RF Demystified—What Is an RF Attenuator?


The attenuator is a management part, the primary perform of which is to scale back the energy of the sign passing by means of it. The sort of part is usually used to stability sign ranges within the sign chain, to increase the dynamic vary of a system, to supply impedance matching, and to implement numerous calibration methods ultimately software design.

Introduction

To proceed the collection of quick discourses on RF for non-RF engineers, we are going to talk about IC attenuators and provides some insights into their sorts, configurations, and specs. This text goals to help engineers with getting began with all kinds of IC merchandise and selecting the best one for an finish software. Associated articles on this collection embrace: “A Information for Selecting the Proper RF Amplifier for Your Utility,” “The best way to Simply Choose the Proper Frequency Era Element,” and “RF Demystified–Understanding Wave Reflections.”

Kinds of Attenuators

From the important thing practical perspective, attenuators could be labeled as fastened attenuators with an unchanging degree of attenuation and variable attenuators with an adjustable degree of attenuation. Relying on the type of attenuation management supported by variable attenuators, they’ll in flip be additional labeled as voltage variable attenuators (VVAs), that includes analog management, and digital step attenuators (DSAs) which might be managed digitally.

VVAs present steady adjustment of attenuation ranges that may be set to any worth throughout the given vary. Analog variable attenuators are often employed for computerized achieve management circuits, calibration corrections, and different processing features the place clean and exact management of a sign is required.

DSAs function a set of discrete attenuation ranges permitting for sign energy changes with a predetermined attenuation step dimension. Digitally managed RFIC attenuators provide a management interface suitable with microcontrollers and supply a very good answer to keep up practical integrity in complicated designs.

Design Configurations

Attenuator ICs could be realized in GaAs, GaN, SiC, or CMOS applied sciences utilizing resistors, PIN diodes, FETs, HEMTs, and CMOS transistors. Determine 1 exhibits three primary topologies that underly numerous kinds of attenuator design configurations: T-type, π-type, and bridged-T networks.

Attenuator

Determine 1. Primary attenuator topologies: (a) T-type, (b) π-type, (c) bridged-T networks.

Fastened worth attenuators make use of those core topologies realized with resistors in skinny movie and thick movie hybrid applied sciences to supply fastened ranges of attenuation.

VVAs usually use a T-type or π-type configuration with a diode or transistor parts operated in a nonlinear resistance area. The resistance traits of the bottom parts are exploited to regulate the required degree of attenuation by various the management voltage.

DSAs often make use of a number of cascaded models representing particular person bits that may be switched in or out to attain the required degree of attenuation. Just a few configuration examples used for DSA designs are proven in Determine 2. They embrace configurations with built-in SPDT switches that toggle enter and output ports with the attenuating pad and a by means of line, switched-scaled machine designs with transistors or diodes used as switchable resistances, switched-resistor configuration the place the resistors could be switched in or out of the circuit, and device-embedded kind design with a transistor or a diode as an integral a part of the design.

Attenuator

Determine 2. DSA design configuration examples: (a) π-type configuration with built-in switches, (b) switched-scaled FET configuration, (c) switched-resistor configuration, (d) FET-embedded configuration.

Attenuator topologies could be organized right into a reflection- or balanced-type design schematically proven in Determine 3. Reflection-type gadgets use equal attenuators related to the output of a 3 dB quadrature coupler and usually provide a big dynamic vary. Balanced configurations mix a pair of equivalent attenuators utilizing two 3 dB quadrature couplers and supply good VSWR and energy dealing with functionality.

Attenuator

Determine 3. (a) Reflection-type and (b) balanced-type attenuator design topologies.

Along with the primary design configurations described on this article, there are different kinds of circuits utilized for realization of IC attenuator elements; nonetheless, their consideration is past the scope of this quick article.1,2

Key Specs

To pick out the precise kind of an attenuator for the top software, an engineer will need to have a very good understanding of its key specs. Other than the attenuation capabilities and a few elementary parameters similar to insertion and return loss, there are different numerous traits used to explain attenuator elements of which the important thing ones embrace:

  • Frequency vary (Hz): the frequencies over which the IC maintains its specified traits
  • Attenuation (dB): the quantity of suppression in extra of the insertion loss
  • Frequency response: variation of the attenuation degree (dB) throughout the frequency vary (Hz)
  • Attenuation vary (dB): the entire worth of attenuation provided by the part
  • Enter linearity (dBm): it’s often expressed when it comes to the third-order intercept level (IP3) defining a hypothetical level for the enter energy degree at which the facility of the corresponding spurious elements would attain the identical degree of the basic part
  • Energy dealing with (dBm): it’s usually described when it comes to the enter 1 dB compression level defining the enter energy degree at which the insertion lack of the attenuator decreases by 1 dB; the facility dealing with attribute is commonly specified for the typical and peak enter energy ranges for the regular state and hot-switching modes
  • Relative section (levels): a shift in section launched to a sign by the attenuator part

Along with these widespread parameters, variable attenuators are additionally described by their switching traits which might be usually expressed in ns when it comes to rise and fall time, on and off time, and the amplitude and section settling time of the RF output sign.

There are additionally particular traits inherent to every kind of variable attenuators.

For VVAs, they’re associated to their analog management operation and embrace:

  • Voltage management vary (V): the voltages required to regulate the attenuation degree throughout the attenuation vary
  • Management traits often expressed when it comes to the attenuation slope (dB/V) and the efficiency curves exhibiting the extent of attenuation as a perform of management voltage

For DSAs, their inherent traits in flip embrace:

  • Attenuation accuracy (also referred to as the state error) (dB): the restrict of variation within the attenuation degree relative to the nominal worth
  • Attenuation step dimension (dB): the delta between any two successive attenuation states
  • Step error (dB): the restrict of variation within the attenuation step dimension relative to the nominal worth
  • Overshoot, undershoot (dB): the extent of sign transients (glitches) throughout state transitions

A great attenuator part is usually required to ship flat attenuation efficiency and good VSWR throughout the operational frequency vary, to supply enough accuracy and energy dealing with functionality, and to make sure clean glitch-free operation with little sign distortions throughout state transitions or to supply linear management attribute.

Conclusion

The broad range of IC attenuator elements is definitely not restricted to solely these mentioned on this article. We will acknowledge different kinds of ICs together with frequency-dependent and phase-compensated attenuators, temperature variable attenuators, programmable VVAs with an built-in DAC, and others. Nonetheless, on this article, we thought of the commonest classes of IC attenuators in addition to mentioned their principal topologies and key specs, which may help an RF designer to decide on the precise part for an finish software.

Analog Gadgets gives the broadest portfolio of built-in RF elements within the business. ADI’s attenuators ICs can be found in a variety of architectures and type components, giving designers the flexibleness to pick out an element that greatest aligns with their system necessities. The elements are designed to supply the best-in-class efficiency and extremely dependable operation to deal with probably the most demanding necessities throughout a variety of purposes in instrumentation, communications, army, and aerospace markets.

Anton Patyuchenko, RF Specialist, Analog Gadgets Inc.

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