Balanced Modulator

BALANCED MODULATOR

Diode Lattice Modulator

Figure 3.1(a) At Positive half cycle

Explaination:

·  Carrier sine wave is used as a source of forward bias and reverse bias for the diodes. The carrier turns the diode off and on at a high rate of speed.

·  Diodes acts like switches that connect the modulating signal at secondary T1 to primary T2.

·  When the polarity of carrier is at the positive half as shown above, D1 and D2 are forward biased. D3 and D4 are reversed biased (open circuits).

·  Currents are divided equally in the upper and lower portions of the primary winding of T2.

·  The current in the upper part of the winding produce an equal but opposite magnetic field as the current at the lower half of the secondary.

·  Thus, magnetic field cancels out each other and no output is induced in the secondary. Thus carrier is effectively suppressed.

Figure 3.2(b) At negative half cycle

Explaination:

·  When the polarity of carrier is at the negative half as shown above, D3 and D4 are forward biased. D1 and D2 are reversed biased (open circuits).

·  Currents are divided equally in the upper and lower portions of the primary winding of T2.

·  The current in the upper part of the winding produce an equal but opposite magnetic field as the current at the lower half of the secondary.

·  Thus, magnetic field cancels out each other and no output is induced in the secondary T2.Thus zero carrier output. The carrier is balanced out.

Notes:

1.  Degree of carrier suppression depends upon degree of precision with which the transformers are made and placement of the center taps to ensure perfectly equal upper and lower currents and magnetic field cancellation.

2.  Degree of carrier attenuation also depends upon the diodes, where the diode characteristics must be perfectly matched.

BALANCED MODULATOR(continued)

·  When info signal is applied to primary T1, it will appear across the secondary T1.

·  The diode switches will connect the secondary T! to primary T2 at different times depending on the polarity of the carrier.

·  When the carrier polarity’s is at positive half (as shown in 3.1(a)), diode D1 and D2will conduct and act as closed switches. Thus modulating signal is applied to primary T2 through D1 and D2.

·  When carrier polarity is in the negative half (as shown in 3.1(b)), diodes D3 and D4 will conduct as closed switches. Again, a portion of modulating signal will be applied to primary of T1 but this time the leads have been effectively reversed because of the connections of D3 and D4.

·  The result is a 1800 phase reversal. Thus, if modulating signal is positive, the output at the secondary winding of T2 is negative, and vice versa.

·  The carrier is operating at higher frequency, thus the diode will switch off and on at a high rate of speed, causing portions of modulating signal to be passed through the diodes at different times.

AMPLITUDE DEMODULATOR

DIODE DETECTOR

Explaination:

·  The AM signal is usually transformer coupled. It is applied to a basic half wave rectifier circuit consisting of D1 and R1.

·  Diode D1 conducts when the positive half cycle of the AM occurs.

·  The diode is reversed biased during the negative half cycle. Thus the voltage across R1 is a series of positive pulses whose amplitude varies with the modulating signal.

·  A capacitor is connected across R1 to recover back the original modulating signal. Its value is chosen to have a low impedance at the carrier frequency and high impedance at the modulating signal.

·  As a result, the capacitor filter outs the carriers, leaving only the modulating signal.

·  Because the diode detector recovers the envelope of the AM signal, the circuit is sometimes known as envelope detector.

Output waveform:

Across R1:

Diode current (no filter)

Across capacitor: