Attenuator Circuit Designs: Passive to Programmable
Key Takeaways
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Passive attenuators use resistor networks for signal reduction without power, while active attenuators can include components like MOSFETs and PIN diodes for adjustable attenuation levels.
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Fixed attenuators provide a constant level of attenuation; step attenuators offer precise control with pre-calibrated steps; continuously variable attenuators allow for manual adjustment; programmable attenuators are computer-controlled for dynamic adjustments.
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When designing an attenuator, it's crucial to consider whether it will be balanced or unbalanced, the configuration (T, Pi, L, H, or O), and the specific application needs, such as frequency range, attenuation level, and power handling. Online tools can assist in calculating resistor values for desired attenuation amounts.
Passive resistor-divider attenuators. Top: L, T, and π-type unbalanced attenuator circuits. Bottom: T, and π-type balanced attenuator circuits, also known as H and O, respectively.
An attenuator reduces the magnitude of signals while maintaining their waveform. This two terminal circuit block is mostly used after signal generator circuits, ensuring signals are at an appropriate strength before feeding into antenna circuits. The simplest version is constructed with resistors, but can come in various forms, including fixed attenuators, which offer a constant level of attenuation, and variable attenuators, which allow for an adjustable attenuation level. Read on as we delve into attenuator design notes for your own devices.
Attenuator Designs and Key Features
|
Type |
Description |
Key Features |
|
Fixed |
Features a resistor network set to a specific attenuation value. |
- Surface mount, waveguide, or coaxial designs - Use of thermally conductive resistive substrates in chip-based designs |
|
Step |
Similar to fixed attenuators, but includes a push-button to select from pre-calibrated steps. |
- Also known as switched RF attenuators - Available in chip, waveguide, or coaxial formats - Facilitates precise control with increments like 1, 2, 4, 8 dB |
|
Continuously Variable |
Allows manual adjustment within a specified range. |
- Utilizes solid-state elements (MOSFETs, PIN diodes) for finer resolution instead of resistor networks - Adjusting the voltage across FET or current through diode allows for finer control over attenuation |
|
Programmable |
Controlled via an external computer signal for adjusting attenuation. |
- Also referred to as Digital Step Attenuator - Uses TTL logic circuits for step sizes ranging from 2 to 32 - Operates on voltage levels to control switches |
|
DC Bias |
Features capacitors at both input and output ports to block DC voltages. |
- Allows RF signals to pass while blocking DC voltages - Serves dual functions of attenuation and DC isolation |
|
DC Blocking |
Blocks DC signals without providing an alternative path to the output port. |
- Similar to DC Bias type but completely blocks DC signals |
|
Optical |
Designed for light waves, uses various methods to achieve attenuation without altering the waveform. |
- Available as fixed, variable, or programmable - Fixed types use doped fibers for dispersion - Variable and programmable types mirror the functionality of their RF counterparts |
Attenuator Design Notes
The functionality of an attenuator circuit design is characterized by its linearity and reciprocity. Depending on its specific application, an attenuator can be designed to be either unidirectional or bidirectional. For symmetric designs, there is no discernible difference between the input and output ports, allowing for uniform signal reduction regardless of the signal direction.
Passive Attenuators
Passive attenuators are resistor networks organized into a voltage divider network to achieve the desired attenuation amount. In the attenuator design process, consider whether you require your attenuator as balanced or unbalanced. Specifically, attenuators that are part of coaxial lines are generally unbalanced, while those used with twisted-pair cables are usually balanced.
There are several common layouts for attenuators, including the T, Pi, and L configurations, which are considered unbalanced types. For balanced applications, the T and Pi configurations transform into the H and O configurations, respectively. The key difference between these types lies in their circuit symmetry: balanced configurations are symmetrical, allowing for even signal distribution and management, whereas unbalanced configurations are asymmetrical, designed for specific directional signal flow. You can use online attenuator resistor-value calculators to find a combination of resistors that meets your desired attenuation amount.
Left: Attenuator symbol. Right: Variable attenuator symbol
Attenuator Design Parameters
In attenuator design, specific care should be taken to meet the design requirements, usually measured in the following parameters.
Attenuator Design Parameters and Description
|
Parameter |
Description |
|
Frequency Range (Hz) |
The spectrum of frequencies over which the IC consistently maintains its specified operational characteristics. |
|
Attenuation (dB) |
The level of signal suppression beyond the basic insertion loss introduced by the component. |
|
Frequency Response |
The variation of the attenuation level (dB) across the attenuator’s operational frequency range. Usually large near infinite for purely passive attenuators but non-infinite for continuously variable attenuators or any that use active components. |
|
Attenuation Range (dB) |
The total amount of attenuation that the component can provide. |
|
Input Linearity (dBm) |
Defined by the third-order intercept point (IP3), representing the input power level at which the power of spurious components equals the power of the fundamental signal. |
|
Power Handling (dBm) |
Described by the input 1 dB compression point, indicating the input power level at which the component's insertion loss decreases by 1 dB. |
|
Relative Phase (degrees) |
The phase shift introduced to a signal by the attenuator component. |
|
Switching Characteristics |
Relevant only for variable attenuators, these parameters include rise and fall time, on and off time, and the amplitude and phase settling time of the RF output signal, typically expressed in nanoseconds (ns). |
Variable Attenuator Design Parameters
Variable attenuators can either be voltage variable attenuators (VVAs) with analog control or digital step attenuators (DSAs) controlled digitally.
- VVAs allow for precise, continuous adjustment within a specified range, ideal for applications requiring fine control; such as automatic gain control circuits and calibration corrections.
- DSAs, suitable for integration with microcontroller-based systems, achieve desired attenuation levels through cascaded units representing bits that are digitally switched.
VVAs are characterized by their voltage control range, indicating the voltages necessary to adjust attenuation levels, and control characteristics detailed by the attenuation slope (dB/V) and performance curves, which depict attenuation as a function of control voltage. DSAs, on the other hand, are defined by several additional key parameters:
- Attenuation accuracy (state error in dB), which is the permissible variation from the nominal attenuation level.
- Attenuation step size (dB), the difference between successive attenuation levels.
- Step error (dB), the allowable deviation in attenuation step size.
- Overshoot and undershoot (dB), representing signal transients during state changes.
Cadence AWR Software Supports Attenuator Design
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