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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