FM Detector

DETECTOR OF FM SIGNAL

Abstract: The main purpose of this assignment to find out detail about detector of FM signal. There are four major types of FM detectors. The Foster-Seeley Discriminator along with its variation, the Ratio Detector, are obsolescent but still very commonly found in older receivers and new receivers built to older design. More modern types of demodulators include the Quadrature Detector and the Phase Locked-Loop (PLL).

1.0 Introduction

The detector, also called the demodulator, is the part of the receiver that recovers the baseband signal. It performs the inverse operation to the transmitter modulator. The circuitry use for this varies, of course, with the modulation scheme. The FM demodulator must convert frequency variations of the input signal into amplitude variations at the output. The amplitude of the output must be proportional to the frequency deviation of the input.

The S-curve explains the characteristic for many FM detectors. The output voltage proportional to frequency deviation over a range at least equal to 2δ, since the deviation is the distance the signal frequency moves above and below the carrier frequency. This type of curve explains why the tuning is relatively critical with FM receivers, once the detector begin to operate in the non-linear portion of the S-curve, severe distortion results. The sensitivity of a detector is the slope of the straight line portion of the S-curve. It can be seen that the frequency variations of the signal are converted into voltage variations which can be amplified by an audio amplifier before being passed into headphones, a loudspeaker, or passed into other electronic circuitry for the appropriate processing.

Figure 1: S-curve characteristic of FM detectors

2.0 Contents

There are two techniques can be use in FM demodulation which are direct and indirect. Slope detector or discriminator is a part of direct method. In hardly practical system the slope detector is perhaps the easiest type of FM detector to understand. This technique is required to convert FM signal to AM

signal and then by using demodulation circuit is to get back the information signal.

Figure2:Block diagram of the slope detection circuit.

There are three major types of indirect FM detectors. The Foster-Seeley discriminator, along with variations, the ratio detector are obsolescent but still very commonly found in older receivers and new receivers built to older designs. These detectors are effective but require a good number of discrete components, including a specially designed transformer, and must be adjusted by hand. More modern type of demodulators includes the quadrature detector and the phase locked-loop (PLL).

2.1 Foster-Seeley Discriminator

The Foster-Seeley Discriminator is widely used FM detector. The detector consists of a special center-tapped transformer feeding two diodes in full wave DC rectifier circuit. These circuit convert frequency changes first to phase shifts and then to amplitude variation. The conversion from frequency to phase modulation is achieved by using the fact that the phase angle between voltage and current in tuned circuit changes as the applied frequency goes through resonant. Recall that a series tuned circuit is capacitive for frequencies below resonance, resistive at resonant, and inductive above resonant.

The transformer is doubled tuned, with both primary (L1-C1) and secondary (L2-C2) circuits resonant at the carrier frequency.

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FM Detector

FIGURE 2: Foster-Seeley Discriminator

Figure shows the primary and secondary voltages add. The vector sum of the primary voltage and one half the secondary voltage is applied to one diode detector (consisting of D1 and C4), and the vector difference between the primary voltage and one half the secondary voltage goes to the other detector (D2 and C5). The output will be the difference between the rectified and filtered voltages at the output of the two detectors, so it will be the difference between the amplitudes of the two phasors representing the voltages applied to the two detectors.

When the incoming signal frequency is equal to the resonant frequency of the tuned circuits, the voltages applied to the two detectors are equal in magnitude. This can be seen by observing that the vectors V4 and V5 are the same length. Therefore, the net output voltage is zero.

V4

V2

If

If frequency increases, the secondary voltages have a leading phase angle, and the relative lengths of these vectors change. Vector V4 is now greater than V5, as shown in figure. The output voltage becomes positive. Reducing the frequency below resonance causes a similar but opposite phase

change, causing V5 to be less than V4 and producing a negative output voltage.

V4

V2

V1 V5 V3

Input frequency > fc

A change in primary voltage amplitude (due to noise, for example) will also cause a change in output voltage. This undesirable effect can be reduced by using limiters before the detector. However, detectors that are less sensitive to amplitude variations would increase the effectiveness of the limiters.

2.2 Ratio Detector

The ratio detector is a variant of the Foster-Seeley discriminator, but one diode conducts in an opposite direction. It greatly reduces sensitivity to amplitude variations, at the cost of a 50 percent reduction in output voltage.

V4 V2

V1

V5

V3

2

V3V5

Input frequency= fc

Input frequency < fc

FM Detector

FIGURE 3: Ratio Detector

Its circuit is similar to that of the discriminator but can be recognized at a glance by the fact that one of the diodes has been reversed, so that the two outputs across C4 and C5 add rather than subtract. Rather than going to the output, the sum of the two voltages is applied to an additional capacitor shown as C6 in the diagram. This capacitor has a long discharge time constant in combination with R1 and R2 provides a reference level. Its voltage changes only with long-term variations in signal strength and not with brief amplitude variations, such as those due to noise. As the vectors change in the way described above, the ratio between the two voltages produced by the diode detectors across C4 and C5 changes, but sum is fixed by the capacitor voltage.

The output of this circuit cannot be taken between the two diodes as it is for the discriminator, since this voltage does not vary with modulation. Instead, it is taken between the center point of the load resistor and the junction of C4 and C5. The output voltage will be one-half of the difference between the voltages across C4 and C5.

Both the ratio and Foster-Seeley detectors are expensive to manufacture. Wound components like coils are not easy to produce to the required specification and therefore they are comparatively costly. Accordingly these circuits are rarely used in modern equipment.

2.3 Quadrature Detector

Compare angle A and B FM signal

Tuned Circuit Low-pass filterFIGURE 4: Quadrature Detector

Another form of FM detector or demodulator that can be these days is called the quadrature detector. It has the advantage over the ratio and Foster-Seeley detectors that it only requires a simple tuned circuit. So, it’s less expensive than others. In quadrature detector, in incoming signal is applied to one input of a phase detector. Quadrature FM detectors use a high-reactance capacitor (C1) 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 (L1 and C2). Therefore, it causes no phase shift at the carrier frequency but does cause a phase shift at other frequency that adds to or subtracts from the basic 90 degree shifted caused by C1.

The output of the phase-shift network is applied to the second input of the phase detector. When the input frequencies changes, the angle of phase shift in the quadrature circuit also varies, as the resonant circuit becomes inductive or capacitive. Low pass filtering the output recovers the modulation. As is usual with detector low-pass filters, the cutoff frequency should be well above the highest modulating frequency and well below the receiver intermediate frequency. The phase detector can be an analog multiplier or a digital gate (either an AND or an exclusive-OR gate).

The detector is able to operate with relatively low input levels, typically down to levels of around 100 microvolt and it is very easy to set up requiring only the phase shift network to be tuned to the centre frequency of the expected signal. It also provides good linearity enabling very low levels of distortion to be achieved.

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FM Detector

2.4 Phase Lock Loop (PLL)

FIGURE 5: Phase Locked-Loop (PLL)

Analog PLLs are generally built of a phase detector, low pass filter and voltage-controlled oscillator (VCO) placed in a negative feedback closed-loop configuration There may be a frequency divider in the feedback path or in the reference path, or both, in order to make the PLL's output signal frequency an integer multiple of the reference. A non integer multiple of the reference frequency can be created by replacing the simple divide-by-N counter in the feedback path

The incoming FM signal is used to control the frequency of the VCO. As incoming frequency varies the PLL generates a control voltage to change the VCO frequency. This control voltage varies at the same rate as the frequency of the incoming signal. So it can be used directly as the output of the circuit. PLL must have a short time constant so that it can follow the modulation. Free-running frequency of the VCO is set equal to the signal’s carries frequency at the detector. The lock range must be at least twice the maximum deviation of the signal.

It is found that the linearity of this type of detector is governed by the voltage to frequency characteristic of the VCO. As it normally only swings over a small portion of its bandwidth, and the characteristic can be made relatively linear, the distortion levels from phase locked loop demodulators are normally very low. The use of a PLL to demodulate FM signals is very straights forward. PLL is the best among the other types of detector.

3.0 Applications

Believe it or not, an oscillator is an FM detector.  If an FM signal is coupled into the tuned circuit of an oscillator, there will be an additive effect

between the FM signal and the oscillator's signal when the two match exactly in frequency.  This additive effect will show up as slightly stronger oscillator signal amplitude.  As the FM signal swings away from this perfectly matching frequency, the additive effect will diminish.  Just like with slope detection, the amplitude variations can be used to create an audible signal.

In a video disc player, color picture information recorded in FM carrier form is recovered from the output of player's signal pickup apparatus by an FM demodulator. It for developing during playback of a video disc record an FM signal.

For Quadrature Detector it is suitable to use in audio detector for television. The need for receivers that can be used for many different modulation types, in cellular radio, and personal communications systems, for instance, has resulted in great interest in a receiver in which many of the functions are provided in software running on specialized fast microprocessors called digital signal processors (DSP).

4.0 Conclusion

The following summary will refresh memory of demodulation, its basic principles, and typical circuitry required to accomplish this task. Demodulation also called detection is the process of recreating original modulating frequencies is restored. From the types of FM detector it can be summarize that Phase Locked-Loop (PLL) and Quadrature Detector is the best types of FM detector.

Acknowledge

We would like to thank our beloved lecturer, PM Dr. Abu Sahmah bin Mohd Supaat and our colleagues at UTM on their co-operation and guide to contribute in our report for principles of communication (SEE 3533).

References1. Roy Blake, “ Electronic Communication

System”, 2nd Ed., Delmar, pg 245-249, 2002.2. http://en.wikipedia.org/wiki/phase-locked_loop ,

15 March 2008.3. http://www.radio-electronics.com/info/

receivers/fm_demodulation.php , 16 March 2008.

4. http://www.r-type.org/static/add059.htm , 16 March 2008.

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Phase frequency detector

Low-pass filter

Bias generator

Voltage controlled oscillator

Output converter

÷ N

VCO