Balanced Modulator

Balanced Modulator  •

Balanced Modulator uses the nonlinear devices.

•

Balanced Modulator balances out or removes the carrier from the output so it is called the balanced modulator.

•

If there are two non linear devices such as FET and we are giving one FET the signal (v1 +v2) and the other FET (v1-v2) where v1 is carrier  signal and v2 is message signal or modulating signal. This means that modulating signal voltage appears 180 degree out of phase at the gates and carrier signal is applied in phase to the FET’s gate.We take both the devices symmetrical i.e.

•

If the current equation for nonlinear device is i= a+bv+cv2 then constants a , b and c are same for both the devices.

•

In the output the arrangement is such as the current due to both transistors or FET’s are in opposite direction

• The current equation for the first FET is Id1= a+b(v1+v2)+c(v1+v2)2 a+bv1+bv2+cv12+cv22+2cv1v2 • The current equation for second FET is id2= a+b(v1-v2)+ c(v1-v2)2 id2= a +bv1-bv2+cv12+cv22-2cv1v2 The resultant current is I1=Id1-id2= 2bv2+4cv1v2

V1 is carrier signal. v1=Vcsinwct V2 is message or modulating signal. v2=Vmsinwmt I1=2bVmsinwmt+4cVmVcsinwctsinwmt =2bVmsinwmt+2cVmVc[cos(wc-wm)t-cos(wc+wm)t] The output voltage is proportional to current.let the constant of   proportionality is . V0= i1 =2



bVmsinwmt+2



cVmVc[cos(wc-wm)t-cos(wc+wm)t]

V0=2



bVmsinwmt+2



cVmVc[cos(wc-wm)t-cos(wc+wm)t]

Simplifying it Let P=2



bVm and Q=2



cVmVc

So V0=Psinwmt+Qcos(wc-wm)t-Qcos(wc+wm)t

Ring Modulator 

Ring Modulator 

• Ring modulator removes the modulating signal also.

•

The diodes are paired and are turned on and off during the positive and negative half of the carrier frequency cycle.

During the positive half, D1 and D4 are on and the other two are off. The circuit will therefore look as follows

During the negative half, D2 and D3 are on and the other two are off. The resulting circuit is shown.

The current produced by the carrier signal is split at the centre taps of the transformers and flow in opposite directions. This leads to their cancellation as they produce magnetic fields, which are equal in magnitude but opposite in phase.