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Table of Contents
Diode Ring Modulator
Objective
Materials
Background
In electronic communications, a balanced modulator is a circuit that produces double-sideband suppressed-carrier (DSBSC) signals: It suppresses the radio frequency carrier thus leaving the sum and difference frequencies at the output. The output waveform lacks the carrier, but still contains all the information a traditional AM signal has. This results in power saving during signal transmission.
One of the most prevalent balanced modulators is the Diode Ring Modulator, otherwise known as Lattice Modulator. It comprises of four diodes originally fashioned as a “ring”, thus the moniker, and input and output transformers. The modulator has two inputs: a single frequency carrier and a modulating signal, which can be a single frequency or a complex waveform. The carrier is applied at the center taps of the input and output transformers and the modulating signal at the primary of the input transformer. The output, however, is measured at the secondary of the output transformer. Figure 1 shows the diode ring modulator in two different circuit orientations.
Figure 1. Diode Ring Modulator
Also, the diode ring modulator is one of the most extensively used circuits in electronic communications. In addition to producing DSBSC signals, it is also used in frequency and phase modulation systems as well as in digital modulation systems, such as PSK and QAM.
The orientation of the diodes in a ring modulator must not be mistaken with that of a diode bridge rectifier. They may take the similar “ring” shape; however, the ring modulator has all its diodes face either clockwise or counterclockwise while the bridge rectifier has its diodes facing either left or right.
Operation
The diodes used in the diode ring modulator can either be silicon, silicon Schottky-barrier or gallium-arsenide. They serve as switches that control whether the input signal is passed with or without a 180° phase reversal. The carrier signal is the one that sets the diodes on and off at a high rate of speed. It is important to know that for the modulator to operate, the carrier’s amplitude must be adequately greater than the modulating signal’s, about six to seven times greater.
Figure 2. Positive Half-cycle Operation
During the positive half-cycle, D1 and D2 are forward biased and on, and D3 and D4 are reverse biased and act as open circuits. The carrier current is then equally divided at the center tap of the input transformer’s secondary and flows in opposite directions through the upper and lower halves of the winding. The currents in the upper and lower parts each produce a magnetic field that is both equal and opposite with each other therefore, the magnetic fields produced cancel out and the carrier is suppressed. Thus, the modulating signal is passed from the input to the output transformers through D1 and D2 without phase reversal. Figure 2 shows the positive half-cycle operation of the modulator.
Figure 3. Negative Half-cycle Operation
Figure 3 illustrates the negative half-cycle operation of the diode ring modulator. Diodes D1 and D2 are reversed biased and off while D3 and D4 are forward biased and on.
university/courses/electronics/electronics_lab_diode_ring_modulator.1553136232.txt.gz · Last modified: 21 Mar 2019 03:43 by Hannah Rosete