AM–DSB-SC Mathematical Representation & Generation using Balanced FET Modulator( EC8491-UNIT-1)
EC8491-Communication Theory- Syllabus ,UNIT I AMPLITUDE MODULATION (9) Amplitude Modulation- DSBSC, DSBFC, SSB, VSB - Modulation index, Spectra, Power relations and Bandwidth - AM Generation – Square law and Switching modulator, DSBSC Generation – Balanced and Ring Modulator, SSB Generation – Filter, Phase Shift and Third Methods, VSB Generation – Filter Method, Hilbert Transform, Pre-envelope & complex envelope –comparison of different AM techniques, Superheterodyne Receiver, Anna University-2017R, AM-Power Efficiency, Bandwidth
Balanced Modulator Method
Balanced modulators are the building blocks from which a wide variety of frequency mixers, modulators, and demodulators are built.
two matched FETs connected in a differential amplifier, which acts a balanced modulator
Carrier voltage is applied in phase to the two gates of FET
Modulating signal appears 1800out of phase at the gate. This is because of input transformer T1 is a center-tapped transformer.
Mode -1 (only carrier signal applied and message signal =0)
Both FETs conduct simultaneously due to the in-phase carrier voltage applied to their gates
Their drain currents are equal in magnitude but opposite in direction and cancel each other. The net output is Zero.
Mode =2 (both carrier and message signal both present)
The drain currents of the two FETs flow due to the combined effect of carrier and message signal.
The current due to message are equal but not opposite so they do not cancel out.
Disadvantages of FET balanced modulator
100% of carrier suppression is not possible.
Because both FETs must have identical characteristics and the transformer center taps must be exactly at the center of the windings.
Carrier will be heavily suppressed but not completely removed.
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