Ratiodetector

Ratiodetector, also known as a ratio detector, is an AM demodulation circuit used in radio receivers to extract audio from amplitude-modulated signals with reduced distortion compared to a simple envelope detector. It was developed as an alternative demodulator for improving fidelity when dealing with higher modulation indices and carrier fluctuations.

The circuit typically consists of a center-tapped transformer secondary feeding two diodes that rectify the positive and negative halves of the AM signal. The diode outputs are connected to a load network that includes an RC filter, and the demodulated audio appears across this load. The transformer winding and diode arrangement create a controlled ratio between the two rectified currents, which the circuit uses to recover the modulating signal while minimizing the effects of carrier amplitude variations.

Operating principle: a ratio detector produces an output voltage that is related to the instantaneous ratio of the diode currents rather than their absolute values. This makes the detector less sensitive to changes in carrier level and amplitude that accompany modulation, which helps reduce distortion when the modulation index is high. The detector’s performance depends on accurate transformer symmetry and stable diode behavior; deviations can introduce distortion or reduced audio fidelity.

Advantages and limitations: the ratio detector offers improved linearity and distortion performance over a simple envelope detector, particularly under higher modulation. However, it is more complex, bulkier, and more sensitive to component tolerances and transformer quality. It is commonly found in older or specialized AM receivers and may be chosen when fidelity is prioritized and the design can accommodate its component requirements.

See also: envelope detector, product detector, discriminator.