A 30 dB 5W RF attenuator, DC–18GHz, with N-type coaxial connectors
Electronic symbol
100 Watt power attenuator

An attenuator is an electronic device that reduces the power of a signal without appreciably distorting its waveform.

An attenuator is effectively the opposite of an amplifier, though the two work by different methods. While an amplifier provides gain, an attenuator provides loss, or gain less than 1. An attenuator is sometimes referred to as a "pad" in certain fields.

Construction and usage

Attenuators are usually passive devices made from simple voltage divider networks. Switching between different resistances forms adjustable stepped attenuators and continuously adjustable ones using potentiometers. For higher frequencies precisely matched low VSWR resistance networks are used.

Fixed attenuators in circuits are used to lower voltage, dissipate power, and to improve impedance matching. In measuring signals, attenuator pads or adapters are used to lower the amplitude of the signal a known amount to enable measurements, or to protect the measuring device from signal levels that might damage it. Attenuators are also used to 'match' impedance by lowering apparent SWR (Standing Wave Ratio).

Attenuator circuits

π-type unbalanced attenuator circuit
π-type balanced attenuator circuit
T-type unbalanced attenuator circuit
T-type balanced attenuator circuit

Basic circuits used in attenuators are pi pads (π-type) and T pads. These may be required to be balanced or unbalanced networks depending on whether the line geometry with which they are to be used is balanced or unbalanced. For instance, attenuators used with coaxial lines would be the unbalanced form while attenuators for use with twisted pair are required to be the balanced form.

Four fundamental attenuator circuit diagrams are given in the figures on the left. Since an attenuator circuit consists solely of passive resistor elements, it is both linear and reciprocal. If the circuit is also made symmetrical (this is usually the case since it is usually required that the input and output impedance Z1 and Z2 are equal), then the input and output ports are not distinguished, but by convention the left and right sides of the circuits are referred to as input and output, respectively.

Various tables and calculators are available that provide a means of determining the appropriate resistor values for achieving particular loss values. One of the earliest was published by the NAB in 1960 for losses ranging from 1/2 to 40 dB, for use in 600 ohm circuits.[1]

Attenuator characteristics

An RF microwave attenuator

Key specifications for attenuators are:[2]

RF attenuators

Radio frequency attenuators are typically coaxial in structure with precision connectors as ports and coaxial, micro strip or thin-film internal structure. Above SHF special waveguide structure is required.

Important characteristics are:

The size and shape of the attenuator depends on its ability to dissipate power. RF attenuators are used as loads for and as known attenuation and protective dissipation of power in measuring RF signals.[3]

Audio attenuators

A line-level attenuator in the preamp or a power attenuator after the power amplifier uses electrical resistance to reduce the amplitude of the signal that reaches the speaker, reducing the volume of the output. A line-level attenuator has lower power handling, such as a 1/2-watt potentiometer or voltage divider and controls preamp level signals, whereas a power attenuator has higher power handling capability, such as 10 watts or more, and is used between the power amplifier and the speaker.

Component values for resistive pads and attenuators

This section concerns pi-pads, T-pads and L-pads made entirely from resistors and terminated on each port with a purely real resistance.

Reference figures for attenuator component calculation

This circuit is used for the general case, all T-pads, all pi-pads and L-pads when the source impedance is greater than or equal to the load impedance.
The L-pad computation assumes that port 1 has the highest impedance. If the highest impedance happens to be the output port, then use this figure.
Unique resistor designations for Tee, Pi and L pads.

The attenuator two-port is generally bidirectional. However, in this section it will be treated as though it were one way. In general, either of the two figures applies, but the first figure (which depicts the source on the left) will be tacitly assumed most of the time. In the case of the L-pad, the second figure will be used if the load impedance is greater than the source impedance.

Each resistor in each type of pad discussed is given a unique designation to decrease confusion.

The L-pad component value calculation assumes that the design impedance for port 1 (on the left) is equal or higher than the design impedance for port 2.

Terms used

Symbols used

Passive, resistive pads and attenuators are bidirectional two-ports, but in this section they will be treated as unidirectional.

Symmetric T pad resistor calculation

see Valkenburg p 11-3[4]

Symmetric pi pad resistor calculation

see Valkenburg p 11-3[4]

L-Pad for impedance matching resistor calculation

If a source and load are both resistive (i.e. Z1 and Z2 have zero or very small imaginary part) then a resistive L-pad can be used to match them to each other. As shown, either side of the L-pad can be the source or load, but the Z1 side must be the side with the higher impedance.

see Valkenburg 1998, pp. 11_3-11_5

Large positive numbers means loss is large. The loss is a monotonic function of the impedance ratio. Higher ratios require higher loss.

Converting T-pad to pi-pad

This is the Y-Δ transform[5]

Converting pi-pad to T-pad

This is the Δ-Y transform[5]

Conversion between two-ports and pads

T-pad to impedance parameters

The impedance parameters for a passive two-port are

It is always possible to represent a resistive t-pad as a two-port. The representation is particularly simple using impedance parameters as follows:

Impedance parameters to T-pad

The preceding equations are trivially invertible, but if the loss is not enough, some of the t-pad components will have negative resistances.

Impedance parameters to pi-pad

These preceding T-pad parameters can be algebraically converted to pi-pad parameters.

Pi-pad to admittance parameters

The admittance parameters for a passive two port are

It is always possible to represent a resistive pi pad as a two-port. The representation is particularly simple using admittance parameters as follows:

Admittance parameters to pi-pad

The preceding equations are trivially invertible, but if the loss is not enough, some of the pi-pad components will have negative resistances.

General case, determining impedance parameters from requirements

Because the pad is entirely made from resistors, it must have a certain minimum loss to match source and load if they are not equal.

The minimum loss is given by[4]

Although a passive matching two-port can have less loss, if it does it will not be convertible to a resistive attenuator pad.

Once these parameters have been determined, they can be implemented as a T or pi pad as discussed above.

See also


  1. ^ NAB Engineering Handbook, Table 9-3 Resistive Pads (PDF) (5th ed.). National Association of Broadcasters. 1960. pp. 9–10.
  2. ^ "Attenuators, Fixed | Keysight (formerly Agilent's Electronic Measurement)". Retrieved 2018-08-31.
  3. ^ About RF attenuators Archived October 30, 2013, at the Wayback Machine – Herley General Microwave
  4. ^ a b c Valkenburg 1998, pp. 11_3
  5. ^ a b Hayt & Kemmerly 1971, p. 494