The ratio detector is a type of detector circuit, commonly used in radio receivers for demodulating frequency modulated (FM) signal. [1] [2] [3] [4]
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.
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.
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 of the carrier wave is varied in proportion to that of the message signal, such as an audio signal. This technique contrasts with angle modulation, in which either the frequency of the carrier wave is varied, as in frequency modulation, or its phase, as in phase modulation.
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the power of a signal. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is a circuit that has a power gain greater than one.
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 is performed by the inverter.
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The Hartley oscillator is an electronic oscillator circuit in which the oscillation frequency is determined by a tuned circuit consisting of capacitors and inductors, that is, an LC oscillator. The circuit was invented in 1915 by American engineer Ralph Hartley. The distinguishing feature of the Hartley oscillator is that the tuned circuit consists of a single capacitor in parallel with two inductors in series, and the feedback signal needed for oscillation is taken from the center connection of the two inductors.
An envelope detector is an electronic circuit that takes a (relatively) high-frequency amplitude modulated signal as input and provides an output, which is the demodulated envelope of the original signal.
In electronics, a mixer, or frequency mixer, is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals are applied to a mixer, and it produces new signals at the sum and difference of the original frequencies. Other frequency components may also be produced in a practical frequency mixer.
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. The antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, and finally recovers the desired information through demodulation.
The Foster–Seeley discriminator is a common type of FM detector circuit, invented in 1936 by Dudley E. Foster and Stuart William Seeley. The Foster–Seeley discriminator was envisioned for automatic frequency control of receivers, but also found application in demodulating an FM signal.
A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.
An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not to be confused with electrical elements, which are conceptual abstractions representing idealized electronic components and elements.
A grid leak detector is an electronic circuit that demodulates an amplitude modulated alternating current and amplifies the recovered modulating voltage. The circuit utilizes the non-linear cathode to control grid conduction characteristic and the amplification factor of a vacuum tube. Invented by Lee De Forest around 1912, it was used as the detector (demodulator) in the first vacuum tube radio receivers until the 1930s.
A radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. Radio waves are electromagnetic waves with frequencies between about 30 Hz and 300 GHz. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves. Transmitters are necessary parts of all systems that use radio: radio and television broadcasting, cell phones, wireless networks, radar, two way radios like walkie talkies, radio navigation systems like GPS, remote entry systems, among numerous other uses.
Ripple in electronics is the residual periodic variation of the DC voltage within a power supply which has been derived from an alternating current (AC) source. This ripple is due to incomplete suppression of the alternating waveform after rectification. Ripple voltage originates as the output of a rectifier or from generation and commutation of DC power.
In radio, a detector is a device or circuit that extracts information from a modulated radio frequency current or voltage. The term dates from the first three decades of radio (1888-1918). Unlike modern radio stations which transmit sound on an uninterrupted carrier wave, early radio stations transmitted information by radiotelegraphy. The transmitter was switched on and off to produce long or short periods of radio waves, spelling out text messages in Morse code. Therefore, early radio receivers had only to distinguish between the presence or absence of a radio signal. The device that performed this function in the receiver circuit was called a detector. A variety of different detector devices, such as the coherer, electrolytic detector, magnetic detector and the crystal detector, were used during the wireless telegraphy era until superseded by vacuum tube technology.
Most high power transmitter amplifiers are of valve construction because of the high power required.
A valve RF amplifier or tube amplifier (U.S.) is a device for electrically amplifying the power of an electrical radio frequency signal.
In electronics, a plate detector is a vacuum tube circuit in which an amplifying tube having a control grid is operated in a non-linear region of its grid voltage versus plate current transfer characteristic, usually near plate current cutoff, to demodulate amplitude modulated carrier signal. This differs from the grid leak detector, which utilizes the non-linearity of the grid voltage versus grid current characteristic for demodulation. It also differs from the diode detector, which is a two-terminal device.