The Basics Of A Digital Attenuator
The digital attenuator is also known as stepped attenuator. It is an electronic device which reduces or weakens the amplitude or power level produced by a signal generator. This device reduces signal power or amplitude without significantly distorting its waveform. The digital attenuator is usually a passive device manufactured from simple voltage divider networks.
The digital attenuator is not really digital at all. It is used to provide more than one step in attenuation. The basic topology of this kind of attenuator mainly consists a transmission line with diode spacing optimized over a desired frequency range, and the digital-to-analog drive circuitry. The digital-to-analog drive circuit is used to control the variable analog attenuator. Microstrip line is the preferred media in the RF design of the attenuator because it allows access to the circuitry while being measured. This provides more precise tuning capability. Unlike a continuously variable attenuator, the stepped attenuator switches in discrete, finite attenuation states, usually in binary steps. Switching can be achieved at high speed using semiconductor devices such as metal oxide semiconductor field effect transistors (MOSFETs), gallium arsenide metal semiconductor field effect transistors (MESFETs) and PIN diodes.
Digital attenuators are specified in terms of the number of bits of attenuation. The bits do not command equal amounts of attenuation. Instead, they select increasing amounts of attenuation. The least-significant bit (LSB) chooses the smallest amount of attenuation offered by the component. Meanwhile, the most-significant bit (MSB) selects the largest amount of attenuation. When all the bids are selected, the attenuator provides its maximum rated attenuation.
Some of the manufactures of digital attenuators are Mini-Circuits, M/A-COM Technology Solutions, Skyworks and National Semiconductor. Price varies depending on techniques used to develop the attenuators. Mini-Circuits' series of digital step attenuators (DAT family) are manufactured using Super Radio Frequency Complementary Metal Oxide Semiconductor (RF CMOS) technology. The company claims that the DAT family has a combination of linearity, accuracy, programmability, electrostatic discharge (ESD) tolerance, and wide bandwidth in a small surface mount package.
M/A-COM Technology Solutions offers radio frequency power attenuator, optical attenuator and microwave digital attenuators. Its AT65-0413 is a gallium arsenide field effect transistor (FET) 4-bit digital attenuator. It features attenuation ranging from 1.0 dB steps to 15 dB. It also has low direct current power consumption and an integral transistor-transistor logic (TTL) driver. It also has 50 ohm impedance. The device is best suited for applications where accuracy, fast speed and low cost are required. Skyworks’ AA106-86 is a 5-bit, single positive control gallium arsenide FET attenuator. It has an LSB of 0.5 dB and a total attenuation of 15.5 dB. It requires external DC blocking capacitors, five individual bit control voltages, and positive supply voltage.
Meanwhile, National Semiconductor’s LM1972 digital audio attenuator is fabricated on a CMOS process. Each channel has attenuation steps of 0.5 dB from 0dB-47.5 dB, 1.0 dB steps from 48 dB -78 dB with a mute function attenuating 104 dB. This digital attenuator’s logarithmic attenuation curve can be customized depending on the desired application.
