Ratio detector

The ratio detector is a type of detector circuit, commonly used in radio receivers for demodulating frequency modulated (FM) signal.[1][2][3][4]

A ratio detector using solid-state diodes

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

A large value reservoir capacitor (C4) and bleeding resistor(s) (R1, R2) are connected across the diodes. The 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 small 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 damps the tuned circuit, thereby compensating for the amplitude increase of the signal.

Similarly, a decrease in the signal amplitude results in decreased 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
  1. US 2497840, Seeley, Stuart Williams, "Angle Modulation Detector", issued February 14, 1950
  2. US 2561089, Anderson, Earl I., issued July 17, 1951
  3. Report L.B.-645: "Ratio detectors for FM receivers" (15 September 1945) issued by the Radio Corporation of America, RCA Laboratories Industry Service Division, 711 Fifth Avenue, N.Y., N.Y. Reprinted in Radio, pages 18-20 (October 1945).
  4. Seeley, Stuart W.; Avins, Jack (June 1947), "The ratio detector", RCA Review, 8 (2): 201–236
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.