Basic Principles of Balanced Modulators

At the heart of every radio frequency transmitter is an array of electronic circuitry generating a RF signal that carries useful information, or intelligence. First, an oscillating circuit generates a continuous, steady state signal, referred to as the carrier wave, fixed to a discrete frequency in the radio frequency spectrum. Second, useful intelligence is inserted into the carrier wave. Intelligence can be generated in many different ways, including signals originating in the audio frequency spectrum, pulses containing discrete information within them, and frequencies shifting in phase relative to the carrier wave. The process of inserting intelligence into the carrier wave is termed “modulation.”

Modulation can take several different forms, the most familiar forms being amplitude modulation (AM), where the amplitude of the carrier wave fluctuates according to the strength and frequency of the modulating signal, and frequency modulation (FM), which alters the frequency of the carrier wave. Generally, pulse modulation and phase modulation are regarded as special forms used for transmitting data or in radio detection and ranging applications.

Regardless of the type and form of modulation, generating and transmitting a useable RF signal that preserves the original information as closely as possible is not without problems. Modulating signals originating in the audio frequency spectrum, such as speech and music, are full of noise and harmonics. In the process adding information to the carrier wave, the modulating signal both adds to and subtracts from the amplitude (AM), or the frequency (FM) of the carrier wave.

The combined frequencies of the carrier wave and the modulating signals are referred to as “sidebands”. Modulating frequencies that add to the carrier wave frequency are termed “upper side band, or USB”, while frequencies that subtract from the carrier wave frequency are termed “lower side band, or LSB”. Since the useful information is carried only in the sidebands, it is practical to design transmitters that transmit radio frequencies exclusively and efficiently in only the USB or LSB frequency range. Furthermore, the original carrier wave frequency is not required, since the carrier wave contains no information by itself, and, since both sidebands contain identical information, only one sideband is required.

Additionally, not all of the energy present in the modulating signal is required for efficient communication. Energy spikes of varying duration above the level required to efficiently transmit information can be removed, thereby cleaning up and compacting the frequency spectrum and allowing final power amplifiers to pump energy more efficiently into a narrowly concentrated frequency spectrum.

Modulators are electronic circuits designed to enhance the efficient combination of the modulating signal and carrier wave according to its intended application. In a radio frequency transmitter, a balanced modulator circuit would consist of a diode bridge and carefully tuned filters designed to remove all the unnecessary frequencies, being the carrier wave and one or the other sidebands. A diode bridge consists of a combination of inter-connected RF diodes, or an integrated circuit, that controls the level of energy necessary for efficient modulation of the carrier wave. Working in combination with high- or low-pass filters tuned to the desired frequency, the carrier wave and the unnecessary side band are removed. Only the desired sideband frequency, containing the useful intelligence, remains to be amplified, ultimately delivered to the transmitter’s transmission line and antenna.

Selection of the most efficient RF diodes is a critical step is designing a balanced modulator. Careful consideration should be given to the diode’s design specifications and the temperature and frequency range it will be operating in. Depending of the frequency spectrum, the lead length and diameter of the diode’s connectors, as well as their configuration and proximity to other components in the circuit may also be a consideration.