RF Attenuator Basics and Tutorial

- for use in radio frequency circuits including receivers and transmitters, etc

RF attenuators includes:

    •  RF attenuator basics
    •  Resistor attenuator
    •  Attenuator resistor values
    •  Balanced attenuator
    •  Variable PIN diode attenuator
    •  Building / construction of RF attenuators

RF attenuators are a universal building block within the RF design arena. As the name implies RF attenuators reduce the level of the signal. This may be required to protect a stage from receiving a signal level that is too high, 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.

Types of RF attenuator

RF 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 sued 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 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.

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