Arduino RFpainter - Abusing FMCW to Paint Cats on Your Spectrogram

A few years ago, drmpeg wrote a nice gnuradio block which is called gr-painter. This block works very well and I had some fun with it. Now, I don’t have SDR which can transmit anymore, only receive (an el-cheapo RTL SDR dongle), and I want to with gr-painter again, so, I can’t. I have some AD9851 DDS modules lying around, so I decided to implement an FMCW version of gr-painter (because I can’t figure out how to implement OFDM on the DDS module, yet). It won’t be as good as gr-painter does (which is using OFDM) but it’s good enough for me. Sure, the DDS module is controlled using an Arduino UNO and some rather simple code.

Note, FMCW : Frequency Modulated Continuous Wave, OFDM : Orthogonal Frequency Division Multiplexing

Requirements

• Arduino UNO
• AD9851 DDS module
• RTL-SDR dongle (to see your RF output)
• GQRX GitHub repo
• Breadboard and some jumper wires

Precaution

This project will emit radio waves. If you don’t have any proper radio license, please do this at very low power, don’t put any RF power amplifier with it, and do at your own risk!

Principle of Operation

• Cat picture is stored in the Arduino PROGMEM in the form of binary value ASCII art. (white pixel is represented by space and black pixel is represented by alphanumeric)
• This cat ascii-art is stored in the form of 2-dimensional array[height, width].
• This array is scanned from the [height-1, 0] index to [height-1, width-1], continue to [height-2, 0] to [height-2, width-1], and so on until [0, width-1].
• When the array element value is a space, then the DDS AD9850 is programmed to output very low frequency which we don’t care about (in this case, 1 kHz). When the element value is an alphanumeric, then the DDS AD9850 is programmed to output some RF frequency + some offset, proportional to the width position value.
• Then, FMCW is generated.

The detail of how one can program the AD9851 is presented in the datasheet.

Arduino UNO to DDS AD9851 connection:

Very Important: Don’t forget to pull up the DDS D0 and D1 pins to 5V thru a 10k resistor and ground the DDS D2 pin. Otherwise, the serial programming wouldn’t work (please refer to the datasheet, page 15, figure 18).

How to Put Your Own Cat (or other pics)

• Prepare your image, which is black and white.
• Convert your image into an Ascii Art using this tool : http://picascii.com/.
• Replace the non-alphanumeric characters with any alphanumeric.
• Put your Ascii-Art in the Arduino Skecth in the form of a 2 dimensional array and using PROGMEM attribute.
• Write the definitions of your image width and heigth (from the ascii art perspective).
• Call your ascii-art in the drawCat function.

Signal Properties

• Main RF Carrier is at 35.0 MHz (be cautious), configureable using void drawCat(unsigned long in_freq, int scale, int repeat) function.
• FM Bandwidth is adjustable using parameter int scale.
• Scan repetition is alse adjustable using parameter int repeat.

Examples

Arduino-AD9850-Ascii-Art_Cats

This example will run the drawCat function with the scale = 2 and repeat = 2. Using RTL SDR and GQRX, set the RTL SDR center freq at 35.04 (approx.), FFT size = 32768, rate = 20fps, and Time span = auto.

Arduino-AD9850-Ascii-Art_Cats-SLOW

This example will run the drawCat function with the scale = 2 and repeat = 20. Rather usefull to generate more “hi-res” Spectrogram image. Using RTL SDR and GQRX, set the RTL SDR center freq at 35.04 (approx.), FFT size = 32768, rate = 20fps, and Time span = 5min.

Acknowledgement

• Ron Economos, drmpeg who wrote gr-painter for the inspiration.
• Alexandru csete, for the GQRX

Contributing

1. Fork it https://github.com/handiko/Arduino-RFpainter/fork
2. Create new branch (git checkout -b add-blah-blah)
3. Do some editing / create new feature
4. Commit your works (git commit -m "Adding some blah blah blah..")
5. Push to the branch (git push -u origin add-blah-blah)
6. Create a new Pull Request