Resistive attenuators (also known as "pads") are two-port passive networks consisting entirely of resistance elements. They are usually used between sections of a system where signal levels need adjusting or impedances need to be matched. Since they are made up of pure resistive elements, they are inherently broadband and are widely used between parts of a system where impedance levels can change with frequency (antennas or amplifiers, for example). The downside, as you might surmise from the name, is that they also attenuate the signal. The greater the attenuation, the better the isolation, and less load variability is seen by the driving system. The tradeoff, then, is one of isolation vs. signal attenuation through the pad (sometimes called "insertion loss").
There are three basic pad configurations: PI, T and L. The designation stems from the shape formed by the resistor symbols when the pad diagram is drawn in schematic form. Inputs and outputs are Ports A and B in the diagrams.
The choice of whether to use a PI or T configuration in any application largely comes down to the values of the resistors involved. More convenient values may derive from one or the other.
Besides matching unequal impedances, the PI and T pads can be designed for equal output and input impedances. When this is the case, any degree of attenuation can be realized with these pads. If input and output impedances are unequal, there is an upper limit on how much attenuation can be achieved. L pads can only be designed to match unequal impedances, however.
H and O pads are simply balanced versions of the T and PI pads, respectively, where the series resistances are divided equally between the two series arms of the pad.
One of the most useful applications of resistive attenuators is to reduce the effect of impedance mismatches between parts of a system. When a pad is inserted between a signal source and a mismatched load impedance, the SWR is lowered and the return loss as seen by the generator is improved (that is, increased) by twice the value of the attenuation of the pad. The improvement can be dramatic when considered from a SWR viewpoint.
For instance, suppose we have a 50-ohm source which we want to connect to a load impedance of 200 ohms. This would result in a 4:1 SWR, or a return loss of 4.44 db. By inserting a nominal 6 db pad the return loss in increased by 12 db, to 16.44 db, and the corresponding SWR is lowered to 1.18:1. Of course, this is desirable only if the loss of signal amplitude can be tolerated. Click here to see spread sheet calculations for a range of load parameters and the isolating effects of a variety of pad sizes.
When the source and load resistances are known, the computer program available here calculates resistor values for a variety of resistive pad configurations and for any desired value of attenuation. You can have the results displayed as exact values, or with 1% or 5% tolerance resistors. If you ask for non-exact values the maximum attenuation error will be displayed.Click here to download the resistive pad calculator program