RF Attenuators: basics, types, symbols

RF attenuators have a surprising number of uses: reducing high power signals for test equipment, level control; impedance matching.

RF Attenuators Includes:
Attenuator basics     Attenuator specs     Resistive attenuator design     Attenuator resistor values table     Balanced resistive attenuator pads     Variable PIN diode attenuator     Construction guidelines    

RF attenuators can be used for a variety of different purposes within many RF systems. These RF attenuators can be fixed, switched or even continuously variable.

There are many different types of RF attenuator, but the simpler ones using just resistors can be designed very easily, especially of they are for low power use.

What is an attenuator?

As the name implies RF attenuators reduce the level of the signal, i.e. they attenuate the signal. Typically the attenuation is defined in decibels, and fixed attenuators may be available in a variety of levels.

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 equipment.
  • 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 variable attenuators 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, but an analogue voltage is available - often in feedback loops controlling a level.

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 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 an attenuator symbol that has been widely adopted.

Attenuator circuit symbol

For some applications a variable attenuator symbol may be required:

Variable attenuator symbol showing arrow indicating its variable capability
Variable attenuator circuit symbol

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. One particular use is for testing high power RF signals, e.g. in transmitters where the signal level needs to be reduced before it can be applied to an item of test equipment.
  • 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.

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