DSB DSB-SC Transmissions

  1. To attain two side bands with carrier. The carrier should be modulated with audio signal by the simple modulator (Not Balanced Modulator). Let the carrier frequency is 100 KHz and the audio frequency range is about 100 Hz to 5 KHz. After modulation, the result is LSB, USB and Carrier. LSB is ranging between 95,000 Hz and the USB is ranging between 100100 Hz to 105,000 Hz. The carrier is of 100 KHz.
  2. These two side bands and the carrier is then fed to the mixer where they mixed with the carrier of high frequency ( say 15 MHz depends upon transmission). The output frequency thus being raised to the value desired for transmission.
  3. Then these high frequency double side bands with carrier are amplified in driver and power amplifier stages and then fed to the aerial for the transmission
DSB Transmission Block Diagram
DSB Transmission Block Diagram

DSB-SC Transmission

DSB-SC (Double side band suppressed carrier) transmission is achieved by modulating a carrier with an audio signal through a balanced modulator. The block diagram is shows such transmission.

  1. Message signal after amplification is fed to the balanced modulator whose frequency range is between 100 Hz to 5 KHz.
  2. A balanced modulator receives two input signals: the message signal and the carrier signal. The function of the balanced modulator is to generate an amplitude modulated signal having two side bands, at same time canceling out the carrier signal.
  3. The upper side band USB range includes the carrier plus the message frequencies; The lower side band LSB range contains the carrier minus the message frequencies. This action is displayed in figure.
  4. Notice that the separation between the lower and upper side bands in the modulated signal is only 200 Hz in this example. the lower frequency in the message signal is the factor that determines the separation. From the figure, it is clear that the separation is two times the lowest message signal frequency.
  5. Then both side bands are fed to the balanced mixer where the frequencies of two side bands are yet raised with the crystal oscillator or synthesizer.
  6. Then the signal is fed to the linear amplifiers to achieve sufficient power for transmission. This signal is then radiated through aerial to air.
DSB-SC Transmission Block Diagram
DSB-SC Transmission Block Diagram