Attenuators (aka pads) are useful for many things and are easy to make. The ones I've made are known as "pi" attenuators because the resistors are arranged in the shape of the Greek letter π.
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| Pi attenuator |
There are numerous websites around that can calculate the resistor values needed for a given value of attenuation and impedance (I work exclusively at 50 Ω). Just be sure to consider the power rating of the resistors (especially R1 since it bears the brunt of the dissipation at high attenuation values). Also, for RF projects you need to use resistors with low inductance values (ie. don't use wirewound) to minimize high frequency issues. One site that I found useful allows you to enter your parameters into an Excel spreadsheet (power-attenuator-calculator).
I built a fixed 20dB attenuator (10W input) and two switchable attenuators (1W input) using metal film resistors. I honestly spent more time and effort on the enclosures than the actual circuits themselves. Once assembled they were easy to check out. After terminating the output with a 50 Ω dummy load, I simply applied a known DC voltage to the input (being careful not to exceed the power rating of the input resistors) and measured the output voltage. Plugging those two values into the equation below gave the expected attenuation:
attenuation (-dB) = 20 * log (Vout/Vin)
