analog & digital communicationa lab

Expt No. 1

AMPLITUDE MODULATION AND DEMODULATION

Date:

A I M : To construct an amplitude modulation circuit and measure the Modulation Index. To recover the modulating signal from the AM wave by using a Diode Detector circuit.

TES T RI G S:

S.NO. NAME RANGE QNTY

1 DSO 0 – 30 MHZ 1

2 FUNCTION GENERATORS 0 – 2 MHZ 2

3 VARIABLE POWER SUPPLY 0 – 30 V ( DUAL ) 1

CO MPO N EN TS:

S.NO. NAME PART NUMBER QNTY

1 RESISTORS 33K, 39K, 47K, 68E, 68E, 100K, 1K 1 each

2 CAPACITORS 10uF, 0.01uF, 0.1uF 1, 2, 1 each

3 DIODE OA79 1

4 BREAD BOARD 1

5 POTENTIOMETERS 100K 1

6 IC CA3080 1

TH EOR Y:

Amplitude modulation (AM) is a method of impressing a message signal onto an alternating-current (AC) carrier waveform. The highest frequency of the modulating data is normally less than 10 percent of the carrier frequency. The instantaneous amplitude (overall signal power) varies depending on the instantaneous amplitude of the modulating signal.

In AM, the carrier itself does not fluctuate in amplitude. Instead, the modulating signal appears in the form of signal components at frequencies slightly higher and lower than that of the carrier. These components are called sidebands. The lower sideband (LSB) appears at frequencies below the carrier frequency; the upper sideband (USB) appears at frequencies above the carrier frequency. The LSB and USB are essentially "mirror images" of each other in a graph of signal amplitude versus frequency. The sideband power accounts for the variations in the overall amplitude of the signal.

An Amplitude Modulated Signal is represented as follows.

Vam(t) = [ Ec + EmSin(2πfmt) ] × [ Sin(2πfct) ]

Where Ec + EmSin(2πfmt) is the Amplitude Modulated WaveEm is the peak change in the amplitude of the Envelope ( volts )fm is the frequency of the modulating signal.

4 57

P U R PO S E :

This experiment demonstrates the principle of Multiplier operation using the CA3080 Operational Transconductance Amplifier. A simple demodulator demonstrates one method of recovering an amplitude modulated signal using a diode detector known as envelope detector. The modulation index m is calculated that indicates by how much the modulated variable varies around its unmodulated level. It relates to the variations in the amplitude of the carrier signal. The value of m is within the range of 1.

P I N O U T D I A G R A M OF C A 3 0 80:

P I N O U T D I A G R A M OF O A 79 ( g e rm a n i u m d i o d e )

MODU L ATO R :

+9VR7

100K Of f set Null

-9VR247K

+9V

Carrier R1Signal3 + AM Out

62

33K

R468E

R368E R5

39K

R6100K

-9V

Modulating Signal0

DEMODU L AT OR :

AM In D1 R8 C3 Message Out

OA79 1K 0.1uF

C10.01uF

C20.01uF

0

M O DU L A T I O N G R A P H :

PROC ED UR E:

1. Connections are made for the AM Modulator and Demodulator as shown in the circuit diagrams.2. Frequency of the input carrier is fixed at constant amplitude of 1 volt and 150 KHz.3. A message signal of 1 KHz at 0.5 volt amplitude is applied at the modulating signal input.4. The Vmax and Vmin are measured and tabulated to calculate the Modulation Index m5. The amplitude of the message signal is varied in steps till the Vmin reaches the minimum.6. The same set of amplitude values are used for two or three modulating frequencies and values

tabulated.7. The maximum value of m is observed to be 1.8. The demodulated message signal is observed from the output of the Envelope Detector and

tabulated in the demodulator side of the tabulation..9. A selection of RC network is important for a faithful recovery of the message signal.10. All optimum parameters like Vcc are noted down.

T ABU L AR CO LUM N:

Vc = 150 KHz @ 1 Volt Amplitude

S.No.Modulator Side Demod Side

Fm Vm Vmax Vmin m Fo Vo

1 1 KHz

0.5

1

1.5

2

2 2 KHz

0.5

1

1.5

2

3 3 KHz

0.5

1

1.5

2

RE FER ENC ES:

1. CA3080/CA3080A DATASHEET.2. OA79 DATASHEET.3. Understanding and Using OTA Op-Amp Ics ( Nuts and Volts Magazine )4. Electronic Communications Systems V Edition by Wayne Tomasi – Pearson Education.5. Communication Lab Manual – ECE Department – SSIT, Tumkur6. Communication Lab Manual – ECE Department – Easwari Engg College, Chennai.

R ES UL T : Thus the modulation Index of an Amplitude Modulation Circuit was calculated and the message signal was recovered using an Envelope Detector.

Expt No. 2

FREQUENCY MODULATION AND FSK GENERATION

Date:

A I M : To construct a Frequency Modulation circuit using a sinusoidal input waveform and to measure theModulation Index. To use the same circuit for FSK generation with a square waveform.

TES T RI G S:


1 DSO 0 – 30 MHZ 1

2 FUNCTION GENERATOR 0 – 2 MHZ 1


CO MPO N EN TS:


1 RESISTORS 1.5K, 5.6K, 10K 1 EACH

2 CAPACITORS 1 nF 2

3 INTEGRATED CIRCUIT NE/SE566D 1

4 BREAD BOARD 1

TH EOR Y:

F R EQ U E N C Y M O D U L A T I O N :

Frequency modulation (FM) is a method of impressing data onto an alternating-current (AC) wave by varying the instantaneous frequency of the wave. This scheme can be used with analog or digital data.

In analog FM, the frequency of the AC signal wave, also called the carrier, varies in a continuous manner. Thus, there are infinitely many possible carrier frequencies. In narrowband FM, commonly used in two-way wireless communications, the instantaneous carrier frequency varies by up to 5 kilohertz (kHz, where 1 kHz = 1000 hertz or alternating cycles per second) above and below the frequency of the carrier with no modulation.

In wideband FM, used in wireless broadcasting, the instantaneous frequency varies by up to several megahertz (MHz, where 1 MHz = 1,000,000 Hz). When the instantaneous input wave has positive polarity, the carrier frequency shifts in one direction and when the instantaneous input wave has negative polarity, the carrier frequency shifts in the opposite direction. At every instant in time, the extent of carrierfrequency shift (the deviation) is directly proportional to the extent to which the signal amplitude is positive or negative.

In digital FM, the carrier frequency shifts abruptly, rather than varying continuously. The number of possible carrier frequency states is usually a power of 2. If there are only two possible frequency states, the mode is called frequency-shift keying (FSK). In more complex modes, there can be four, eight, or more different frequency states. Each specific carrier frequency represents a specific digital input data state. Frequency Modulation is widely used in communication systems. The most well known use is in FM broadcasting. Digital FM is used in modems and multi tone selective signaling systems.

The general definition of frequency modulated signal SFM(t) is given by the formula:

S FM ( t )

A C cos( 2 f C t ( t )) A C cos( 2 f C t 2 K f

t

m ( ) d )

where,

m( )

A C

fC

K f

is the modulating signal.

is the amplitude of the carrier.

is the carrier frequency.

is the frequency deviation constant measured in Hz/V.

76

C1

R1

18

GN

DV

+

FR EQUEN C Y SHI FT K EYI N G:

Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK (BFSK). BFSK literally implies using a pair of discrete frequencies to transmit binary (0s and 1s) information. With this scheme, the "1" is called the mark frequency and the "0" is called the space frequency. The time domain of an FSK modulated carrier is illustrated in the waveforms.

PUR POS E:

This experiment demonstrates some of the principles of VCO operation using the NE566 integrated circuit by implementing a Frequency Modulation Circuit. The modulation index h is calculated that indicates by how much the modulated variable varies around its unmodulated level. It relates to the variations in the frequency of the carrier signal.

h = ∆f/Fm where ∆f is the frequency deviation and Fm is the modulating frequency.

An FSK signal is generated by replacing the sine wave input with a square wave. The MARK andSPACE frequencies are observed for 2 KHz signal.

PI NO UT DI AG R AM OF NE 566:

Figure 1

CI RC UI T DI AG R AM S:

FM & FSK MOD UL AT OR :

VCC12 V+

M ODULATING SIGNAL INPUT

C2

R2 R11. 5K ?

C3

0.01UF 0. 001UF TRIANGLE WAV E OUTPUT

U1A

5 OUT TRI OUT

4

NE5563

SQ OUT

SQUARE WAV E OUTPUT

R3

10K?

C1

0

Figure 2

The NE/SE566 Function Generator is a general purpose voltage-controlled oscillator designed for highly linear frequency modulation. The circuit provides simultaneous square wave and triangle wave outputs at frequencies up to 1MHz. A typical connection diagram is shown in Figure 2. The control terminal (Pin 5) must be biased externally with a voltage (Vc) in the range

V+ ≤ Vc ≤ V+

where VCC is the total supply voltage. In Figure 2, the control voltage is set by the voltage divider formed with R2 and R3. The modulating signal is then AC coupled with the capacitor C2. The modulating signal can be direct coupled as well, if the appropriate DC bias voltage is applied to the control terminal. The frequency is given approximately by

fo =

and R1 should be in the range 2k< R1<20k. A small capacitor (typically 0.001uF) should be connected between Pins 5 and 6 to eliminate possible oscillation in the control current source. The value of C1 is1nF.

W AV EFORM S:

F R EQ U E N C Y M O D U L A T I O N :

FR EQUEN C Y SHI FT K EYI N G

PROC ED UR E:

1. Connections are made as per the circuit diagram of Figure 2

2. Without applying the modulating signal, measure the carrier frequency Fc at the output of NE566.

3. Apply a modulating signal Fm of 1 KHz toNE566 and observe the FM wave.

4. Note down the change in the frequency of the waveform δ=Fc ~ fFM

5. Vary the modulating frequency upto 5 KHz and tabulate the readings.

6. Calculate the modulation Index h = δ/Fm7. Calculate the BW = 2 ( Fm + δ )8. To generate an FSK waveform, the input

frequency of 2 KHz should be a square wave.

9. The MARK and SPACE frequency are tabulated.

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S.No. ModulatingSignal Fm

FrequencyModulation f

Frequencydeviation δ=Fc ~ f

ModulationIndex h=δ/Fm

BW =

2 ( Fm + δ )

1 1 KHz

2 2 KHz

3 3 KHz

4 4 KHz

5 5 KHz

T ABU L AT I ON FOR FM:

FM FM

T ABU L AT I ON FOR FS K:

Signal Amplitude(V) Time period(sec)

Modulating signal

Carrier signal

Mark Frequency

Space Frequency

Res ul t: Thus an FM and FSK modulation signal were generated and the properties were tabulated.

Expt No. 3

BALANCED MODULATOR

Date:

A I M : To construct a Balanced Modulator and note down its working principle.

TES T RI G S:


1 DSO 0 – 25 MHZ 1



CO MPO N EN TS:


1 RESISTORS 100K 2

2 CAPACITOR 220pF 1

3 DIODES IN4148 4

4 BREAD BOARD 1

5 INDUCTOR Decade InductanceBox

1

TH EOR Y:

A Balanced Modulator generates a DSB signal. The inputs to a balanced modulator are the carrier and a modulating signal. The output is the upper and lower sidebands. A balanced modulator suppresses the carrier, leaving only the sum and the difference frequencies at the output. The output of a balanced modulator can be further processed by filters or phase-shifting circuitry to eliminate one of the sidebands, thereby resulting in an SSB signal. One of the most popular and widely used balanced modulator is thediode ring or lattice modulator illustrated in figure 1. Figure 1.

D4 D1

T11 5

Modulating 6Signal

4 8

D1N4148 D1N4148

5 T2

6

8

1

DSB Output

4

TR ANSFORMER CT D2 D3 TRANSFORMER CT

D1N4148 D1N4148

V1

Carrier Signal



It consists of an input transformer T1, an output transformer T2, and four diodes connected in a bridge configuration. The carrier signal is applied to the center taps of the input and output transformers. The modulating signal is applied to the input transformer T1. The output appears across the secondary of the output transformer T2.

The carrier sine wave, which is usually considerably higher frequency and amplitude than the modulating signal is used as a source of forward and reverse bias for the diodes. The carrier turns the diodes off and on at a high rate of speed. The diodes act like switches which connect the modulating signal at the secondary of T1 to the primary of T2.

The greatest carrier suppression will occur when the diode characteristics are perfectly matched. A carrier suppression of 40 dB is achievable with well-balanced components.

CI RC UI T DI AG R AM:

R1 R2 Figure 2.100k

D1V1

100k

D3 DSB OutputP R O C E D U R E:

Modulating Signal

D1N4148V1

Carrier Signal

D2

D1N4148

D1N4148

D4

D1N4148

L1 2.2mH

C1

220pF

1. Connections aremade as shown in figure 2.

2. A carrier wave of200KHz at 1 voltpp amplitude and a message signal of

3. 1 KHz at less than 1 Volt pp are applied as shown in the circuit diagram.4. The output waveform is noted to have a suppressed carrier.5. The output frequency of the carrier is noted and tabulated as shown.6. The carrier frequency is varied and the inductance value is tuned to have maximum amplitude.7. All the parameters are tabulated for fc = 150 KHz and 100 KHz.

T ABU L AT I ON :

S.No. CarrierFrequency fc

ModulatingFrequency fm

Amplitude offm

Output CarrierFrequency Fo Vo

1 200 KHz 1 KHz 0.5 V

2 150 KHz 1 KHz 0.5 V

3 100 Khz 1 KHz 0.5 V

R ES UL T : Thus a simple balanced modulator is built to understand its working principle.

RE FER ENC E:

1. COMMUNICATION LABORATORY MANUAL, UNIVERSITY OF FLORIDA2. DIGITAL AND ANALOG COMMUNICATION SYSTEMS, Leon W. Couch



Expt No. 4

PRE-EMPHASIS & DE-EMPHASIS

Date:

A I M : To design a Pre-Emphasis and De-Emphasis circuit for a desired roll-over frequency and compare the practical output with theoritical calculations.

TES T RI G S:


1 DSO 0 – 30 MHZ 1



CO MPO N EN TS:


1 RESISTORS 100E, 820E, 2.2K, 15K 1 each

2 CAPACITORS 100nF 1

3 INTEGRATED CIRCUIT LM741 1

4 BREAD BOARD 1

TH EOR Y:

When an FM system is compared to an AM system with a modulation index of 1 operating under similar noise conditions, then it can be shown that the FM signal has a signal to noise ratio which is 3m2

better than the AM system. m here is the modulation index or deviation ratio for the FM signal.

In an FM system the higher frequencies contribute more to the noise than the lower frequencies. Because of this all FM systems adopt a system of pre-emphasis where the higher frequencies are increased in amplitude before being used to modulate the carrier.

The transfer function sketched above is used for a pre-



47

V-

V+

emphasis circuit for FM signals in the FM band. The Time T = 75µs. For FM systems in the FM band m ~5 resulting in a S/N improvement of 19dB. With pre-emphasis this can be increased by 4dB for a total of23dB.

At the receiver the higher frequencies must be de- emphasized in order to get back the original baseband signal. The transfer function of the de-emphasis circuit is shown above.

P U R PO S E :

This experiment demonstrates the use of Pre-Emphasis and De-Emphasis circuits in FM Transmitters and Receivers respectively as discussed in the Theory part of this experiment. A separate circuit and its design are used to build the circuits and to observe their performances.

The Frequencies F1 and F2 are selected according to the desired levels of frequency response at the source and the destination. Usually the values of 100 Hz and 20 KHz are selected as the Audio spectrum and the boost and cut frequencies are used in the design.

P I N O U T D I A G R A M OF L M 7 4 1:

CI RC UI T DI AG R AM S: PR E- EM PH ASI S:

R1 RfFigure 1 D ESI GN :

0 + V c c Given: F1 = 2.1 KHz and F2 = 15 KHz

C

r

Vi1Vac0Vdc

U12

-

LM7413

+

R

$PIN 6 5

OU T 6 Vo

OS1 1

-V cc

F1 = 1/(2πrC) and F2 = 1/(2πRC)

Choose C = 100nF, then r = 820E and R = 100E

Also r/R = Rf/R1, then R1 = 2.2K and Rf = 15K

0 0



47

V-

V+


Input Vi = 500 mV

S.No. FrequencyOutputVo Gain = 20 Log V0/Vi

1 100

2 250

3 500

4 1k

5 2.5k

DE-E MPH ASI S:

R1 Rf

0 +V ccFigure 2 DESIGN:

Fc = 1/(2πRdCd)Rd

Vi1Vac0Vdc

0

2

C 3

0 0

U1

-

LM741

+

$PIN6 5

OUT 6

OS1 1

-V cc

Vo

15K

Choose Cd = 100nF and Fc = F1 = 2.1 KHz

Then Rd = 820E

Also r/R = Rf/R1, then R1 = 2.2K and Rf =


Input Vi = 500 mV

S.No. Frequency Output Vo Gain = 20 Log V0/Vi

1 100

2 250

3 500

4 1k

5 2.5k



PROC ED UR E:

1. The values of the Resistors and the Capacitors are calculated using the design formulas for lower cut-off frequency F1 and Upper cut-off frequency F2 for Pre-Emphasis circuit.

2. Similarly the cut-off frequency F1 for De-Emphasis circuit is used to arrive at the values of the passive components.

3. Connections are made as shown in the figures of Pre-Emphasis and De-Emphasis circuits.4. A minimum of 500 mV is applied as Vi to the inputs.5. The frequency is varied in steps throughout the audio range and the corresponding readings are

tabulated.6. The gain is calculated as shown in the Tabular Columns.7. A graph of Frequency versus Output Voltage is drawn on a Semi-Log Graph Sheet.

GR AP H S:

The Graphs for both the circuits will resemble as shown in the theory part.

RE FER ENC ES:

1. LM741 DATASHEET2. Electronic Communications Systems V Edition by Wayne Tomasi – Pearson Education.3. Communication Lab Manual – ECE Department – SSIT, Tumkur4. Communication Lab Manual – ECE Department – Easwari Engg College, Chennai.

R ES UL T : Thus a circuit to improve the frequency response of FM receivers was studied using Pre- Emphasis and De-Emphasis.

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Expt No. 5

PHASE LOCKED LOOP AND APPLICATIONS

Date:

A I M : To construct a Phase Locked Loop and to observe the locking frequency range.

TES T RI G S:


1 DSO 0 – 30 MHZ 1



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Expt No. 6

PWM GENERATION AND DETECTION

Date:

A I M : To generate a Pulse Width Modulation signal for a sinusoidal message signal and to compare the detected message signal with the input signal.

TES T RI G S:


1 DSO 20 MSPS 1



CO MPO N EN TS:


1 RESISTORS 1.5K, 10K,1K 2,1,1

2 CAPACITORS 100nF, 10nF, 1.5uF 1,1,2

3 BREAD BOARD 1

4 IC LM741 1

5 POTENTIOMETER 100K 1

TH EOR Y:

Pulse-width modulation (PWM) is a digital modulation technique, which converts an analog signal into a digital signal for transmission. The modulator converts an audio signal (the amplitude-varying signal) into a sequence of pulses having a constant frequency and amplitude, but the width of each pulse is proportional to the Amplitude of the audio signal.

In this experiment, a square-wave generator or a monostable multivibrator can be used to generate the PWM signal, whose output pulse width is determined by the values of R4, C3, and Vin(+). The LM741 operational amplifier acts as a voltage comparator.

The reference voltage at Vin(+) input (pin 3) is determined by the resistor values of R3 and R5. The combination of R4 and C3 provides the path for charging and discharging. When no audio signal is applied, the dc reference voltage at Vin(+) input can be changed by adjusting the R5 value.

If dc level of R5 is fixed and an audio signal is applied to the audio input, the audio signal is added to the fixed dc level and the reference voltage will be changed with the change of audio amplitude. The resultingPWM signal presents at the output of the comparator.



47

V-

V+

P I N D I A G R A M O F L M 7 4 1:

CI RC UI T DI AG R AM S:

PU LSE W I D TH MO DUL AT I O N C I R CUI T:

+12V

R 3

C4

100nF

R5100K

1k

U13

+ OS2 5

OUT 6

2 - OS1

1

PWM Output

1KHz Sine Wave LM741R 4

10k

-12V

C3

10nF

0

Figure 1

DEMODU L AT I ON C I RC UI T:

R1 R2

1.5k

PWM InputC2

1uF

1.5k

C1

1uFModulatingSignal

0

Figure 2

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M O DU L A T I O N G R A P H :

PROC ED UR E:

1. The Circuit of Figure 1 and 2 are connected and cascaded to form a PWM Modulator andDemodulator.

2. A 1 KHz Sine Wave with an amplitude of 1 Volt is applied to the input. The non-inverting input is adjusted to have a zero DC bias by varying the 100K potentiometer R5 to have a 50% duty cycle.

3. The output PWM is noted for its ON time and OFF time and tabulated.

4. If the amplitude of the input signal generates a fairly good PWM, then the amplitude is fixed and the values are tabulated for different frequency inputs.

5. The corresponding demodulated sine wave is verified.

T AB U L AR CO LU MN:

S.No. INPUT FREQUENCY ( KHz ) AMPLITUDE (Vin)PWM

T ON T OFF

1 1 KHz

2 2 KHz

3 3 KHz

4 4 KHz

5 5 KHz

6 6 KHz

RESU L T:

Thus a Pulse Width Modulation signal for a sinusoidal message signal was generated and the detected message signal was compared with the input signal.



Expt No. 7

AGC CHARACTERISTICS

Date:

A I M : To study the principle of an Automatic Gain Control circuit and its performance characteristics.

TES T RI G S:


1 DSO 100 MSPS 1



CO MPO N EN TS:


1 RESISTORS 470E, 1K, 10K, 33K, 120K, 240K, 100K 1 each, 240K – 2

2 CAPACITORS 10uF 3

3 TRANSISTOR J176, 2N3904 1 each

4 BREAD BOARD 1


6 INTEGRATED CIRCUIT LM358 1

TH EOR Y:

Automatic gain control (AGC) is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels. For example, without AGC the sound emitted from an AM radio receiver would vary to an extreme extent from a weak to a strong signal; the AGC effectively reduces the volume if the signal is strong and raises it when it is weaker.

AGC algorithms often use a PID controller where the P term is driven by the error between expected and actual output amplitude. Automatic Gain Control or AGC is a circuit design which maintains the same level of amplification for sound or radio frequency. If the signal is too low the AGC circuit will increase (amplify) the level and if it is too high, will lower it to maintain a constant level as possible.

The Automatic Gain Control principle is widely used in AM receivers and sometimes AGC is called a compressor-expander.



HO W I T W O RKS?:

Using the circuit presented here, we can construct a very inexpensive AGC amplifier with the following features: a dynamic range greater than 50 dB; negligible distortion to the output waveform; fast attack and slow decay; an adjustable output level from 0 to 1.2 V p-p; operation from a single 5-V supply; less than 1-mA current drain; and low cost.

Referring to the circuit diagram, J1 (a Pchannel JFET), coupled with R2 and the equivalent resistance of R3 and R4, form a voltage divider to the input signal source. With input levels below 40 mV p-p, the input is evenly divided between R2 (120k) and R3 ¦ ¦ R4 (120k). The output amplitude of U1A isn’t large enough to turn on J1, which acts as a positive peak detector. The gate of the JFET is pulled to +5 V, pinching its channel off and creating a very high resistance from drain to source. This essentially removes it from the circuit.

At input levels above 40 mV p-p, Q1 is turned on at the positive peaks of the output of U1A, lowering the JFETs gate to source voltage. The channel resistance decreases and attenuates the input signal to maintain the output of U1A at approximately 1.2 V p-p.

The circuit, as shown, was tested with a sine-wave input ranging from 300 Hz to 30 kHz at 40 mV to 20 V p-p, a 54-dB range. It maintained the output level at 1.2 V p-p, ±0.5 dB, with no visible distortion when comparing it with the input waveform. With a 40 mV to 20 V p-p input signals, the amplitude of the signal across the JFET (VDS) measured less than 20 mV p-p.

Other JFETs with VGS(OFF) of 5V or under, such as the 2N5019 or 2N5116, should work equally well in this circuit, although they haven’t been tried. To use JFETs with higher VGS(OFF), such as the2N3993 (it was tried and worked equally well), increase the supply voltage to 12 V.

PUR POS E:

The use of AGC circuits in radio receivers have now being integrated into the monolithic receiver ICs. Hence this simple to implement design technique is studied which can be used in any other circuit where constant amplitude output is necessary.

P I N D I A G R A M O F L M 3 5 8:



84

V+

V-

P I N D I A G R A M O F J 1 7 6:


P I N D I A G R A M OF 2 N 3 9 0 4 :

C2 R5 R6

Audio Input R2

10uF

0

C1

470

5v

R3240k

2

LM358

3

33k

0

-U1A 1

OUT

0-20v @ 1KHz 120k

R11k

10uF 5v

R8

+

R4240k

5vC3

R710k

0D

100k 0 10uF

0 GJ1

J176

S C410uF

Q12N3904

Audio OutputP1 10k

0-1.2v

0

PROC ED UR E:

1. Connections are made for the AGC circuit as shown in the circuit diagram.

2. The frequency of audio input signal is fixed at constant frequency of 1 KHz.

3. The Amplitude of the input is made to vary in steps from 1 Volt.

4. The output is noted to be constant at 1.2 Volts irrespective of the input amplitude increment in steps

5. The input versus the output amplitude is tabulated as given below.

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T ABU L AT I ON :

Input Frequency = 1 KHz

S.No.Input

Amplitude Vi

Output Amplitude

Vo

1 1Less than1.2V

2 2 1.2

3 3 1.2

4 4 1.2

5 5 1.2

6 6 1.2

7 7 1.2

8 8 1.2

9 9 1.2

10 10 1.2

RE FER ENC ES:

1. DATASHEETS OF LM358, J176, 2N3904.2. Electronic Communications Systems V Edition by Wayne Tomasi – Pearson Education.3. ECE1352 Analog Integrated Circuits I, University of Toronto.4. h t t p : / / ele c t ron i c de s i g n . c o m / ar t i c l e / ana l o g-a n d - m i x e d- s ign a l / e ff e c t i v e-ag c -a m p l i f i e r - c a n - b e-b u i l t - a t -

a-nominal-.aspx

R ES UL T : Thus an automatic gain control circuit was rigged up and its performance characteristics was studied.



http://electronicdesign.com/article/analog-and-mixed-signal/effective-agc-amplifier-can-be-built-at-

Expt No. 8

FM DETECTOR

Date:

A I M : To demodulate an FM signal using a PLL FM Demodulator.

TES T RI G S:


1 DSO 0 – 25 MHZ 1

2 DUAL DIGITAL SYNTHESIZER FUNCTION GENERATOR 0 – 2 MHZ 1


CO MPO N EN TS:


1 RESISTORS 680E, 2.2K 1 each

2 CAPACITORS 1uF, 1nF, 10nF, 750pF 1 each

3 INTEGRATED CIRCUITS NE565 1 each

4 BREAD BOARD 1


TH EOR Y:

There are a number of circuits that can be used to demodulate FM. Each type has its own advantages and disadvantages, some being used when receivers used discrete components and others now that ICs are widely used.

Below is a list of some of the main types of FM demodulator or FM detector. In view of the widespread use of FM, even with the competition from digital modes that are widely used today, FM demodulators are needed in many new designs of electronics equipment.

Slope FM detectorFoster-Seeley FM detectorRatio detectorPLL, Phase locked loop FM demodulatorQuadrature FM demodulatorCoincidence FM demodulator

Each of these different types of FM detector or demodulator has its own advantages and disadvantages.



Phase locked loop, PLL FM demodulator or detector is a form of FM demodulator that has gained widespread acceptance in recent years. PLL FM detectors can easily be made from the variety of phase locked loop integrated circuits that are available, and as a result, PLL FM demodulators are found in many types of radio equipment ranging from broadcast receivers to high performance communications equipments.

The way in which a PLL FM demodulator operates is quite straightforward. The loop consists of a phase detector into which the incoming signal is passed, along with the output from the voltage controlled oscillator (VCO) contained within the phase locked loop. The output from the phase detector is passed into a loop filter and then used as the control voltage for the VCO.

Phase locked loop (PLL) FM demodulator

With no modulation applied and the carrier in the centre position of the pass-band the voltage on the tune line to the VCO is set to the mid position. However if the carrier deviates in frequency, the loop will try to keep the loop in lock. For this to happen, the VCO frequency must follow the incoming signal, and in turn for this to occur the tune line voltage must vary. Monitoring the tune line shows that the variation in voltage corresponds to the modulation applied to the signal. By amplifying the variations in voltage on the tune line it is possible to generate the demodulated signal.

PLL FM demodulator performance

The PLL FM demodulator is normally considered a relatively high performance form of FM demodulator or detector. Accordingly they are used in many FM receiver applications.

The PLL FM demodulator has a number of key advantages:

Linearity: The linearity of the PLL FM demodulator is governed by the voltage to frequency characteristic of the VCO within the PLL. As the frequency deviation of the incoming signal normally only swings over a small portion of the PLL bandwidth, and the characteristic of the VCO can be made relatively linear, the distortion levels from phase locked loop demodulators are normally very low. Distortion levels are typically a tenth of a percent.Manufacturing costs: The PLL FM demodulator lends itself to integrated circuit technology. Only a few external components are required, and in some instances it may not be necessary to use an inductor as part of the resonant circuit for the VCO. These facts make the PLL FM demodulator particularly attractive for modern applications.

PLL FM demodulator design considerations

When designing a PLL system for use as an FM demodulator, one of the key considerations is the loop filter. This must be chosen to be sufficiently wide that it is able to follow the anticipated variations of the frequency modulated signal. Accordingly the loop response time should be short when compared to the anticipated shortest time scale of the variations of the signal being demodulated.



A further design consideration is the linearity of the VCO. This should be designed for the voltage to frequency curve to be as linear as possible over the signal range that will be encountered, i.e. the centre frequency plus and minus the maximum deviation anticipated.

In general the PLL VCO linearity is not a major problem for average systems, but some attention may be required to ensure the linearity is sufficiently good for hi-fi systems.

DEMODU L AT OR u si ng N E/ SE565 :

P i n D esc ri p t i o n o f N E 5 65:

F i g u r e 1 .

N E 5 6 5 as F M D e t e c t o r :

The 565 Phase-Locked Loop is a general purpose circuit designed for highly linear FM demodulation. During Lock, the average DC level of the phase comparator output signal is directly proportional to the frequency of the input signal. As the input frequency shifts, it is this output which causesthe VCO to shift its frequency to match that of the input.

Consequently, the linearity of the phase comparator output with frequency is determined by the voltage- to-frequency transfer function of the VCO. Because of its unique and highly linear VCO, the 565 PLL can lock to and track an input signal over a very wide bandwidth with high linearity. A typical connection is shown in figure 4. The VCO free-running frequency is given approximately by

and should be adjusted to be at the center of the input signal frequency range. C1 can be any value from 220pF to 750pF, but R1 should be within the range of 2000 to 20000 ohms with an optimum value of the order of 4000 ohms. A small capacitance should be connected to Pin7 and 8 to eliminate possible oscillation in the control current source.

MOD EL G R AP H:

Modulating Signal

Carrier Signal

FM Signal

Demodulated Signal Fig ur e 2.



98

T C

AP

T R

ES

110

-VC

C+V

CC


+V cc

10K

R3R1 C2

2.2k 1n

FM Input C1

1uF

R2680E

U1

2IN

3IN2

4

LM565

VCON 7

C310n

M e s s age Out

VOUT

5 VIN

REF 6

0

C4

750pF

-V cc Fi g ur e 3.

PROC ED UR E:

1. Connections are made as shown in figure 3 using the PLL IC NE565.2. With the absence of the input FM signal, the output at pin 4-5 should be a square wave.3. Adjust the square wave to have an output frequency of 100 KHz by adjusting the 10K POT.4. An FM signal is generated with the following settings – Fo=100KHz, Fm=1KHz, Fd=10KHz or

more and less than 30 KHz, Wave=Sine and Amplitude=500mV using a Dual Digital Synthesizer Function Generator. This is the input signal to the demodulator.

5. The output is a demodulated waveform of 1KHz.6. Decreasing or increasing the frequency deviation results in a distorted output.7. The frequency of FM carrier signal is varied and tabulated.

T ABU L AT I ON :

S.No. Fo KHz Fm KHz Fd KHz VFM Vo in KHz

1 75 1 20 100 mV

2 100 1 20 101 mV

3 125 1 20 102 mV

R ES UL T : Thus an FM detector circuit was rigged up to extract a message signal from an FM signal. An undistorted message signal was captured at an optimum frequency deviation setting of 20 KHz.

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Expt No. 9

PAM AND VERIFICATION OF SAMPLING THEOREM

Date:

A I M : To rig up a circuit to generate a Pulse Amplitude Modulation Signal and to verify the sampling theorm for appropriate message signal to avoid Aliasing.

TES T RI G S:


1 DSO 100 MSPS 1


CO MPO N EN TS:


1 RESISTORS 3.3K, 4.7K, 22K 1 each

2 CAPACITORS 100nF 1

3 TRANSISTOR SL100 1

4 BREAD BOARD 1

TH EOR Y:

Pulse-amplitude modulation, acronym PAM, is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses.

Example: A two bit modulator (PAM-4) will take two bits at a time and will map the signal amplitude to one of four possible levels, for example −3 volts, −1 volt, 1 volt, and 3 volts.

Demodulation is performed by detecting the amplitude level of the carrier at every symbol period.

Pulse-amplitude modulation is widely used in baseband transmission of digital data. Some versions of the widely popular Ethernet communication standard are a good example of PAM usage. In particular, the Fast Ethernet 100BASE-T2 medium (now defunct), running at 100 Mbit/s, utilizes 5 level PAM modulation (PAM-5) running at 25 mega pulses/sec over two wire pairs.

Pulse Amplitude Modulation has also been developed for the control of Light Emitting Diodes especially for lighting applications. LED drivers based on the PAM technique offer improved energy efficiency over systems based upon other common driver modulation techniques such as Pulse Width Modulation as the forward current passing through an LED is relative to the intensity of the light output and the LED efficiency increases as the forward current is reduced.



Figure: 1 Pulse Amplitude Modulation

PUR POS E:

Pulse Amplitude Modulation is the most simplest of all Digital Modulation Techniques currently available. A simple circuit around a single driver transistor is implemented and the aliasing effect is noted.


Modulating Signal

V1VOFF = 3 V DCVAMPL = 3V FREQ = 100Hz

V2

R2

22K

R 1

4.7K

PAM R3

3.

3K Q1

SL100

Message C1

100nF

5KHz @ 5V Carrier Pulse

0 Figure: 2 Circuit Diagram

FI L TE R DESI G N:

1. Let the Cut off frequency of the filter fo >> fm2. Choose Fo = 500 Hz = 1/( 2×π×R3×C1)3. Choose C1 = 100nF, therefore R3 = 3.3K4. Rc = 4.7K, Rb = 22K

PROC ED UR E:

1. The circuit of figure 2 is rigged up with modulating frequency connected to the collector of SL100 with amplitude of 3 volts and at 100 Hz sine wave.

2. An offset of 1 to 3 Volts DC bias is adjusted in the function generator which supplies the modulating signal of 100 Hz.

3. A Carrier signal of 1 to 5 KHz with minimum amplitude of 1 V is applied as shown in the circuit.4. The output PAM is available at the collector of Q1.5. Adjust the amplitude of both the carrier and the modulating signal to get a pure PAM signal.6. The low-pass filter comprising of R3 and C1 demodulates the PAM signal.7. All the signals are tabulated.8. To verify the Sampling Theorem, the frequency of the carrier and the modulating signals are

altered asa. Fc < 2Fm b. Fc = 2Fm c. Fc > 2Fm

And the output PAM signals is verified with sampling theorem as shown in figure 3.



T ABU L AT I ON :

S.No.Vc

(pp) Volts

Fc inHz

Vm (pp) Volts

ReconstructedSignal

VoVolts

Fo inHz

1

2

3

4

5

6

P I N O U T D I A G R A M OF S L 100:

V E R I F I C AT I ON OF S A M P L I N G T H E O R E M :

Figure 3. Verification of Sampling Theorem

RE FER ENC ES:

1. Datasheet of SL1002. Electronic Communications Systems V Edition by Wayne Tomasi – Pearson Education.3. Communications Lab Manual, ECE Department, S.S.I.T., Tumkur – 5721054. Communications Lab Manual, ECE Department, Easwari Engg College, Chennai – 89.

R ES UL T : Thus a PAM circuit was built and sampling theorem was verified under three conditions of Fm& Fc.



Expt No. 10

PULSE CODE MODULATION ENCODER AND DECODER

Date

A I M : To rig up a Pulse Code Modulation Circuit and to observe the message pulses at the output of theDetector. This is to be carried out with the help of trainer kit.

TES T RI G S:


1 DSO 0 – 30 MHZ 1



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Ex. No. 11

DELTA MODULATION AND DEMODULATION

Date

A I M : To understand the theory of Delta Modulation by rigging up a suitable circuit and to demodulate the message signal.

TES T RI G S:


1 DSO 100 MSPS 1



CO MPO N EN TS:


1 RESISTORS 150K, 100K 1,3

2 CAPACITORS 100nF, 10nF 1 each

3 INTEGRATED CIRCUITS LM741, CD4013, CD4016 1 each

4 BREAD BOARD 1

TH EOR Y:

A 1-bit DPCM coder is known as a delta modulator (DM). In other words, DM codes the differences in the signal amplitude instead of the signal amplitude itself. Yet another name for DM is pulse width modulation

A delta-modulation encoder is shown in Figure 1; it is known as a single integration modulator.

Figure 1. Delta Modulation Encoder



The input signal is compared to the integrated output pulses and the delta (difference) signal is applied to the quantizer. The quantizer generates a positive pulse when the difference signal is negative, and a negative pulse when the difference signal is positive.

This difference signal moves the integrator step by step closer to the present value input, tracking the derivative of the input signal.

For example if we consider 1.5 kHz sinusoidal input signal with maximum amplitude 1 and delta is chosen to be 0.125 which is equivalent to 4 bit quantization i.e. 16 quantization levels.

To achieve a resolution equivalent to 4 bit quantization with 4 kHz sampling rate an oversampling ratio of 16 is needed i.e. (4^2)/(1^2)*4kHz=64 kHz. In Figure 2, 32 times oversampling is used and the output of the integrator tracks nicely the input signal.

Figure 2. 32 times oversampled DM signal

A delta modulation decoder has to integrate the modulated signal and low pass filter the output of the integrator as shown in figure 3.

Figure 3. DM Decoder

PUR POS E:

This experiment demonstrates the principle of Delta Modulation from its first principles as described in the theory section. An active Integrator circuit can be built using an Op-Amp followed by aLow Pass Filter to decode the message signal usually voice.

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47

714

R4


Clock In 10 KHzSquare Wave C1

100n

+V cc

Message In250 Hz

+V cc

U13

LM741

R2100k

-V cc

5

6

U2A1

Q2 Q3

CLK5

VDD Q2

!Q2CLK2

Out

1413121110

+V cc

R4

100k

U313 X1

1 1

5

4

6

1 2

3CD4016

mr(t) 2 16 D 4013 R2 9 0 97 S D2 8 8

R3150k -V cc

R1100k

GND S2

12-V cc 10

11

10n C2 0

Note: Vcc = 5V-Vcc = - 5V -V cc

Fi g ur e 4. D el ta M od ul ator

DESC RI PTI ON OF TH E C OMPONE N TS US ED :

L M 741:

The amplifiers offer many features which make their application nearly foolproof: overload protection on the input and output, no latch-up when the common mode range is exceeded, as well as freedom from oscillations.

CD 40 1 3 :

The CD4013B dual D-type flip-flop is a monolithic complementary MOS (CMOS) integrated circuit constructed with N- and P-channel enhancement mode transistors. Each flip-flop has independent data, set, reset, and clock inputs and “Q” and “Q” outputs.

These devices can be used for shift register applications, and by connecting “Q” output to the data input, for counter and toggle applications. The logic level present at the “D” input is transferred to the Q output during the positive- going transition of the clock pulse. Setting or resetting is independent of the clock and is accomplished by a high level on the set or reset line respectively.



TR U T H T A B L E :

CD 401 6:

The CD1016 Series Types are Quad Bilateral Switches intended for the Transmission or Multiplexing of Analog or Digital Signals. Each of the four independent bilateral switches has a single control signal input which simultaneously biases both the P and N device in a given switch ON or OFF.

Applications include

Analog Signal switching / multiplexing / Signal gating / Squelch Control / ChopperModulator / DemodulatorDigital Signal Switching / MultiplexingCMOS logic implementationA to D & D to A Conversion

Digital Control of Frequency, Impedance, Phase, and Analog-Signal Gain

PUR POS E:

This experiment demonstrates the principle of Delta Modulation and how the various building blocks are implemented from the block diagram representations. The demodulation part is left to the student to design and test its functionality.

PREC AU TI ONS:

Clock input should be between 0 and +Vdd for CD4013.The ICs should be populated on to the Bread Board in location where they have the best fit.



HO W I T W O RKS:

The LM741 is operated open loop as a comparator between the input signal m(t) and the feedback error signal mr(t).The function of CD4013 is to hold the value of the quantized error signal constant at + o r – V c c during the sampling period.Both the ICs CD4013 and CD4016 are enabled by the same clock input. A DC integrator is used in the feedback loop.The propagation delay in the Flip-flop is considered negligible.

T ABU L AT I ON :

S.No.Carrier Input ( Square Wave ) Message Input ( Sine Wave )

Frequency in KHz Amplitude in V Frequency in Hz Amplitude in V

1 10 5 250 1

2 10 5 1000 1

PROC ED UR E:

1. The circuit of figure 3 is rigged up with all precautionary measures.2. The clock signal input is a square wave of 10 KHz with an amplitude of 5V.3. The message signal input is a sine wave at 250Hz with amplitude of 1V.4. The DSO is used to observe the message signal and the output DM signal.5. The amplitude of the carrier and the message signal are adjusted to produce a distortion less DM.6. Observe the error signal at pin 3 of U1.7. Tabulate the observation.

PS:

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RE FER ENC ES:

1. DATASHEETS of LM741, CD4013, CD4016.2. DIGITAL COMMUNICATIONS LAB, UNIVERSITY OF CENTRAL FLORIDA.

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Expt No. 12

DIGITAL MODULATION TECHNIQUES USING TRAINERS

Date

A I M : To study the various Digital Modulation Techniques and to observe the waveforms using AdvancedDigital Communication Trainer Kit.

TES T RI G S:


1 DSO 0 – 30 MHZ 1

2 Advanced Digital Communication Trainer 10 Experiments 1

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ec emsin

electronic

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low pass filter

instantaneous

carrier frequency

amplitude