The following summary will refresh your memory of demodulation, its basic principles, and typical circuitry required to accomplish this task.
DEMODULATION, also called DETECTION, is the process of re-creating original modulating frequencies (intelligence) from radio frequencies. The DEMODULATOR, or
DETECTOR, is the circuit in which the original modulating frequencies are restored. A CW DEMODULATOR is a circuit that is capable of detecting the presence of rf energy.
HETERODYNE DETECTION uses a locally generated frequency to beat with the cw carrier frequency to provide an audio output.
The REGENERATIVE DETECTOR produces its own oscillations, heterodynes them with anincoming signal, and detects them
The SERIES- (VOLTAGE-) DIODE DETECTOR has a rectifier diode that is in series with theinput voltage and the load impedance
SHUNT- (CURRENT-) DIODE DETECTOR is characterized by a rectifier diode in parallel with the input and load impedance
The COMMON-EMITTER DETECTOR is usually used in receivers to supply a detected and amplified output
The COMMON-BASE DETECTOR is an amplifying detector that is used in portable receivers.
The SLOPE DETECTOR is the simplest form of frequency detector. It is essentially a tank circuit tuned slightly away from the desired fm carrier.
The FOSTER-SEELEY DISCRIMINATOR uses a double tuned rf transformer to convert frequency changes of the received fm signal into amplitude variations of the rf wave.
The RATIO DETECTOR uses a double-tuned transformer connected so that the instantaneous frequency variations of the fm input signal are converted into instantaneous amplitude variations
The GATED-BEAM DETECTOR uses a specially-designed tube to limit, detect, and amplify the received fm signal.
PHASE DEMODULATION may be accomplished using a frequency discriminator or a quadrature detector.
PEAK DETECTION uses the amplitude, or duration, of a pulse to charge a holding capacitor and restore the modulating waveform.
A LOW-PASS FILTER is used to demodulate pdm by averaging the pulse amplitude over the entire period between pulses.
PULSE CONVERSION is used to convert ppm, pdm, or pcm to pdm or pam for demodulation.
Amplitude modulation detectors
One major technique is known as envelope detection. The simplest form of envelope detector is thediode detector that consists of a diode connected between the input and output of the 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.
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.
Frequency and phase modulation detectors
AM detectors cannot demodulate FM and PM signals because both have a constant amplitude. 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.
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 and the phase or frequency modulated signal is applied to the other. The output is a signal that is proportional to the phase difference between the two inputs.
In phase demodulation the information is contained in the amount and rate of phase shift in the carrier wave.
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.
The ratio detectoris a variant of the Foster-Seeley discriminator, but one diode conducts in an opposite direction. 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 distinct from the Foster-Seeley discriminator, the ratio detector will similarly not respond to AM signals, however the output is only 50% of the output of a discriminator for the same input signal.
In quadrature detectors, the received FM signal is split into two signals. One of the two signals is then passed through a high-reactance capacitor, which shifts the phase of that signal by 90 degrees. This phase-shifted signal is then applied to an LC circuit, which is resonant at the FM signal's unmodulated, "center," or "carrier" frequency. If the received FM signal's frequency equals the center frequency, then the two signals will have a 90-degree phase difference and they are said to be in "phase quadrature" — hence the name of this method. The two signals are then multiplied together in an analog or digital device, which serves as a phase detector; that is, a device whose output is proportional to the phase difference between two signals. In the case of an unmodulated FM signal, the phase detector's output is — after the output has been filtered; that is, averaged over time — constant; namely, zero. However, if the received FM signal has been modulated, then its frequency will vary from the center frequency. In this case, the resonant LC circuit will further shift the phase of the signal from the capacitor, so that the signal's total phase shift will be the sum of the 90 degrees that's imposed by the capacitor and the positive or negative phase change that's imposed by the LC circuit. Now the output from the phase detector will differ from zero, and in this way, one recovers the original signal that was used to modulate the FM carrier.
This detection process can also be accomplished by combining, in an exclusive-OR (XOR) logic gate, the original FM signal and a square wavewhose frequency equals the FM signal's center frequency. The XOR gate produces an output pulse whose duration equals the difference between the times at which the square wave and the received FM signal pass through zero volts. As the FM signal's frequency varies from its unmodulated center frequency (which is also the frequency of the square wave), the output pulses from the XOR gate become longer or shorter. (In essence, this quadrature detector converts an FM signal into a pulse-width modulated (PWM) signal.) When these pulses are filtered, the filter's output rises as the pulses grow longer and its output falls as the pulses grow shorter. In this way, one recovers the original signal that was used to modulate the FM carrier.
Other FM detectors
Less common, specialized, or obsolescent types of detectors include :
- Travis or double tuned circuit discriminator using two non-interacting tuned circuits above and below the nominal center frequency
- Weiss discriminator which uses a single LC tuned circuit or crystal
- Pulse count discriminator which converts the frequency to a train of constant amplitude pulses, producing a voltage directly proportional to the frequency