Detector (radio)
From Wikipedia, the free encyclopedia
A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse Code, and it was only necessary to "Detect" the presence (or absence) of a radio wave using a device such as a coherer; not necessarily making it audible.
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[edit] Amplitude modulation detectors
[edit] Envelope detector
One major technique is known as envelope detection. The simplest form of envelope detector is the diode detector. A diode detector consists of a diode connected between the input and output in a circuit, with a resistor and capacitor in parallel from the output of the circuit to the ground. If the resistor and capacitor are correctly chosen, the output of this circuit will approximate a voltage-shifted version of the original signal.
[edit] Microwave (crystal) detectors
[edit] Product detector
A product detector is a type of demodulator used for AM and SSB signals. Rather than converting the envelope of the signal into the decoded waveform like an envelope detector, the product detector takes the product of the modulated signal and a local oscillator, hence the name. This can be accomplished by heterodyning. The received signal is mixed, in some type of nonlinear device, with a signal from the local oscillator, to produce an intermediate frequency, referred to as the beat frequency, from which the modulating signal is detected and recovered.
[edit] FM and PM detectors
Because FM and PM signals have constant amplitude they are incompatible with AM detectors. However an AM radio may detect the sound of an FM broadcast by the phenomenon of Slope Detection which occurs when the radio is tuned slightly above or below the nominal broadcast frequency. Frequency variation on one sloping side of the radio tuning curve gives the amplified signal a corresponding local amplitude variation, to which the AM detector is sensitive. Slope Detection gives inferior distortion and noise rejection compared to the following dedicated FM detectors that are normally used.
[edit] Phase detector
A phase detector is a nonlinear device whose output represents the phase difference between the two oscillating input signals. It has two inputs and one output: a reference signal is applied to one input. The phase or frequency modulated signal is applied to the other and the output is a signal that is proportional to the difference between the two inputs.
In phase demodulation the information is contained in the amount and rate of phase shift in the carrier wave.
[edit] The Foster-Seeley discriminator
The Foster-Seeley discriminator is a widely used FM detector. The detector consists of a special center-tapped transformer feeding two diodes in a full wave DC rectifier circuit. When the input transformer is tuned to the signal frequency, the output of the discriminator is zero when there is no deviation of the carrier; both halves of the center tapped transformer are balanced. As the FM signal swings in frequency above and below the carrier frequency, the balance between the two halves of the center-tapped secondary are destroyed and there is an output voltage proportional to the frequency deviation.
[edit] Ratio detector
The ratio detector is a variant of the Foster-Seely discriminator, but, the diodes conduct in opposite directions. 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, which eliminates AM noise in the ratio detector output. While unlike the Foster-Seely discriminator, the ratio detector will not respond to AM signals, however the output is only 50% of the output of a discriminator for the same input signal.
[edit] Quadrature detector
Quadrature detectors use a high-reactance capacitor to produce two signals with a 90 degree phase difference. The phase-shifted signal is then applied to an LC-tuned resonant at the carrier frequency. If the frequency changes the phase will also vary and so will the output voltage.
This can also be accomplished by combining the signal and a square wave at the carrier frequency in an XOR gate. The square wave is thus phase shifted 90 degrees with respect to the input carrier modulation. The time interval between the zero crossing of the square wave and the FM signal will depend on the instantaneous frequency, which makes the gate output a pulse whose width depends on the time interval. In essence, the quadrature detector converts an FM signal to a PWM (pulse width modulation) signal. The original audio signal is recovered by passing the PWM resultant through a low-pass filter.
[edit] Phase Locked Loop detector
The phase locked loop detector requires no frequency-selective LC network to accomplish demodulation. In this system, a voltage controlled oscillator (VCO) is phase locked by a feedback loop into following the deviation of the incoming FM signal. The low frequency error voltage that forces the VCO to track is the demodulated output.
[edit] Level detector
One form of level detector is a comparator circuit that compares one inputs with a preset level and provides a DC output indicating when the input signal exceeds this threshold.
Other forms of level detector rectify the input signal and give a DC output proportional the peak or RMS level of the input signal.
