Ratio Detector

A ratio detector demodulates FM signals while naturally rejecting amplitude variations caused by noise. Early commercial FM radios used this to avoid needing a separate limiter stage.

Core Concept

The ratio detector shares the basic transformer structure of the Foster-Seeley circuit. However, it completely reverses one diode direction. A ratio detector adds a massive stabilizing capacitor across the output.

This large 10 uF capacitor holds a very steady DC voltage. It aggressively absorbs any rapid amplitude spikes caused by atmospheric noise or interference. This provides built-in AM rejection.

The audio output depends only on the ratio of voltages across the two smaller capacitors. This ratio changes only with frequency, providing excellent natural noise immunity without external limiters.

Key Equations

Output voltage ratio: Vout = (VC1 - VC2) / 2

Total stabilizing voltage: Vtotal = VC1 + VC2 = Constant

Here, VC1 and VC2 represent the voltages across the individual high-frequency filter capacitors. The sum of these voltages is held constant by the 10 uF electrolytic capacitor.

Given:
Voltage across capacitor C1 = 6 V
Voltage across capacitor C2 = 4 V

Why this formula:
The output voltage is exactly half the difference of the individual capacitor voltages.

Formula:
Vout = (VC1 - VC2) / 2

Substitution:
Vout = (6 - 4) / 2

Calculation:
Vout = 2 / 2

Final Answer:
The audio output voltage is 1 V.

Exam Tip: A ratio detector provides its own amplitude limiting. A Foster-Seeley discriminator does not. This is the single most tested difference between the two circuits.

Key Properties

• Features excellent built-in immunity to amplitude modulation.
• Uses two diodes connected in series to charge a common large capacitor.
• Eliminates the cost and complexity of a separate limiter amplifier stage.
• Has slightly worse linearity compared to a well-designed Foster-Seeley circuit.
• Output audio voltage is taken from the center point between the two tank capacitors.

Quick Revision

• Ratio detectors have one reversed diode.
• A large 10 uF capacitor provides AM limiting.
• The output depends on a voltage ratio.
• They are very cheap to manufacture.
• They were the standard in old television sets.
• Exam trap: Believing a ratio detector needs a hard limiter stage before it.