Digital Communication Lab Manual

DC Digital Communication Laboratory Manual for Under-Graduate course in Electronics and Communication.

Bharath P

Contents 1. Time division multiplexing 2. Amplitude Shift Keying 3. Frequency Shift Keying 4. Binary Phase Shift Keying 5. Propagation loss in Optic fiber 6. Bending loss in Optic fiber 7. Numerical Aperture of Optic fiber 8. Antennas 9. Branch line Coupler 10. 3dB Power Divider 11. Ring Resonator

Prerequisites:

As an exercise, it is recommended to go through the basics of operation of Digital modulation techniques1, Fiber optics2, Transmission lines3, S-Parameter analysis3, wave propagation & radiation in Micro-strips3 and Antenna & its parameters4.

References: [1] Simon Haykin: Digital Communications, John Wiley. [2] John Crisp: Introduction to Fiber Optics, Newnes. [3] David M Pozar: Microwave engineering, John Wiley. [4] John D Kraus: Antennas, McGraw-Hill.

Note: - Always ensure good grounding facilities. - Always check chip number and pin details, keep datasheets close by. - Do not touch Capacitors. - Never look into LASER emitter. - Isolate microwave apparatus from Wi-Fi, Bluetooth, Mobile devices etc. to assure proper Readings. Caution: Important Note Static sensitive device Radiations (LASER) High Intensity Radiation High Voltage

Advanced Communication circuits

1. Time Division Multiplexing

Components: LF398, SL100, 47kΩ, 15kΩ, 27kΩ, 1kΩ, 0.01μF.

Circuit Diagram: MODULATION

+15 V -15 V Message Input 1 3 1 A1sin (2f1t) 4 47 kΩ LF398 5 PAM-TDM Message Input 2 6 7 A2sin (2f2t) 8 Square wave (Sampling frequency) Waveforms: m1(t) t, [fm1] m2(t) t, [fm2] c(t) t, [fc] s(t) TDM

1 2 1 2 1 2 1 2

1. Connect the circuit as shown in the figure. PROCEDURE:

2. Apply the input voltage m1(t) as 2v at 1kHz and m2(t) as 3v at 10kHz. Apply carrier square pulse as 4v at 3kHz. 3. Observe the waveforms for PAM-TDM. 4. Use the demodulation circuit to obtain m1(t) and m2(t) back.

Circuit Diagram: DEMODULATION:

+15 V -15 V PAM-TDM 3 1 4 5 LPF Recovered 47 kΩ LF398 Cutoff f1 Hz message1 6 7 8 Square wave (Sampling frequency) +15 V -15 V 3 1 4 5 LPF Recovered 47 kΩ LF398 Cutoff f2 Hz message2 PAM-TDM 6 7 8 Square wave (Sampling frequency)

2. Amplitude Shift keying Components: SL100, 47kΩ, 1kΩ, BY127, 1μF, IC741, 10kΩ potentiometer

Circuit Diagram: MODULATION:

c (t), 1 kHz 1 kΩ ASK 47 kΩ m (t), 500 Hz SL 100 Waveforms: m(t) t c(t) t s(t) t (ASK)

1. Connect the circuit as shown in the figure. PROCEDURE:

2. Apply the input voltage m(t) as 500Hz. Apply carrier square pulse as 1kHz. 3. Observe the waveforms for ASK 4. Use the demodulation circuit to obtain m(t) back.

Circuit Diagram: DEMODULATION:

Vcc 2 7 ASK BY 127 IC 741 Square R C 1 kΩ pot 3 1 wave Vc2 4 Vee R=1kΩ C=1μF

3. Frequency Shift Keying Components: LF398, IC741, 47kΩ, 10kΩ, 10kΩ potentiometer.

Circuit Diagram: MODULATION:

+15 V -15 V Sinusoidal Carrier 1 3 1 C1 (t) 4 47 kΩ LF398 5 FSK Sinusoidal Carrier 2 6 7 C2 (t) 8 Binary data (Square wave, 1 kHz) Waveforms: Binary Data t c1(t) t c2(t) t s(t) t FSK

1. Connect the circuit as shown in the figure. PROCEDURE:

2. Apply the input voltage c1(t) at 50kHz and c2(t) at 100kHz. Apply Binary data at 1kHz. 3. Observe the waveforms for FSK. 4. Use the demodulation circuit to obtain Binary data back. DEMODULATION : Circuit Diagram: 10 kΩ Vcc 10 kΩ FSK output 7 2 1 IC 741 Recovered Binary data C1 (t) 3 or C2 (t) 4 10 kΩ pot 10 kΩ Vee

4. Binary Phase Shift Keying Components: LF398, IC741, Center-tap Transformer, 47kΩ, 10kΩ, 10kΩ potentiometer. MODULATION : Circuit Diagram: +15 v -15 v c(t) 3 1 4 Sinusoidal carrier (20 kHz) 47 kΩ LF398 5 BPSK 6 7 8 -c(t) Binary Data (Square wave 1 kHz) Waveforms: Binary Data t c(t) t s(t) t BPSK PROCEDURE: 1. Connect the circuit as shown in the figure. 2. Apply the input voltage c(t) at 20kHz. Apply Binary data at 1kHz. 3. Observe the waveforms for BPSK. 4. Use the demodulation circuit to obtain Binary data back.

DEMODULATION: Circuit Diagram:

10 kΩ 10 kΩ Sinusoidal carrier

c(t) Vcc 2 7

BPSK 6 Waveform IC 741 ASK waveform

10 kΩ 3

4 Vee

Optic Fibers

5. Propagation loss in Optic fiber Components: Optic fiber trainer, Optic fiber. Schematic Diagram:

Source Emitter Detector Amplifier Optic fiber

Output

(LED or Laser Diode) (Photo-diode or PIN Diode) Waveforms: Vin(t) t Vout(t) t PROCEDURE: 1. Connect the circuit as shown in the figure. 2. Connect the function generator of the trainer kit to the CRO. Use the 1 kHz sine wave as Input. Note down the amplitude. 3. Connect the output to the CRO and note down the amplitude. 4. Use the formula to calculate the attenuation constant.

= Transmitted Power, Watts = Input Power, Watts

α = Attenuation constant, Nepers/meter L = Length of the fiber, meters V = Voltage, Volts

6. Bending Loss in Optic fiber Components: Optic fiber trainer, Optic fiber, Mandrel. Schematic diagram:

Source Emitter Optic fiber Detector Amplifier

Output PROCEDURE: 1. Connect the circuit as shown in the figure. 2. Connect the function generator of the trainer kit to the CRO. Use the 1 kHz sine wave as input. Note down the amplitude. 3. Wind the Fiber using the mandrel and count the number of turns 4. Connect the output to the CRO and note down the amplitude. 5. Observe the attenuation with respect to number of turns and bending angle.

7. Numerical Aperture of Optic fiber Components: Optic fiber trainer, Optic fiber, fiber mount with lens, scale, aperture scale. Schematic diagram: Screen 25mm

20mm Lens 15mm Optic 10mm Fiber w feed w

length,

SIDE VIEW FRONT VIEW PROCEDURE: 1. Connect the circuit as shown in the figure. 2. Connect lens to the optic fiber as shown. 3. Focus the beam on to any circle as shown. 4. Measure the distance from the screen and calculate numerical aperture. 5. Observe consistency of the value on various circles.

Antennas & Microwave devices

8. Antennas Components: Microwave powers source, attenuator, detector VSWR meter, Antennas, Measuring tape. DIRECTIVITY Schematic diagram:

Source VSWR Meter

Transmitting Antenna Receiving Antenna

Attenuator Schottky Detector

Distance, PROCEDURE: 1. Connect the circuit as shown in the figure with antennas in E-plane orientation. 2. Connect High frequency source to the reference antenna. 3. Mount the receiving antenna at a distance satisfying far field radiation (i.e. , where is the maximum dimension of the antenna and is the wavelength of the radiation). 4. Rotate the antenna about its axis and note down the received power for various angles. 5. Repeat procedure by mounting the receiving antenna in the H-plane orientation. 6. Plot the pattern in polar or semi log graph; obtain the Half Power Beam Width. 7. Use suitable formula to obtain Directivity.

Directivity:

Or

-where , and are the Maximum Power, Average Power and

Normalized Power densities respectively.

Directivity (Approximate):

Where is half power beam width of E-plane in degrees, is half power beam width of H-plane in degrees. Directivity (Decibel):

........ (dBi, decibels over isotropic) GAIN

The Experimental setup is the same as that used for Directivity. PROCEDURE: 1. Connect the circuit as shown in the figure with antennas in E-plane orientation. 2. Connect High frequency source to the reference antenna. 3. Mount the receiving antenna at a distance satisfying far field radiation (i.e. , where is the maximum dimension of the antenna and is the wavelength of the radiation). 4. Keep the antennas in the direction of maximum power radiation. 5. Make note of Input (≈ Transmitted) power, the distance, d and the VSWR reading (≈ Received power). 6. Repeat procedure for various values of d and tabulate the Power readings against it. 7. Obtain the ratio of input to output power (i.e. ); plot a graph of distance, d versus and obtain the slope to calculate gain.

Gain: Friis Transmission Formula

….. (W, Watts)

where and are gains of transmitting and receiving antennas respectively. For identical antennas it is equated to . Hence

Plot to find Gain – ↑

Δ

Δd

→ d

1. Dipole Antenna E-Plane H-Plane

2. Micro strip patch Antenna

E-Plane H-Plane 3. Yagi-Uda Antenna (5 Element)

E-Plane H-Plane

9. Branchline Coupler Component: Microwave Power source, attenuator pad, detector, VSWR meter, 3dB power divider Schematic Diagram: Microwave Source VSWR Meter

Attenuator Branchline Coupler Schottky Detector 1 2

3 4

Matched Load Matched Load

PROCEDURE: 1. Connect the circuit as shown in the figure. 2. Connect the source to the input port of the power divider. 3. Assume one of the output ports as the terminal port and connect a matched load to it. 4. Make note of the power received in the other output port for various frequencies of input power. 5. Similarly carry out the measurements by interchanging the output ports. 6. Repeat the procedure by terminating the input port to matched load and considering one of the output ports as input . 7. Obtain the S-parameters S12, S13 and S23 in decibels and plot the graph against the input frequency. Ideal scattering parameters matrix:

w, width

l, length

10. 3dB Power Divider Component: Microwave Power source, attenuator pad, detector, VSWR meter, 3dB power divider Schematic Diagram:

VSWR meter Microwave Source

Power Divider Schottky Detector

2 1

3 Attenuator

Matched Load PROCEDURE: 1. Connect the circuit as shown in the figure. 2. Connect the source to the input port of the power divider. 3. Assume one of the output ports as the terminal port and connect a matched load to it. 4. Make note of the power received in the other output port for various frequencies of input power. 5. Similarly carry out the measurements by interchanging the output ports. 6. Repeat the procedure by terminating the input port to matched load and considering one of the output ports as input . 7. Obtain the S-parameters S12, S13 and S23 in decibels and plot the graph against the input frequency. Ideal scattering parameters matrix:

l, length

11. Ring Resonator Component: Microwave Power source, attenuator pad, detector, VSWR meter, 3dB power divider Schematic Diagram: Microwave Source VSWR meter

Ring Resonator

1 2

Attenuator Schottky Detector PROCEDURE: 1. Connect the circuit as shown in the figure. 2. Connect the source to the input port of the resonator. 3. Make note of the power received in the output port for various frequencies of input power. 4. Obtain the S-parameter in decibels and plot the graph against the input frequency. 5. Evaluate the Resonant frequency, and utilize in suitable formula to obtain the Effective dielectric constant, and eventually Relative dielectric constant, .

Effective Dielectric Constant:

Where is the free space velocity of the radiation,

is the Resonant frequency of the Micro-strip ring resonator, is the Radius of the Micro-strip ring.

Relative Dielectric Constant:

Where is the height of the dielectric substrate,

is the width of the Micro-strip conductor.

Ideal scattering parameters matrix:

R

(Dielectric) (Conducting Strip)

Thickness Thickness

Substrate, Ground Plane