Arduino RF Network Analyzer

What do you get when you combine a direct digital synthesis (DDS) chip, a power detector, and an Arduino? [Brett Killion] did make that combination and wound up with a practical network analyzer.

The project uses an Analog Devices AD9851 DDS chip clocked at 180 MHz which will output a sine wave at any frequency from 0 Hz and 72 MHz. A Butterworth low pass filter processes the DDS signal and then feeds a two-transistor amplifier. The circuit will output about 0dBm into 50 ohms. The power detector is an Analog Devices AD8307 along with a 50-ohm input load. There is no filtering on the power detector so it can measure from very low frequencies to 500MHz.

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A Slew of Open-Source Synthesizers

Hackaday reader [Jan Ostman] has been making microcontroller-based DIY synthesizers for quite a while now. Recently, he’s opened up the source for a lot of them so that you can play along at home. All of these virtual-analog synths and soundmakers can be realized on an Arduino or AVR ATmega328 if you happen to have one lying around.

Extra parts like a keyboard, some pushbuttons, or some potentiometer knobs to twiddle won’t hurt if you’d like to make something more permanent or more obviously playable, like [Jan] does. On the other hand, if you’d just like to get your feet wet, I’ve tweaked his code to be more immediately plug-and-play. The code is straightforward enough that it’s a good learning platform. So let’s take a quick tour through three drum machines and a string synth, each of which you can build on a breadboard in just a few minutes.

To install on an Arduino UNO, fetch the zip file from this GitHub repository, and move each subfolder to your Arduino sketch directory. You’re ready to play along.

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$40 Antenna Analyzer with Arduino and AD9850

If you are a hacker, you might consider ham radio operators as innovative. Most people, however, just see them as cheap. So it is no surprise that hams like [jmharvey] will build an antenna analyzer from a DDS module and an Arduino instead of dropping a few hundred dollars on a commercial unit. As he points out, you probably only need an analyzer for a day or two while you set up an antenna. Unless you are a big time antenna builder, the unit will then sit idle on the shelf (or will wind up on loan to hams even cheaper than you are).

The design is rooted in another proven design, but changed to take advantage of parts he happened to have on hand. Although the build is on a universal circuit board, [jmharvey] used Eagle to lay out the circuit as though it were a PCB. Since placement can be important with an RF circuit, this isn’t a bad idea. It’s always easier to move stuff around on the screen than on the perf board.

Since this is a no frills, unit, you are expected to grab the output from the Arduino and manually put it in a spreadsheet to plot the results. There is another version of the Arduino code that drives an OLED screen, although you still need a PC to kick the process off. One interesting feature of the Arduino code is how it deals with the nonlinear nature of the diodes used in the circuit. After plotting the values with known loads, [jmharvey] broke the diode operation into three regions and used different equations for each region. Even so, he warns that readings higher than 1:1 VSWR are only accurate to 10% or 20% – still good enough for ham shack use.

If you want an antenna analyzer for $40 (or less, if you have a good stock of parts) this looks like a worthwhile project. If, however, you want to repurpose it to Rickroll your neighbor’s AM radio, you might want to go with the commercial unit.

Click past the break to see the analyzer in action.

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Scope Noob: Microcontroller Quirks with DDS

In this installment of Scope Noob I’m working with Direct Digital Synthesis using a microcontroller. I was pleasantly surprised by some of the quirks which I discovered during this process. Most notably, I had a chance to look at errant triggers solved by using holdoff and a few timing peculiarities introduced by my use of the microcontroller. Here’s a video synopsis but I’ll cover everything in-depth after the break.

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Direct Digital Synthesis (DDS) Explained by [Bil Herd]

One of the acronyms you may hear thrown around is DDS which stands for Direct Digital Synthesis. DDS can be as simple as taking a digital value — a collection of ones and zeroes — and processing it through a Digital to Analog Converter (DAC) circuit. For example, if the digital source is the output of a counter that counts up to a maximum value and resets then the output of the DAC would be a ramp (analog signal) that increases in voltage until it resets back to its starting voltage.

This concept can be very useful for creating signals for use in a project or as a poor-man’s version of a signal or function generator. With this in mind I set out here to demonstrate some basic waveforms using programmable logic for flexibility, and a small collection of resistors to act as a cheap DAC. In the end I will also demonstrate an off-the-shelf and inexpensive DDS chip that can be used with any of the popular micro-controller boards available that support SPI serial communication.

All of the topics covered in the video are also discussed further after the break.

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Digital “Crystal” Breathes New Life into Old Radio

[Bill Meara] of the Soldersmoke Podcast has a nice old Drake 2B radio, and wanted to use it for the 12 meter amateur band. These old radios normally make switching tuning bands easy — you just swap out one frequency crystal for another and you’re set.

Only [Bill] didn’t have the 21 MHz crystal that he needed. No problem, because he had a junk crystal, a hacksaw, and a modern direct-digital synthesis (DDS) chip sitting around. So he takes the donor crystal, cuts it open, and solders the two wires directly from the DDS to the crystal’s pins. Now he’s got a plug-in replacement digital oscillator that doesn’t require modifying the nice old Drake receiver at all. A sweet little trick.

The video’s a little bit long, but the money shot comes in around 5:00.

Now, one might worry about simply plugging a powered circuit (the DDS) in place of a passive element (the crystal), but it seems to work and the proof of the pudding is in the tasting. We wonder how far this digitally-controlled-analog-receiver idea could be extended.

An Open Source 1MHz Arbitrary Waveform Generator with an Awesome UI

1MHZ DDS

 

[Herp] just shared a nice 1MHz Arbitrary Waveform Generator (right click -> translate to English as google translation links don’t work) with a well designed user interface. His platform is based around a PIC32, a TFT module with its touchscreen and the 75MHz AD9834 Direct Digital Synthesizer (DDS). Of course the latter could generate signals with frequencies up to 37.5MHz… but that’s only if two output points are good enough for you.

As you can see in the video embedded below, the ‘tiny dds’ can generate many different kinds of periodic signals and even ones that are directly drawn on the touchscreen. The offset and signal amplitude can be adjusted using several operational amplifiers after the DDS ouput and a separate SMA TTL output is available to use a PIC32 PWM signal. The platform can read WAV audio files stored on microSD cards and also has an analog input for signal monitoring. Follow us after the break for the video.

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