Amplitude Demodulator | Diode | Crystal Radio | Synchronous Detection

What is Amplitude Demodulator

Amplitude Demodulator, or detectors, are circuits that accept modulated signals and recover the original modulating information. The Amplitude demodulator circuit is the key circuit in any radio receiver. In fact, Amplitude demodulator circuits can be used alone as simple radio receivers.

Diode Detectors

The simplest and most widely used amplitude demodulator is the diode detector (see Fig. 4-15). As shown, the AM signal is usually transformer-coupled and applied to a basic half wave rectifier circuit consisting of D1 and R1. The diode conducts when the positive half-cycles of the AM signals occur. During the negative half-cycles, the diode is reverse-biased and no current flows through it. As a result, the voltage across R1 is a series of positive pulses whose amplitude varies with the modulating signal. A capacitor C1 is connected across resistor R1, effectively filtering out the carrier and thus recovering the original modulating signal. One way to look at the operation of a diode detector is to analyze its operation in the time domain. The waveforms in Fig. 4-16 illustrate this. On each positive alternation of the AM signal, the capacitor charges quickly to the peak value of the pulses passed

Amplitude Demodulator
Amplitude Demodulator

by the diode. When the pulse voltage drops to zero, the capacitor discharges into resistor R1. The time constant of C1 and R1 is chosen to be long compared to the period of the carrier. As a result, the capacitor discharges only slightly during the time that the diode is not conducting. When the next pulse comes along, the capacitor again charges to its peak value. When the diode cuts off, the capacitor again discharges a small amount into the resistor. The resulting waveform across the capacitor is a close approximation to the original modulating signal. Because the capacitor charges and discharges, the recovered signal has a small amount of ripple on it, causing distortion of the modulating signal. However, because the carrier frequency is usually many times higher than the modulating frequency, these ripple variations are barely noticeable. Because the diode detector recovers the envelope of the signal, which is the original modulating signal, the circuit is sometimes referred to as an envelope detector. Distortion of the original signal can occur if the time constant of the load resistor R1 and the shunt filter capacitor C1 is too long or too short. If the time constant is too long, the capacitor discharge will be too slow to follow the faster changes in the modulating signal. This is referred to as diagonal distortion. If the time constant is too short, the capacitor will discharge too fast and the carrier will not be sufficiently filtered out.

Amplitude Demodulator

The dc component in the output is removed with a series coupling or blocking capacitor, C2 in Fig. 4-15, which is connected to an amplifier. Another way to view the operation of the diode detector is in the frequency domain. In this case, the diode is regarded as a nonlinear device to which are applied multiple signals where modulation will take place. The multiple signals are the carrier and sidebands, which make up the input AM signal to be Amplitude demodulated. The components of the AM signal are the carrier fc, the upper sideband fc + fm, and the lower sideband fc – fm. The diode detector circuit combines these signals, creating the sum and difference signals:

All these components appear in the output. Since the carrier frequency is very much higher than that of the modulating signal, the carrier signal can easily be filtered out with a simple low-pass filter. In a diode detector, this low-pass filter is just capacitor C1 across load resistor R1. Removing the carrier leaves only the original modulating signal. The frequency spectrum of a diode detector is illustrated in Fig. 4-17. The low-pass filter, C1 in Fig. 4-15, removes all but the desired original modulating signal.

Crystal Radio Receivers

The crystal component of the crystal radio receivers that were widely used in the past is simply a diode. In Fig. 4-18 the diode detector circuit of Fig. 4-15 is redrawn, showing an antenna connection and headphones. A long wire antenna picks up the radio signal, which is inductively coupled to the secondary winding of T1, which forms a series resonant circuit with C1. Note that the secondary is not a parallel circuit, because the voltage induced into the secondary winding appears as a voltage source in series with the coil and capacitor. The variable capacitor C1 is used to select a station. At resonance, the voltage across the capacitor is stepped up by a factor equal to the Q of the tuned circuit. This resonant voltage rise is a form of amplification. This higher-voltage signal is applied to the diode.

Amplitude Demodulator

The diode detector D1 and its fi lter C2 recover the original modulating information, which causes current fl ow in the headphones. The headphones serve as the load resistance, and capacitor C2 removes the carrier. The result is a simple radio receiver; reception is very weak because no active amplification is provided. Typically, a germanium diode is used because its voltage threshold is lower than that of a silicon diode and permits reception of weaker signals. Crystal radio receivers can easily be built to receive standard AM broadcasts.

Synchronous Detection

Synchronous detectors use an internal clock signal at the carrier frequency in the receiver to switch the AM signal off and on, producing rectification similar to that in a standard diode detector (see Fig. 4-19.) The AM signal is applied to a series switch that is opened and closed synchronously with the carrier signal. The switch is usually a diode or transistor that is turned on or off by an internally generated clock signal equal in frequency to and in phase with the carrier frequency. The switch in Fig. 4-19 is turned on by the clock signal during the positive half-cycles of the AM signal, which therefore appears across the load resistor. During the negative half-cycles of the AM signal, the clock turns the switch off, so no signal reaches the load or filter capacitor. The capacitor filters out the carrier. A full wave synchronous detector is shown in Fig. 4-20. The AM signal is applied to both inverting and noninverting amplifiers. The internally generated carrier signal operates two switches A and B. The clock turns A on and B off or turns B on and A off. This arrangement simulates an electronic single-pole, double-throw (SPDT) switch. During positive half-cycles of the AM signal, the A switch feeds the noninverted AM output of positive half-cycles to the load. During the negative half-cycles of the input, the B switch connects the output of the inverter to the load. The negative half-cycles are inverted, becoming positive, and the signal appears across the load. The result is full wave rectification of the signal. The key to making the synchronous detector work is to ensure that the signal producing the switching action is perfectly in phase with the received AM carrier. An internally generated carrier signal from, say, an oscillator will not work.

Amplitude Demodulator

Even though the frequency and phase of the switching signal might be close to those of the carrier, they would not be perfectly equal. However, there are a number of techniques, collectively referred to as carrier recovery circuits, that can be used to generate a switching signal that has the correct frequency and phase relationship to the carrier. A practical synchronous detector is shown in Fig. 4-21. A center-tapped transformer provides the two equal but inverted signals. The carrier signal is applied to the center tap. Note that one diode is connected oppositely from the way it would be if used in a full wave rectifier. These diodes are used as switches, which are turned off and on by the clock, which is used as the bias voltage. The carrier is usually a square wave derived by clipping and amplifying the AM signal. When the clock is positive, diode D1 is forward-biased.

It acts as a short circuit and connects the AM signal to the load resistor. Positive half-cycles appear across the load. When the clock goes negative, D2 is forward-biased. During this time, the negative cycles of the AM signal are occurring, which makes the lower output of the secondary winding positive. With D2 conducting, the positive half-cycles are passed to the load, and the circuit performs full wave rectification. As before, the capacitor across the load filters out the carrier, leaving the original modulating signal across the load. The circuit shown in Fig. 4-22 is one way to supply the carrier to the synchronous detector. The AM signal to be demodulated is applied to a highly selective bandpass filter that picks out the carrier and suppresses the sidebands, thus removing most of the amplitude variations. This signal is amplified and applied to a clipper or limiter that removes any remaining amplitude variations from the signal, leaving only the carrier. The clipper circuit typically converts the sine wave carrier into a square wave that is amplified and thus becomes the clock signal. In some synchronous detectors, the clipped carrier is put through another bandpass filter to get rid of the square wave harmonics and generate a pure sine wave carrier. This signal is then amplified and used as the clock. A small phase shifter may be introduced to correct for any phase differences that occur during the carrier recovery process. The resulting carrier signal is exactly the same frequency and phase as those of the original carrier, as it is indeed derived from it. The output of this circuit is applied to the synchronous detector. Some synchronous detectors use a phaselocked loop to generate the clock, which is locked to the incoming carrier. Synchronous detectors are also referred to as coherent detectors, and were known in the past as homodyne detectors. Their main advantage over standard diode detectors is that they have less distortion and a better signal-to-noise ratio. They are also less prone to selective fading, a phenomenon in which distortion is caused by the weakening of a sideband on the carrier during transmission.

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