7/29/2019 ACL-03-04_EXP-03-A1 PR.pdf

1/11

TO PLOT THE DEMODULATION CHARACTERISTIC OF THE FMDEMODULATOR (FOSTER-SEELEY DEMODULATOR).

OBJECTIVE: To plot the Demodulation characteristics of the fm demodulator (Foster-

Seeley Demodulator). To observe the Waveforms of the foster-seeley Demodulated Signal. To Study the Ratio Demodulator. To Study the phase locked loop Detector. To Study the Quadrature Detector. To Study the Detuned resonance detector.

EQUIPMENT: ACL-03 Kit & ACL-04 Kit Power supply. E-lab.

Connective links. Volt meter Frequency meter.

THEORY:

FREQUENCY DEMODULATION:

To demodulate a frequency modulated signal, a circuit is necessary whichsupplies the output with proportional voltage to the frequency deviation of theinput modulated signal.The ideal characteristic of the demodulator is a straight line, also if it is actuallysufficient to obtain characteristics as the one ofFIG-1, which presents a linearbehavior only for a certain frequency range (demodulator usage range). Thefigure reports:

a) The instantaneous frequency f of the modulated signal, oscillating between F1and F2 (Fc is the frequency of carrier).

b) The voltage/ frequency characteristic curve of the demodulator.

c) The detected signal.

SENSITIVITY AND DEMODULATION NON-LINEARITY:

The Sensitivity and Non-Linearity are characteristic parameters of the frequencydemodulator. Both parameters can be detected by the characteristicdemodulation curve, shown in FIG-2.Sensitivity S is defined by

7/29/2019 ACL-03-04_EXP-03-A1 PR.pdf

2/11

S = DV (f)/ df = D V / Dfwhere V(f) is the instantaneous output voltage, function of the instantaneousinput frequency f. If Sc and S1 are the sensitivities calculated respectively incorrespondence to the central frequency and on post 1, the Non-Linearity N. L. inpost1 is defined by:

N.L.= (Sc-S1) / Sc 100

DEMODULATION OF FM SIGNALS:An FM receiver is very similar to an AM receiver. The most significant change isthat the demodulator must now extract the information signal from a frequency,rather than an amplitude, modulated wave.The basic requirement of any FM demodulator is therefore to convert frequencychanges into changes in voltage, with the minimum amount of distortion.To achieve this, it should ideally have a linear voltage/ frequency characteristics,similar to that shown in FIG-4.

A demodulator can also be called discriminator or detector.Any design of circuit that has a linear voltage / frequency characteristics wouldbe acceptable and we are going to consider the five most popular types.In each case, the main points to look for are:1 How do they convert FM signals into AM signals?2 How linear is their response - this determines the amount of distortion in thefinal output?3 How good are they at rejecting noise signals?

FREQUENCY DEMODULATOR CIRCUITS:For the detection of the frequency modulated signals, different circuit solutionshave been used. Some are out of use and others are used at the moment.Among the first ones, we mention:

TRAVIS DISCRIMINATION:It is based on amplitude variation, as a function of frequency, introduced by aresonant circuit. The amplitude variation is detected via the diode.

FOSTER-SEELEY DISCRIMINATOR:It is based on the phase variation as function of frequency, introduced by aresonant circuit. The original modulated signal and the shifted one are properlyadded and the resulting signal is detected with the diode.

RATIO DISCRIMINATOR:It behaves analogous to the Foster-Seeley one, but it is unaffected by themodulated signal amplitude.

QUADRATURE DETECTOR:It is used in integrated circuits. The direct FM signal and the same signal shiftedby 90 are multiplied. The resulting signal is proportional to the frequency

7/29/2019 ACL-03-04_EXP-03-A1 PR.pdf

3/11

deviation of INPUT FM SIGNAL.

PLL DETECTOR:It constitutes one of the applications of the Phase Locked Loop and is, in respectto the last circuits, less sensitive to noise.

DETUNED RESONANCE DETECTOR:This is the simplest form of the demodulator.In this, the parallel tuned circuit is deliberately detuned so that the incoming FMsignal is first converted to AM signal and then using the diode detector circuit, weget back the original signal.

AMPLITUDE LIMITER:The frequency demodulators are generally sensitive to the amplitude variation ofthe input FM signal. The output of the demodulator depends only on thefrequency variation of the input signal, but also on its eventual amplitude

variation (for example, caused by the noise or by disturbances of differentnature). To minimize this inconvenience, insert a limiter circuit before it, whichremoves or reduces the unwanted amplitude variations.The charecteristics curves of the ideal limiter and an actual limiiter are shown inFIG.3 In the first case, the amplitude of the output signal is constant for any inputsignal amplitude. In case of an actual limiter, the output amplitude keepsconstant only if the input signal gets over the minimum value.

FOSTER-SEELEY DISCRIMINATOR:FIG.5 shows the typical circuit of a Foster-Seeley discriminator.The FM signal is inductively coupled to the resonant circuit L2-C2, tuned to thecentral frequency of the modulated signal. The same signal is also taken from C1to the main socket of L2. The diodes D1 and D2, with the respective low passfilters CR, form two envelope detectors.Fo is the frequency at which the circuit L2-C2 is tuned. The operation of thecircuit is analyzed in three situations:

1) Instantaneous frequency f of the input FM signal equal to Fo f = FoIn the two secondaries of L2, we add two voltages . One is the one induced withL1 by the input signal Vfm, the other is the input signal coupled directly via C1.Being at the resonance frequency, the induced voltage Vind will be shifted by 90with respect to the voltage Vfm. The voltage coupled directly through C1 can beconsidered, if the reactance of C1 is small at the signal frequency, in phase withthe input VFM.The voltages reaching D1 and D2 are the vectorial sum of Vfm and +/- Vind/2and have the same amplitude but opposite sign. The output Vo, which is the sumof two detected signals will be null in this case.

2) Instantaneous input frequency higher than Fo f > FoWhen the instantaneous frequency of the input FM signal is superior to Fo, the

7/29/2019 ACL-03-04_EXP-03-A1 PR.pdf

4/11

resonant circuit L2-C2 has an inductive behavior. The voltages across diodeshave in this case different amplitudes and the resulting output voltage will bepositive.

3) Instantaneous input frequency lower than Fo f < Fo

When instantaneous frequency of the input FM signal is lower than Fo, theresonant circuit L2-C2 has a capacitive behavior. The voltages across diodes stillhave different amplitudes, but the resulting voltage will be negative.The main disadvantage of the Foster-Seeley demodulator is that it detectsamplitude variations of the input signal, as the voltage amplitude Vd1 and Vd2 ofthe diodes depends also on amplitude of the input signal. This inconvenience isminimized in the ratio demodulator.

RATIO DISCRIMINATOR:The operation of the circuit concerns with the coupling of the FM signal with thetwo detection circuits and the vectorial diagrams. It is similar to what we have

seen for the Foster-Seeley discriminator. The capacitor C5, with higher value,has the purpose of highly reducing the voltage fluctuations vab, occuring due toamplitude variations of the input signal. In this way, the output voltage vo is notaffected by unwanted amplitude variations. We can write, in fact :

vae + veb vae - vebvo = ________ - veb = ________ =

2 2vae + veb (vae / veb) 1 vab (vae / veb) 1

vo = __________ x ____________ = _____ x ____________2 (vae / veb) + 1 2 (vae / veb) + 1

As veb is practically constant, the output vo depends only on the relation vae /veb, which varies only with the effect of variations of the input frequency of signaland does not cause amplitude variations.

QUADRATURE DETECTOR:FIG.8 shows the functional diagram of the quadrature detector. A quadraturedetector multiplies the direct FM signal with the same signal shiftedby a resonantcircuit LC.At the resonant frequency, corresponding to the central frequency of the FMsignal, the shift is 90. At variation of the input signal, the shift introduced by LCcircuit will vary.

The multiplication of the direct FM signal and the shifted FM signal producesmany components, among which the low frequency component is proportional tothe information. The low pass filter separates this signal.

THE PHASE LOCKED LOOP (PLL) DETECTOR:

This is another demodulator that employs a phase comparator circuit. It is a verygood demodulator and has an advantage that it is available as a self-contained

7/29/2019 ACL-03-04_EXP-03-A1 PR.pdf

5/11

integrated circuit, so no setting is required. You just plug it in and it works. Forthese reasons, it is often used in commercial broadcast receivers. It has very lowof distortion.Altogether a very nice circuit.The overall action of the circuit may, at first, seem rather pointless. As we cansee in FIG.9, there is a voltage-controlled oscillator (VCO). The DC output

voltage from the output of the low pass filter controls the frequency of thisoscillator. Now, this DC voltage keeps the oscillator running at the samefrequency as the original input signal but 90 out of phase.The question often arises why we would want the oscillator to run at the samefrequency and 90 out of phase. And if we did, then why not just adda phaseshifting circuit at the input to give the 90 phase shift?The answer can be got by imagining what happens when the input frequencychanges as it would with an FM signal.If the input frequency increases and decreases, the VCO frequency is made tofollow it. To do this, the input control voltage must increase and decrease. Thesechanges in DC voltage level form the demodulated signal.

The AM signal then passes through a signal buffer to prevent any loading effectfrom disturbing the VCO and then through an audio amplifier if necessary.The Frequency response is highly linear.

DETUNED RESONANCE DETECTOR:This is the simplest form of demodulator.We can easily say that as the input frequency of the circuit changes, theamplitude of the output signal will increase and decrease. For example, if thefrequency of the incoming signal is increased, the operating point will movetowards the right in the diagram. This would cause an increase in the amplitudeof the output signal.An FM signal will therefore result in the amplitude modulated signal at the output.This amplitude-modulated signal is then given to the diode detector (envelopedetector) circuit, which follows the amplitude variations in the signal and the finalaudio output appears across the cathode of the diode.This output is then passed through the low pass filter to remove the unwantedDC component and the ripple.

V

b)f2

f1 fcf

c)V

fa)

t

7/29/2019 ACL-03-04_EXP-03-A1 PR.pdf

6/11

V1

DV

Df

DV

Df

Fcf

Vo

Vi

A) B)

Vo

Vi

7/29/2019 ACL-03-04_EXP-03-A1 PR.pdf

7/11

7/29/2019 ACL-03-04_EXP-03-A1 PR.pdf

8/11

-v"

IND/2

-v

IND/2-v'

IND/2

+v"

IND/2

+v

IND/2+v'

IND/2

v

v

v

v'

v'

v'

v"

v"

v"

D1

FM

D2

D1

FM

D2

D1

FM

D2

a) b) c)

V0

0

90

FM IN0

V0

0

7/29/2019 ACL-03-04_EXP-03-A1 PR.pdf

9/11

10

7/29/2019 ACL-03-04_EXP-03-A1 PR.pdf

10/11

P5 P6

LEVELFREQ.

FM MODULATOR

400KHz-1500KHz 0-2Vpp

500KHz 1500KHz

VF

RF/FMOUT

SW2

FS OUT

RATIO

FM IN

FOSTER SEELEY / RATIO DETECTOR

R

FS

FS

R

FS

R

SWITCH FAULTS

OFF

ON

SF 1

SWITCH FAULTS

OFF

ON

SF 1

( ACL-03 )

( ACL-04 )

BLOCK DIAGRAM FOR STUDY OF FREQUENCY DEMODULATION

PROCEDURE: Refer to the block diagram & Carry out the following connections and

settings. Connect the power supply with proper polarity to the kit ACL-03 and ACL-

04 switch it on. Keep all Switch Faults in OFF position. Keep switch SW2 at 500KHz position.

Using pot P5 keep frequency at 450KHz and using pot P6 keep amplitudeat 1Vpp.

Connect the output of FREQUENCY MODULATOR FM/RF OUT post tothe input of Foster-Seeley Demodulator of ACL-04 FM IN post.

Set the frequency demodulator in Foster-Seeley (jumpers in the FSposition).

7/29/2019 ACL-03-04_EXP-03-A1 PR.pdf

11/11

Connect the Oscilloscope or frequency-meter to the input of thedemodulator FM IN post.

Connect the voltmeter (or the DC oscilloscope) to the output of thedemodulator.

Check that the output voltage is 0 volt.

Vary the input frequency from 400 to 500KHz, in steps of 5KHz, andreport the frequencies and the corresponding output voltages on a table.The output voltage must vary from about 100 mV to about +100mV.

Plot a graph with the measured values.