science

Ratio detector

Ratio detector is a science 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 Ratio detector rather than just read about it. In short: The ratio detector is a type of detector circuit, commonly used in radio receivers for demodulating frequency modulated (FM) signal. The ratio detector is a variant of the Foster–Seeley discriminator, but one diode conducts in an opposite direction, and using a tertiary winding in the preceding transformer.

Key takeaways

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

Reference excerpt

The ratio detector is a type of detector circuit, commonly used in radio receivers for demodulating frequency modulated (FM) signal. The ratio detector is a variant of the Foster–Seeley discriminator, but one diode conducts in an opposite direction, and using a tertiary winding in the preceding transformer. The output in this case is taken between the sum of the diode voltages and the center tap. The output across the diodes is connected to a large value capacitor, forming a dynamic limiter. The ratio detector has the advantage over the Foster–Seeley discriminator that it does not respond to amplitude modulation (AM) signals, thus potentially saving a limiter stage; however, the output is only 50% of the output of a discriminator for the same input signal. The ratio detector has wider bandwidth, but more distortion than the Foster–Seeley discriminator.

Amplitude noise suppression The suppression of the effect of amplitude variation of the incoming signal on the output of the ratio detector is based on the principle of RF dynamic limiting: A large value reservoir capacitor (C4) and bleeding resistor(s) (R1, R2) are connected across the diodes. The RF signal is rectified by the diodes and the rectified DC voltage charges the reservoir capacitor, while a small steady current bleeds through the resistor(s), continuously discharging it. This results in a constant steady load and a moderate damping effect on the tuned circuit. As the amplitude of the signal increases, the higher rectified voltage results in an inrush current towards the reservoir capacitor; the increased load results in an increased damping of the tuned circuit, resulting in nearly constant amplitude on the output despite the amplitude increase of the incoming signal. Similarly, a decrease in the input signal amplitude results in decreased (or ceased) current flow towards the charged reservoir capacitor, resulting in decreased- or no damping on the tuned circuit. The values of the reservoir capacitor and the bleeding resistor(s) are chosen so that the combined time constant of these components is below the audio spectrum.

References

Illustrations

Worked examples

Example 1 — a first encounter with Ratio detector

Start with the simplest possible case. Write down what Ratio detector claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, 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 Ratio detector 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 Ratio detector 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 Ratio detector

In research: Ratio detector appears in science 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 Ratio detector 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: Ratio detector is common in secondary-school and first-year university syllabi. It links to neighbouring topics Demodulation, Detectors, so understanding it makes those chapters shorter.
In everyday life: Look for Ratio detector 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 Ratio detector in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Ratio detector 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 Ratio detector out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Ratio detector in simple terms?

Why does Ratio detector matter?

Because it connects several science 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 Ratio detector?

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 Ratio detector.