The ratio detector is a type of detector circuit, commonly used in

radio receiver In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. Th ...

s for demodulating frequency modulated (FM) signal. The ratio detector is a variant of the

Foster–Seeley discriminator The Foster–Seeley discriminator is a common type of FM detector circuit, invented in 1936 by Dudley E. FosterDudley E. Foster: biographical information and photo: ''Proceedings of the Institute of Radio Engineers'', vol. 29, page 571 (October 1 ...

, but one

diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A 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 Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to ...

(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 A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an inver ...

(C4) and bleeding resistor(s) (R1, R2) are connected across the

s. 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 An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ac ...

. As the

amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...

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

{{reflist Detectors