naturally increases proportionately. Increases in output load capacitance, often from placing increased fan-out loads on a wire, will also increase propagation delay. All of these factors influence each other through an RC time constant: any increase in load capacitance increases C, heat-induced resistance the R factor, and supply threshold voltage increases will affect whether more than one time constants are required to reach the threshold. If the output of a logic gate is connected to a long trace or used to drive many other gates (high fanout) the propagation delay increases substantially.

Networking In computer networks, propagation delay is the amount of time it takes for the head of the signal to travel from the sender to the receiver. It can be computed as the ratio between the link length and the propagation speed over the specific medium. Propagation delay is equal to d / s where d is the distance and s is the wave propagation speed. In wireless communication, s=c, i.e. the speed of light. In copper wire, the speed s generally ranges from .59c to .77c. This delay is the major obstacle in the development of high-speed computers and is called the interconnect bottleneck in IC systems.

See also Contamination delay Delay calculation Latency (engineering) Time of flight Transmission delay

References

Illustrations

Worked examples

Example 1 — a first encounter with Propagation delay

Start with the simplest possible case. Write down what Propagation delay claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Propagation delay before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Propagation delay ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Propagation delay

In research: Propagation delay appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Propagation delay in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Propagation delay is common in secondary-school and first-year university syllabi. It links to neighbouring topics Digital circuits, Electronics concepts, Timing in electronic circuits, so understanding it makes those chapters shorter.
In everyday life: Look for Propagation delay outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Propagation delay in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Propagation delay means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Propagation delay out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Propagation delay in simple terms?

Propagation delay is the time duration taken for a signal to reach its destination, for example in the electromagnetic field, a wire, gas, fluid or solid body. Physics An electromagnetic wave travelling through a medium has a propagation delay determined by the speed of light in that particular med…

Why does Propagation delay matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Propagation delay?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Propagation delay.