RF Attenuator Basics & Tutorial

- attenuators are a useful building block within RF design where they reduce the signal level while providing an accurate match for the characteristic impedance used.

RF attenuators are a universal building block within the RF design arena.

RF attenuators can be fixed, switched or even continuously variable.

Dependent upon their type, they can be designed using just resistors, they may need a switch, either mechanical or solid state, or they may use diodes to make them continuously variable over a given range.

What is an attenuator?

As the name implies RF attenuators reduce the level of the signal, i.e. they attenuate the signal.

This attenuation may be required to protect a circuit stage from receiving a signal level that is too high. Also an attenuator may be used to provide an accurate impedance match as most fixed attenuators offer a well-defined impedance, or attenuators may be used in a variety of areas where signal levels need to be controlled.

There are many used for these RF attenuators and although these may not seem obvious initially when asking what is an attenuator, they are widely used in RF applications.

Types of RF attenuator

Attenuators can be categorised in a number of ways according to their capabilities and the technologies they use.

  • Fixed RF attenuator:   As the name implies fixed attenuators have a specific value and this cannot be changed. They may come in a variety of formats from small in-line items in a similar format to connector adaptors to those in small boxes with connectors on the ends to those incorporated within equipments.
  • Switched RF attenuators:   Switched RF attenuators are widely used in test systems where levels may need to be changed. They are often seen as small boxes with a number of switches, typically with switches for 1, 2, 4, 8, etc dB changes. Switched attenuators may also be found in items of test equipment to change the levels, for example of a signal generator output.
  • Variable RF attenuators:   variable RF attenuators are normally used in applications where it is necessary to continuously vary the level of a signal. Typically they provide a continuous level change by varying an analogue voltage on the input control line. They are normally used where accuracy is not a prime requirement.

There are a number of ways in which attenuators can be designed and made. The two main types are given below.

  • Resistor RF attenuators:   Resistor attenuators are used to provide fixed levels of attenuation. Levels may be varied by switching in different attenuator sections to provide the levels that are required.
  • PIN diode RF attenuators:   PIN diode attenuators are normally used in attenuator designs where a continuously variable level is required.
  • FET RF attenuators:   FET attenuators are normally used in attenuator designs where a continuously variable level is required. Like a PIN diode attenuator, FET attenuators use an analogue control voltage to control the level of attenuation.

These are only broad categories for RF attenuators - they can be categories in a variety of ways according to the application and the type of attenuator technology that is used.

RF attenuator schematic symbol

It is often necessary to show the block symbol in a circuits schematic diagram for an attenuator without necessarily showing all the individual components.

There is a symbol that is widely adopted as the RF attenuator schematic symbol.

The circuit schematic symbol for a fixed RF  attenuator used in reducing the level, typically of RF signals
Fixed RF attenuator circuit symbol

For some applications a variable attenuator may be required. There is a circuit symbol for this:

The circuit schematic symbol for a variable attenuator where the level of attenuation can be changed according to the requirements
Variable RF attenuator circuit symbol

RF attenuator specifications

When designing, purchasing or using an RF attenuator it is necessary to be able to specify it to ensure that an attenuator with the correct performance is obtained. While some of the major specifications are detailed below, for some applications other parameters may need to be specified.

  • Attenuation:   This is the primary specification for an RF attenuator. It is the ratio between the output and the input power levels and is typically quoted in decibels (dB).
  • Attenuation accuracy :   It is often necessary to know the accuracy of the level of the attenuation of the attenuator. Particularly in applications where equipment is being tested, the attenuation accuracy is likely to be important. In these cases a tolerance on the nominal level of attenuation will be given.
  • Frequency response:   The level of attenuation of an attenuator will vary with frequency. This can result from the frequency dependence of the resistors or other components used in the RF attenuator, or where coupling between the input and output may exist as this will be frequency dependent. Some RF attenuators where the absolute level of attenuation is important may be provided with calibration charts measuring the absolute attenuation at different spot frequencies over a frequency band.
  • Impedance :   RF attenuators will be designed for use in systems with a given characteristic impedance. 50 ohms is the most common, although 75 ohms is also used, and it may be possible to obtain RF attenuators with other impedance values should the need arise.
  • Power dissipation:   In order to reduce the signal level, RF attenuators dissipate or absorb the unwanted power. For many small signal applications, power dissipation is not an issue, but for other applications where signal levels are higher, it is necessary to ensure that the RF attenuator will satisfactorily be able to handle the power levels anticipated. Power capabilities for RF attenuators may be quoted in Watts (or milliwatts) or as dBW - decibels relative one Watt (or dBm - decibels relative to a milliwatt)>
  • Mechanical details of the attenuator:   The mechanical details may include aspects such as the size and weight. The connectors may be included in this area of the attenuator specification.
  • Environmental details:   Many applications for attenuators are for use within benign conditions such as a laboratory environment. Environmental conditions would not be an issue. However for some applications it is possible that an environmental is required to detail factors such as vibration, temperature, humidity and the like.

RF attenuator applications

RF attenuators are used in a wide variety of applications in RF circuits. They are a key building block used in many areas of RF design:

  • Reduce signal level:   The basic concept behind an attenuator is to reduce the signal level. This can be required to control levels within a circuit to keep them within the required range.
  • Improve impedance match:   By its very nature an impedance matched RF attenuator will improve the impedance match. This can be very useful when driving RF mixers that are match sensitive and their performance will be degraded if a poor match is seen.
  • Variable level control:   RF attenuators can be used for level control on the output of items such as signal generators. It is far better to be able to generate an accurate fixed level from the basic generator and then used switch attenuators to reduce the signal to the required level.

RF attenuators are used widely within RF circuits for a variety of reasons. Fortunately these attenuators are easy to design and using surface mount technology their performance can be exceedingly good.

By Ian Poole

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