Ring Modulator

Amplitude modulation is a technique in which amplitude of the carrier is varied linearly with the amplitude of the message signal.
However for a single tone modulation of A.M the modulation efficiency of AM for u=1 is 100/3=33.33%
It means,The power required to transmit the carrier is very high when compared to the sidebands.
To increase the modulation efficiency we need to suppress the carrier and only sidebands are transmitted.
So that technique is called Double sideband suppressed carrier wave.

READ MORE ABOUT DSBSC :->> DSBSC
  To generate that wave we use this circuit.

This is the  popular circuit used in Communication systems especially in the generation of DSBSC wave(Double sideband suppressed carrier wave).    Let the input m(t) be Zero.
  In the positive half cycle of carrier D0 diodes are forward biased and D1 diodes are reverse Biased.

The current divides equally in primary winding of T2 transformer.The current in the upper half of T2 transformer produces a magnetic field which is equal and opposite to the magnetic field produced by the current in the lower half of the secondary transformer T2.As the magnetic field are equal and opposite,they cancel each other.So carrier is suppressed.
Following similar logic during negative half of carrier. we can see that carrier is suppressed.

WIth the modulating signal
Positive half cycle.
 A modulating signal applied to the input of T1 transformer. D1 and D2 are forward biased and they will connect secondary of T1 to primary of T2.That is the modulating signal(usually sine wave_) appears across the Secondary of T2 transformer.
In negative half cycle, the diodes D3 and D4 are forward biased and connect the secondary of transformer T1 to the primary of T2 in REVERSE connections.This results in the 180 degree shift in the modulating signal .Thus we get DSBSC wave,.
Ring modulator is more ideal when the carrier is square wave.

In the above figure you can see the 180 degree shift