Circuit: DIY RF Sweep Generator

Discover how a Raspberry Pi, a DDS module, and an ADC come together to form a powerful DIY sweep generator for RF testing. This compact setup lets you generate frequency sweeps and visualize circuit responses with ease.

The Circuit

A sweep generator (or “wobbulator”) is a test instrument used with an oscilloscope to measure the frequency-response characteristics of an RF circuit. It works by feeding a voltage-controlled oscillator (VCO) with a ramp or sawtooth waveform, causing the output frequency to sweep across a preset range. The resulting response of the circuit under test (typically a filter or amplifier) appears on the oscilloscope. Sweep generators are commonly used for aligning the intermediate-frequency (IF) stages of superheterodyne receivers, but they are also useful for evaluating the frequency response of RF filters and other circuitry.
  “While in the dim past a wobbulator was a complex, expensive all-analog instrument (even with vacuum tubes), today we have little computers like the RPi to do the job with simpler hardware when it comes to control, and with the luxury of software, which can be changed and optimized for best results,” noted the designer, Tom Herbison.
  This project replicates the functions of a traditional sweep generator by combining a Raspberry Pi with a Direct Digital Synthesizer (DDS) module and an Analog-to-Digital Converter (ADC) module. The Raspberry Pi’s GPIO interface is programmed to control the DDS module for generating the frequency sweep and to communicate with the ADC module to capture the circuit’s response. A graphical user interface (GUI) enables the user to set the sweep parameters and view the resulting measurements.

The DDS module is based on the AD9850 frequency synthesizer chip.

"Like the MCP3424 the DDS module operates under control of the RPi, which issues all control words necessary for a swept-frequency output signal that appears on the RF OUT connector," Herbison noted. "This signal is fed to the circuit under test, while the output of the circuit under test goes to the LOG or LIN input, depending on the application."

The DIY RF Sweep Generator Project

The original article, “A Raspberry Pi Wobbulator,” appeared in Elektor July/August 2015. You can read the article for free during the two-week period following the publication of this post. Enjoy!

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Editor's Note: This article first appeared in a 2015 edition of Elektor. Given the project’s age, some components might not be readily available. Still, we believe the design will inspire you to start a project of your own in the near future.