1.0 INTRODUCTION OF DISCRIMINATORS

The FM discriminator (detector) extracts the intelligence that has been modulated

onto the carrier via frequency variations. It should provide an intelligence signal

whose amplitude is dependent on instantaneous carrier frequency deviation and

whose frequency is dependent on the carrier’s rate of frequency deviation. A

desired output amplitude versus input frequency characteristic for a broadcast FM

discriminator is provide in Fig. 1.1.0. notice that the respone is linear in the allowed

area of frequency deviation and that output amplitude is directly proportional to

carrier frequency deviation. Keep in mind, however, that FM detection takes the

place following the IF amplifiers, which mean that the ±75-kHz deviation is intact but

that carrier frequency translation (usually to 10.7MHz) has occurred.

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FIGURE 1.1.0 FM discriminator characteristic.

Discriminator (detector) is having 4 types.

1. Slope Detector.

2. Foster-Seely Discriminator.

3. Ratio Detector.

4. Quadrature Detector.

For this assignment, I’ll just present only two (2) types of discriminator.

1. Foster-Seely Discriminator.

2. Ratio Detector.

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2.0 FOSTER-SEELY DISCRIMINATOR

2.1 INTRODUCTION

Foster-Seeley Diacriminator is also known as Phase Shift Discriminator. The two

classical means of FM detection are the Foster-Seely discriminator and the ratio

detector. While their once widespread use is now dimishing because of new techniques

afforded by ICs, they remain a popular means of discrimination using a minimum of

circuitry.

2.2 SCHEMATIC DIAGRAM

Figure 2.2.1 Foster-Seely discriminator.

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2.3 OUTPUT DIAGRAM

Figure 2.3.1 Discriminator Phase Relations

2.4 EXPLANATION

A typical Foster-Seely discriminator circuit is shown in Fig. 2.2.1. in it, the two tank

circuits (L1C1 and (L2+L3)C2) are tuned exactly to the carrier frequency. Capacitors Cc,

C4, and C5 are shorts to the carrier frequency. The following analysis applies to an

unmodulated carrier input:

1. The carrier voltage e1 appears directly across L4 because Cc and C4 are shorts

to the carrier frequency.

2. The voltage es across the transformer secondary (L2 in series with L3) is 180˚ out

of phase with e1 by transformer action as shown in Fig. 2.3.1(a). the circulating

L2L3C2 tank current,is, is in phase with es since the tank is resonant.

3. The current is , folloeing through inductance L2L3, produces a voltage drop that

lags is by 90˚. The individual components of this voltage , e2 and e3, are thus

displaced by 90˚ from is, as shown in Fig. 2.3.1(a) and are 180˚ out of phase with

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each other because they are the voltage from the ends of a center-tapped

winding.

4. The voltage e4 applied to the diode D1 , C3 and R1 network will be the vector sum

of e1 and e2 (fig. 2.3.1 (a)). Similarly, the voltage e5 is the sum of e1 and e3. The

magnitude of e6 is propotional to e4 is proportional to e5.

5. The output voltage , e8, is equal to the sum of e6 and e7 and is zero since the

diodes D1 and D2 will be conducting current equally (since e4 = e5) but in opposite

directions through the R1C1 and R2C2 networks.

The discriminator output is zero with no modulaton (zero frequency deviation), as is

desired. The following discussion now considers circuit action at some instant when the

inout signal e1 is above the carrier frequency. The phasor diagram of Fig. 2.3.1 is used

to illustrate this condition.

1. Voltages e1 and es are as before, but es now sees an inductive reactance,

because the tank circuit is above resonance. Therefore, the circulating tank

current, is, lags es.

2. The voltages e2 and e3 must remain 90˚ out of phase with is , as shown in Fig

2.3.1 (b). the new vector sums of e2+e1 and e3+e1 are no longer equal, so e4

causes a heavier conduction of D1 than exists for D2.

3. The output, es, which is the sum of e6 and e7, will go positive since the current

down through R1C3 is greater than the current up through R2C4 (e4 is greater

than e5).

The output for frequencies above resonance (fc) is therefore positive, while the phasor

diagram in Fig. 2.3.1 (c) shows that at frequencies below resonance the output goes

native. The amount of output is determined by the amount of frequency deviation, while

the frequency of the output is determined by the rate at which the FM input signal varies

around its carrier or center value.

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2.5 CONCLUSION

The tank circuit of Foster Seeley Discriminator has a resistive impedance. Above

resonance, the tank circuit of Foster Seeley Discriminator has an inductive impedance.

Lastly is below resonance. The tank circuit of Foster Seeley Discriminator has

capacitive impedance. Foster-Seeley discriminators are sensitive to both frequency and

amplitude variations, unlike some detectors. Therefore a limiter amplifier stage must be

used before the detector, to remove amplitude variations in the signal which would be

detected as noise. The limiter acts as a Class A amplifier at lower amplitudes; at higher

amplitudes it acts like a Class C amplifier, which clips off the peaks.

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3.0 RATIO DETECTOR

3.1 INTRODUCTION

While the Foster-Seely discriminator just described offers excellent linear response

to wideband FM signals, it also responds to any undesired input amplitude

variations. The ratio detector does not respond to amplitude variations and thereby

minimizes the required limiting before detection.

3.2 SCHEMATIC DIAGRAM

Figure 3.2.1 Ratio Detector.

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3.3 OUTPUT DIAGRAM

Figure 3.3.1 output diagram

3.4 EXPLANATION

The ratio detector, shown in Fig. 3.2.1, is a circuit designed to respond only to

frequency changes of the input signal. Amplitude changes in the input have no

effect upon the output. The input circuit of the ratio detector is identical to that of

the Foster-Seely discriminator circuit. The most immediately obvious difference is

the reversal of one of the diodes.

The ratio detector circuit operationis similar to the Foster-seely. A detailed

analysis will therefore not be given. Notice the large eledtrolytic capacitor, C5,

across R1-R2 combination. This maintains a constant voltage that is equal to the

peak voltage across the diode input. This feature eliminates variations in the FM

signal, thus providing amplitude limiting. The sudden changes in the input signal’s

amplitude are suppressed by the large capacitor. The Foster-seely discriminator

does not provide amplitude limiting. The voltage Es is

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Es = e1 +e2

=

=

When fin = fc, e1=e2 and hence the desired zero output occurs. When fin>fc, e1>e2,

and when fin<fc, e1<e2. The desired frequency-dependent output characteristic

results.

The component values shown in Fig. 3.2.1 are typical for a 10.7-MHz IF FM

input signal. The output level of the ratio detector is one-half that for the Foster-

Seely circuit.

3.5 CONCLUSION

The Ratio detector is not affected by the amplitude variations on the fm wave. The

output of the detector adjusts itself automatically to the average amplitude of the

input signal. The Ratio detector can give good results but the major drawback to this

design is that it requires transformers for coupling and balance.

Modern designs prefer to use ICs for lower cost, higher performance and greater

reliability, yet a transformer cannot be fabricated into an integrated circuit chip

design. This has led to increased use of the quadrature detector and phase-locked

loop. Of these two detector, the ratio detector was the most popular as it offers a

better level of amplitude modulation rejection of amplitude modulation. This enables

it to provide a greater level of noise immunity as most noise is amplitude noise, and

it also enables the circuit to operate satisfactorily with lower levels of limiting in the

preceding IF stages of the receiver.

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4.0 CONCLUSION

Foster-Seely discriminators or ratio detectors (Figs. 2.2.1 and 3.2.1) are used in

older FM receivers. These circuits may drift out alignment over time abd coils or

capacitors may short or open. All cause distorted or ni audio. Alignment is best

done with a sweep generator, but in a pinch apply a fully modulated signal to the

receiver and adjust the tuning for minimum distortion.

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