Attenuator

Also known as: attenuator, RF attenuator, pad

An attenuator (or pad) is a passive network that reduces a signal’s power by a fixed or adjustable number of decibels while keeping the source and load impedance-matched.¹ Where an amplifier adds gain, an attenuator deliberately throws gain away — most often to stop a strong signal from overloading a sensitive stage such as a receiver’s analog-to-digital converter.

A resistive Pi pad drops the signal by a set number of decibels while presenting a matched impedance at both ports.

Overview

An attenuator is defined by three things: its attenuation in dB, its characteristic impedance (usually 50 Ω), and its power handling. Ideally it reduces every frequency by the same amount (flat response) and stays well matched, so it can be dropped anywhere in a 50 Ω line without causing reflections. Because it is purely resistive, the “lost” power is dissipated as heat inside the pad.

How it works

A fixed pad is a small resistor network — typically a Pi (π) or T arrangement of three resistors — chosen so that looking in from either port the impedance is still 50 Ω while a set fraction of the power reaches the output. The dB value maps directly to a power ratio: 3 dB halves the power, 10 dB cuts it to one-tenth, 20 dB to one-hundredth. See decibel for the ratio math. Because the network is symmetric and matched, attenuators can be cascaded (a 10 dB and a 20 dB pad give 30 dB) and reversed.

Variants

Fixed pads — a single value (3, 6, 10, 20, 30 dB), the workhorse form.
Step attenuators — switchable in discrete steps (e.g. 1 dB steps to 31 dB) for bench and automated use.
Variable / continuously adjustable — including voltage-controlled and digital step attenuators used in automatic-gain-control loops.
Power attenuators — heat-sinked units rated to absorb transmitter-level power.

In practice — protecting the receiver

The most common SDR use is overload protection. A receiver has a finite dynamic range: its noise floor at the bottom and, at the top, the point where its front end compresses or its ADC clips (full-scale, expressed in dBFS). In a strong-signal environment a modest pad brings the whole spectrum down out of the clipping region, trading a little sensitivity for freedom from the intermodulation spurs that overload creates. An attenuator is also handy for calibrating levels, protecting a spectrum analyser input, and setting the optimum drive into a mixer.

Relevance to SDR

Attenuators are one of the simplest and most useful accessories for SDR reception. Many RTL-SDR and other dongle users add a switchable pad or a step attenuator when listening near powerful transmitters, because a receiver driven into clipping produces phantom signals across the band that no amount of software can remove. Some SDRs include a built-in switchable attenuator or a variable-gain front end that serves the same purpose under software control.

GopherTrunk contains no attenuator — it is software downstream of the ADC. But attenuation choices upstream directly shape the samples it decodes: too little and strong-signal clipping corrupts the I/Q stream; too much and weak signals sink into the noise floor. GT works best when the front-end gain and any external pad are set so the wanted signal sits comfortably inside the ADC’s range.

Sources

Attenuator (electronics) — Wikipedia, on resistive pad topologies, dB values, matching, and power handling. ↩