Coaxial Attenuators

Fixed Coaxial Attenuators

Attenuators are linear, passive transmission line components designed to reduce the input power in a matched system by a predetermined amount. Fixed coaxial attenuators are intended for laboratory, production, testing and system use. Typical applications include VSWR reduction at the insertion point of measurement setups, range extension of power meters, RF path loss simulation, and isolation or power level reduction of signal sources.

  • An attenuator reduces an input signal to a lower level.
  • The amount of attenuation is specified in decibels (dB). Decibel values are additive for cascaded attenuator sections.
  • A fixed impedance attenuator is matched to source impedance ZI and load impedance ZO. For RF equipment, Z is most commonly 50 Ω.
  • dB from power ratio: dB = 10 log10(PI / PO)
  • dB from voltage ratio: dB = 20 log10(VI / VO)

Standard dB Values

dB Power Ratio   Voltage Attn. Factor
3 0.5 1/2 1.4x
6 0.25 1/4 2x
8 0.16 1/6.25 2.5x
10 0.10 1/10 3.2x
14 0.04 1/25 5x
20 0.01 1/100 10x
30 0.001 1/1000 100x

Important characteristics are:

  • Frequency range
  • Attenuation
  • VSWR
  • Average and peak power handling capability
  • Operating temperature range

All attenuators are production tested to ensure proper operation within published specifications. Therefore, over specification by the user is unnecessary and reduced cost-effectiveness. If the frequency range requirements are 2 to 4 GHz, over-specifying flat performance to 12.4 GHz will increase component cost with no performance advantage.

Attenuators are available in a variety of connector series. The attenuator elements are designed to provide broadband operation with low frequency sensitivity and extremely stable operation at temperature extremes.

Attenuation

The term attenuation is generally used rather loosely and often means insertion loss or, to be even more specific, characteristic insertion loss. Insertion loss is the ratio of the power delivered to a matched load by a matched generator before and after the insertion of a component in the line. Insertion loss is actually a combination of two losses: mismatched loss (reflective) and attenuation (dissipative). Mismatch loss is the ratio of power that would be absorbed by the device if it were perfectly matched to the actual power absorbed by the device with its mismatch in impedance. Attenuation is the ratio of power into a component to the power out under perfectly matched conditions and represents the actual power dissipated within the component. Where a component is perfectly matched to the line and load, the mismatch loss is zero, and the attenuation and insertion loss are the same.

Voltage Standing Wave Ratio (VSWR)

VSWR is the ratio of the maximum to the minimum voltage of a standing wave. Ideal is a figure of 1:1 which means that 100% of the incoming signal passed through the component without any reflection. In that case, there would be no standing wave. A 2:1 VSWR (or mismatch) means that 12% of the incoming signal was reflected.

Average Power

The maximum average power is the maximum specified input power applied for a minimum of one hour at a specified operating temperature (25° C for Cal Test fixed coaxial attenuators) with the output terminated in the characteristic impedance which will not permanently change the specified properties of the attenuator after return to ambient temperature at the power level 20 dB below the maximum specified input power.

Derating is necessary if the attenuator is operated at higher temperatures. See derating curve and specifications.

Peak Power

The maximum peak power at a specified pulse width and average power which when applied for a minimum of one hour while the output is terminated in the characteristic impedance will not permanently change the specified properties of the attenuator. The pulse width used to test Cal Test attenuators is 5 microseconds.

Temperature Coefficient

The maximum change of insertion loss in dB per °C from 20° C over the maximum operating temperature range. To obtain the delta dB, multiply the temperature coefficient by the value in dB and by the temperature change from 20° C in °C. Cal Test’s attenuator’s coefficient is 10-4 dB/dB/°C.