21-05-2013, 04:08 PM
SSB MODULATION AND DEMODULATION
Aim:
To generate the SSB modulated wave
Theory:
An SSB signal is produced by passing the DSB signal through a highly selective band pass filter. This filter selects either the upper or the lower sideband. Hence transmission bandwidth can be cut by half if one sideband is entirely suppressed. This leads to single sideband modulation (SSB). In SSB modulation bandwidth saving is accompanied by a considerable increase in equipment complexity.
Procedure:
1. Switch ON the trainer and measure the output of the regulated power supply i.e. +12V and 8V
2. Observe the output of the RF generator using CRO. There are 2 outputs from the RF generator, one is direct output and another is 90⁰ out of phase with the direct output. The output frequency is 100 KHz and the amplitude is > 0.2Vpp. (Potentiometers are provided to vary the output amplitude).
3. Observe the output of the AF generator, using CRO. There are 2 outputs from the AF generator, one is direct output and another is 90⁰ out of phase with the direct output. A switch is provided to select the required frequency (2 KHz, 4 KHz or 6 KHz). AGC potentiometer is provided to adjust the gain of the oscillator (or to set the output to
good shape). The oscillator output has amplitude 10Vpp. This amplitude can be varied suing the potentiometers provided.
4. Measure and record the RF signal frequency using frequency counter. (Or CRO).
5. Set the amplitudes of the RF signals to 0.1 Vpp and connect direct signal to one balanced modulator and 90⁰ phase shift signal to another balanced modulator.
6. Select the required frequency (2 KHz, 4 KHz or 6 KHz) of the AF generator with the help of switch and adjust the AGC potentiometer until the output amplitude is 10 Vpp (when amplitude controls are in maximum condition).
7. Measure and record the AF signal frequency using frequency counter (or CRO).
8. Set the AF signal amplitudes to 8 Vp-p using amplitude control and connect to the balanced modulators.
9. Observe the outputs of both the balanced modulators simultaneously using Dual trace oscilloscope and adjust the balance control until desired output wave forms (DSB-SC).
10. To get SSB lower side band signal, connect balanced modulator output (DSB-SC) signals to Subtractor.
11. Measure and record the SSB signal frequency.
12. Calculate theoretical frequency of SSB (LSB) and compare it with the practical value. LSB frequency = RF frequency – AF frequency
13. To get SSB upper side band signal, connect the output of the balanced modulator to the summer circuit.
14. Measure and record the SSB upper side band signal frequency.
15. Calculate theoretical value of the SSB (USB) frequency and compare it with practical value. USB frequency = RF frequency + AF frequency.