Thursday, February 18, 2010

FM Demodulation Theory_Detection Theory

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.


3-12Figure 3-9C.—Slope detector. DIODE DETECTOR.Q-21.What is the simplest form of fm detector?Q-22.What is the function of an fm detector?FOSTER-SEELEY DISCRIMINATORThe FOSTER-SEELEY DISCRIMINATOR is also known as the PHASE-SHIFTDISCRIMINATOR. It uses a double-tuned rf transformer to convert frequency variations in the receivedfm signal to amplitude variations. These amplitude variations are then rectified and filtered to provide adc output voltage. This voltage varies in both amplitude and polarity as the input signal varies infrequency. A typical discriminator response curve is shown in figure 3-10. The output voltage is 0 whenthe input frequency is equal to the carrier frequency (fr). When the input frequency rises above the centerfrequency, the output increases in the positive direction. When the input frequency drops below the centerfrequency, the output increases in the negative direction.Figure 3-10.—Discriminator response curve.The output of the Foster-Seeley discriminator is affected not only by the input frequency, but also toa certain extent by the input amplitude. Therefore, using limiter stages before the detector is necessary.Circuit Operation of a Foster-Seeley DiscriminatorView (A) of figure 3-11 shows a typical Foster-Seeley discriminator. The collector circuit of thepreceding limiter/amplifier circuit (Q1) is shown. The limiter/amplifier circuit is a special amplifiercircuit which limits the amplitude of the signal. This limiting keeps interfering noise low by removing

3-13excessive amplitude variations from signals. The collector circuit tank consists of C1 and L1. C2 and L2form the secondary tank circuit. Both tank circuits are tuned to the center frequency of the incoming fmsignal. Choke L3 is the dc return path for diode rectifiers CR1 and CR2. R1 and R2 are not alwaysnecessary but are usually used when the back (reverse bias) resistance of the two diodes is different.Resistors R3 and R4 are the load resistors and are bypassed by C3 and C4 to remove rf. C5 is the outputcoupling capacitor.Figure 3-11.—Foster-Seeley discriminator. FOSTER-SEELEY DISCRIMINATOR.CIRCUIT OPERATION AT RESONANCE.—The operation of the Foster-Seeley discriminatorcan best be explained using vector diagrams [figure 3-11, view (B)] that show phase relationshipsbetween the voltages and currents in the circuit. Let's look at the phase relationships when the inputfrequency is equal to the center frequency of the resonant tank circuit.The input signal applied to the primary tank circuit is shown as vector ep. Since coupling capacitorC8 has negligible reactance at the input frequency, rf choke L3 is effectively in parallel with the primarytank circuit. Also, because L3 is effectively in parallel with the primary tank circuit, input voltage ep alsoappears across L3. With voltage ep applied to the primary of T1, a voltage is induced in the secondarywhich causes current to flow in the secondary tank circuit. When the input frequency is equal to the centerfrequency, the tank is at resonance and acts resistive. Current and voltage are in phase in a resistancecircuit, as shown by is and ep. The current flowing in the tank causes voltage drops across each half of thebalanced secondary winding of transformer T1. These voltage drops are of equal amplitude and opposite
polarity with respect to the center tap of the winding. Because the winding is inductive, the voltage acrossit is 90 degrees out of phase with the current through it. Because of the center-tap arrangement, thevoltages at each end of the secondary winding of T1 are 180 degrees out of phase and are shown as e1 ande2 on the vector diagram.The voltage applied to the anode of CR1 is the vector sum of voltages ep and e1, shown as e3on thediagram. Likewise, the voltage applied to the anode of CR2 is the vector sum of voltages ep and e2,shown as e4 on the diagram. At resonance e3 and e4 are equal, as shown by vectors of the same length.Equal anode voltages on diodes CR1 and CR2 produce equal currents and, with equal load resistors, equaland opposite voltages will be developed across R3 and R4. The output is taken across R3 and R4 and willbe 0 at resonance since these voltages are equal and of appositive polarity.The diodes conduct on opposite half cycles of the input waveform and produce a series of dc pulsesat the rf rate. This rf ripple is filtered out by capacitors C3 and C4.OPERATION ABOVE RESONANCE.—A phase shift occurs when an input frequency higherthan the center frequency is applied to the discriminator circuit and the current and voltage phaserelationships change. When a series-tuned circuit operates at a frequency above resonance, the inductivereactance of the coil increases and the capacitive reactance of the capacitor decreases. Above resonancethe tank circuit acts like an inductor. Secondary current lags the primary tank voltage, ep. Notice thatsecondary voltages e1and e2 are still 180 degrees out of phase with the current (iS) that produces them.The change to a lagging secondary current rotates the vectors in a clockwise direction. This causes el tobecome more in phase with ep while e2 is shifted further out of phase with ep. The vector sum of ep and e2is less than that of ep and e1. Above the center frequency, diode CR1 conducts more than diode CR2.Because of this heavier conduction, the voltage developed across R3 is greater than the voltage developedacross R4; the output voltage is positive.OPERATION BELOW RESONANCE.—When the input frequency is lower than the centerfrequency, the current and voltage phase relationships change. When the tuned circuit is operated at afrequency lower than resonance, the capacitive reactance increases and the inductive reactance decreases.Below resonance the tank acts like a capacitor and the secondary current leads primary tank voltage ep.This change to a leading secondary current rotates the vectors in a counterclockwise direction. From thevector diagram you should see that e2 is brought nearer in phase with ep, while el is shifted further out ofphase with ep. The vector sum of ep and e2 is larger than that of eand e1. Diode CR2 conducts more thandiode CR1 below the center frequency. The voltage drop across R4 is larger than that across R3 and theoutput across both is negative.DisadvantagesThese voltage outputs can be plotted to show the response curve of the discriminator discussedearlier (figure 3-10). When weak AM signals (too small in amplitude to reach the circuit limiting level)pass through the limiter stages, they can appear in the output. These unwanted amplitude variations willcause primary voltage ep [view (A) of figure 3-11] to fluctuate with the modulation and to induce asimilar voltage in the secondary of T1. Since the diodes are connected as half-wave rectifiers, these smallAM signals will be detected as they would be in a diode detector and will appear in the output. Thisunwanted AM interference is cancelled out in the ratio detector (to be studied next in this chapter) and isthe main disadvantage of the Foster-Seeley circuit.Q-23.What type of tank circuit is used in the Foster-Seeley discriminator?Q-24.What is the purpose of CR1 and CR2 in the Foster-Seeley discriminator?Q-25.What type of impedance does the tank circuit have above resonance?
3-15RATIO DETECTORThe RATIO DETECTOR uses a double-tuned transformer to convert the instantaneous frequencyvariations of the fm input signal to instantaneous amplitude variations. These amplitude variations arethen rectified to provide a dc output voltage which varies in amplitude and polarity with the input signalfrequency. This detector demodulates fm signals and suppresses amplitude noise without the need oflimiter stages.Circuit OperationFigure 3-12 shows a typical ratio detector. The input tank capacitor (C1) and the primary oftransformer T1 (L1) are tuned to the center frequency of the fm signal to be demodulated. The secondarywinding of T1 (L2) and capacitor C2 also form a tank circuit tuned to the center frequency. Tertiary(third) winding L3 provides additional inductive coupling which reduces the loading effect of thesecondary on the primary circuit. Diodes CR1 and CR2 rectify the signal from the secondary tank.Capacitor C5 and resistors R1 and R2 set the operating level of the detector. Capacitors C3 and C4determine the amplitude and polarity of the output. Resistor R3 limits the peak diode current andfurnishes a dc return path for the rectified signal. The output of the detector is taken from the commonconnection between C3 and C4. Resistor RL is the load resistor. R5, C6, and C7 form a low-pass filter tothe output.Figure 3-12.—Ratio detector.This circuit operates on the same principles of phase shifting as did the Foster-Seeley discriminator.In that discussion, vector diagrams were used to illustrate the voltage amplitudes and polarities forconditions at resonance, above resonance, and below resonance. The same vector diagrams apply to theratio detector but will not be discussed here. Instead, you will study the resulting current flows andpolarities on simplified schematic diagrams of the detector circuit.OPERATION AT RESONANCE.—When the input voltage ep is applied to the primary in figure3-12 it also appears across L3 because, by inductive coupling, it is effectively connected in parallel withthe primary tank circuit. At the same time, a voltage is induced in the secondary winding and causescurrent to flow around the secondary tank circuit. At resonance the tank acts like a resistive circuit; that is,
3-16the tank current is in phase with the primary voltage ep. The current flowing in the tank circuit causesvoltages e1 and e2 to be developed in the secondary winding of T1. These voltages are of equal magnitudeand of opposite polarity with respect to the center tap of the winding. Since the winding is inductive, thevoltage drop across it is 90 degrees out of phase with the current through it.Figure 3-13 is a simplified schematic diagram of a ratio detector at resonance. The voltage applied tothe cathode of CR1 is the vector sum of e1 and ep. Likewise, the voltage applied to the anode of CR2 isthe vector sum of e2 and ep. No phase shift occurs at resonance and both voltages are equal. Both diodesconduct equally. This equal current flow causes the same voltage drop across both R1 and R2. C3 and C4will charge to equal voltages with opposite polarities. Let’s assume that the voltages across C3 and C4 areequal in amplitude (5 volts) and of opposite polarity and the total charge across C5 is 10 volts. R1 and R2will each have 5 volts dropped across them because they are of equal values. The output is taken betweenpoints A and B. To find the output voltage, you algebraically add the voltages between points A and B(loop ACB or ADB). Point A to point D is -5 volts. Point D to point B is + 5 volts. Their algebraic sum is0 volts and the output voltage is 0 at resonance. If the voltages on branch ACB were figured, the sameoutput would be found because the circuit branches are in parallel.Figure 3-13.—Current flow and polarities at resonance.When the input signal reverses polarity, the secondary voltage across L2 also reverses. The diodeswill be reverse biased and no current will flow. Meanwhile, C5 retains most of its charge because of thelong time constant offered in combination with R1 and R2. This slow discharge helps to maintain theoutput.OPERATION ABOVE RESONANCE.—When a tuned circuit (figure 3-14) operates at afrequency higher than resonance, the tank is inductive. The secondary current ilags the primary voltageep. Secondary voltage e1 is nearer in phase with primary voltage e, while e2 is shifted further out of phasewith ep. The vector sum of e1 and ep is larger than that of e2 and ep. Therefore, the voltage applied to thecathode of CR1 is greater than the voltage applied to the anode of CR2 above resonance.Figure 3-14.—Current flow and polarities above resonance.
3-17Assume that the voltages developed above resonance are such that the higher voltage on the cathodeof CR1 causes C3 to charge to 8 volts. The lower voltage on the anode of CR2 causes C4 to charge to 2volts. Capacitor C5 remains charged to the sum of these two voltages, 10 volts. Again, by adding thevoltages in loop ACB or ADB between points A and B, you can find the output voltage. Point A to pointD equals -2 volts. Point D to point B equals +5 volts. Their algebraic sum, and the output, equals +3 voltswhen tuned above resonance. During the negative half cycle of the input signal, the diodes are reversebiased and C5 helps maintain a constant output.OPERATION BELOW RESONANCE.—When a tuned circuit operates below resonance (figure3-15), it is capacitive. Secondary current is leads the primary voltage ep and secondary voltage e2 is nearerin phase with primary voltage ep. The vector sum of e2 and ep is larger than the sum of e1 and ep. Thevoltage applied to the anode of CR2 becomes greater than the voltage applied to the cathode of CR1below resonance.Figure 3-15.—Current flow and polarities below resonance.Assume that the voltages developed below resonance are such that the higher voltage on the anode ofCR2 causes C4 to charge to 8 volts. The lower voltage on the cathode of CR1 causes C3 to charge to 2volts. Capacitor C5 remains charged to the sum of these two voltages, 10 volts. The output voltage equals-8 volts plus +5 volts, or -3 volts, when tuned below resonance. During the negative half cycle of theinput signal, the diodes are reverse biased and C5 helps maintain a constant output.Advantage of a Ratio DetectorThe ratio detector is not affected by amplitude variations on the fm wave. The output of the detectoradjusts itself automatically to the average amplitude of the input signal. C5 charges to the sum of thevoltages across R1 and R2 and, because of its time constant, tends to filter out any noise impulses. BeforeC5 can charge or discharge to the higher or lower potential, the noise disappears. The difference in chargeacross C5 is so slight that it is not discernible in the output. Ratio detectors can operate with as little as100 millivolts of input. This is much lower than that required for limiter saturation and less gain isrequired from preceding stages.Q-26.What is the primary advantage of a ratio detector?Q-27.What is the purpose of C5 in figure 3-12?GATED-BEAM DETECTORAn fm demodulator employing a completely different detection principle is the GATED-BEAMDETECTOR (sometimes referred to as the QUADRATURE DETECTOR). A simplified diagram of a



Amplitude modulation detectors

[edit]Envelope detector

A simple envelope detector

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.

An early form of envelope detector was the cat's whisker, which was used in the crystal set radio receiver.

[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]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.

[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 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.

[edit]The Foster-Seeley discriminator

The Foster-Seeley discriminator[1][2] 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

A ratio detector using solid-state diodes

The ratio detector[3][4][5][6]is 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.

[edit]Quadrature detector

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.

[edit]Other FM detectors

Less common, specialized, or obsolescent types of detectors include [7]:

  • Travis[8] 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


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