Mumbai University > Computer Engineering > Sem 3 > Electronic Circuits and Communication Fundamentals

Marks: 10 Marks

Year: Dec 2013

• The phaseing method of SSB generation uses a phase shift technique that causes one of the side bands to be conceled out. A block diagram of a phasing type SSB generator is shown in fig.

• The carrier signal is V_cSin2πf_ct the modulating signal is $V_m\sin2πf_{mt}$.

  Balanced modulator produces the product of these two signals.

  $(V_m\sin2πf_{mt})( V_c\sin2πf_{ct})$

• Applying a trigonometric identity.

  $(V_m \sin2πf_{mt})( V_c\sin2πf_{ct})=1/2[\cos(2πf_c-2πf_m)t-\cos(2πf_c+2πf_m)t]$

• Note that these are the sum and different frequencies or the upper and lower side bands.

• It is important to remember that a cosine wave is simply a sine wave shifted by 90. A cousine wave has exactly the same shape as a sine wave, but it occurs 90

• The 90 phase shifters create cousine waves of the carrier and modulating signal which are multiplied in balanced modulator to produce.

  $(V_m-\cos2πf_{mt})(V_c\cos2πf_{ct})\\ (V_m-\cos2πf_{mt})(V_c\cos2πf_{ct})$

• Another common trigonometric identity translates this to $(V_m \cos2πf_{mt})( V_c \cos2πf_ct) \dfrac{1}{2}[\cos(2πf_c-2πf_m)t+\cos(2πf_c+2πf_m)t]$

• Now if you add these two expressions together the sum frequencies cancel while the difference frequencies add producing only the lower side band.

  $\cos(2πf_c-2πf_m)t$

Carrier and unwanted side band suppression

Carrier and unwanted side band can be suppressed using a spectrum analyzer with a bandwidth narrow enough to see both sidebands and your suppressed carrier.

This task can be done with a separate receiver and a calibrated S meter. Transmit a steady tone on 14.2 MHz USB at low power and then tune the receiver until you can hear your transmission.

If you then switch to LSB on your receiver and tune around you may be able to hear your unwanted sideband. If you measure a signal of S7 then your unwanted sideband is 32dB down.