ATTENUATORS

Attenuators, also know as pads, are allot simpler than they may seem. Though attenuator can mean a variety of things, when referred to in an RF or Microwave system it is a broadband resistive device that drops the amplitude of the signal going in by the designated amount, generally given in dB. Attenuators are most commonly designed for 50 ohm systems since the vast majority of RF and Microwave systems, outside of the cable TV and video industry, are 50 ohm systems. Attenuators are designed to maintain the impedance of the system into which they are connected. If terminated in a 50 ohm termination, an attenuator will provide a 50 ohm input impedance. If terminated in some other impedance different from 50 ohm the impedance will deviate from 50 ohms but will be closer to 50 ohms than the original termination. This is actually one of the common applications of attenuators, improving missmatches which improves VSWR.

One common form of attenuator looks electrically like a PI network shown below:

Resistors R1 and R3 are equal for a standard attenuator. For very small attenuation values R1 and R3 approach infinite resistance and R2 approaches 0 ohms. For very large attenuation values, R1 and R3 approach 50 ohms and R2 approaches infinite resistance. In between very small and very large attenuation values are more practical values but in all cases when terminated in 50 ohms it will read 50 ohms on the other side. There are very simple equations that are used to derive the values for these resistors.

If we express the attenuation as a ratio of input to output voltage and call it A then we have:
A = V_out / V_in and the attenuation or loss is given as: L(dB) = 20 X Log A

To calculate for R₁, R₂ and R₃ for an attenuator in a 50 ohm system:
R1 = 50(1+A)/(1-A) and R₃ = R₁
R2 = 50R₁(1-A)/(A(50 + R₁))

Take a common value of 3dB; R1 and R3 would be 292.4 ohms and R2 would be 17.6 ohms. If you add a 50 ohm resistor across one end and calculate the voltages you will get a .7071 drop in voltage and impedance looking into the attenuator would be 50 ohms. The .7071 drop in voltage, when squared, results in a power drop of .5 which corresponds to 3 dB.

The configuration shown above is frequently implemented with discrete resistors for lower frequency board level designs but is implemented with chip level resistor networks for higher frequency applications. Microwave frequency attenuators are commonly supplied in small connectorized assemblies.

The 50 ohm impedance of these devices can easily be calculated for low frequencies but at higher microwave frequencies the actual impedance is impacted by physical construction and the dimensions within the device. The most common way to characterize these devices at microwave frequencies is by use of VSWR which indicates how well the port matches 50 ohm based on reflected power from a 50 ohm transmission line.