Red Pitaya Blog

Home » , »

Being on Air with Red Pitaya

When we (Walter Sandel (DD8UU) and Ulrich Habel (DF4IAH) saw the hardware specs of this grand concept we were joking about how fun would it be to have fun with this thing and build a radio station. We didn’t realize, this was the first step to our Red Pitaya project!

So, after the initial idea was born on September 2015, just a few months later, we built the transmitter. The main voice modulation variants were running and we thought for Xilinx FPGA to do the main work.

At that point we set the driver software to turn the knobs and switches for setting FPGA registers. However, FPGA makes all the work without CPU intervention ever after.

The first stage

The first stage is up and running for radio amateur, as well as for students to play around with modulation and frequencies. At the front-end the user is able to connect the RF output 1, RF output 2 and the LED bar to any point internal of the FPGA structure. This allows easy understanding and gives the ability to progress to high frequency technologies and modulation

The state on December, 31st of 2015 is shown on the block diagram. It shows the input modulation from the bottom side coming up the MIC_MUXer and the MIC_GAIN amplifier. From there on it depends on modulation, which blocks are excited and streaming
data up to the carrier mixer. More details will be presented in the following blogs.


In case you lack an external modulation source, the MOD_OSC can optionally be a signal source for the modulation variants: AM, FM and PM. For SSB that oscillator is used for the weaver beat frequency of 1700 Hz.

The road map

The road map is planned like that:
– adding a receiver part, also
– adding digital signal encoders and decoders
– possibly with multi signal handling
– all 48 kSPS audio signals should be connected with the Linux kernel sound system as sources and sinks

How does that feel like? Here is a new generation of enthusiastic radio-HAMs and engineers @ work.

Let’s have a quick view above the shoulder of the experimenter:

Here we see SSB modulation at work. The purple and the cyan signals represent the I/Q-signals before being added with each other. The middle trace show the sum of both with results to the USB (upper-side-band).

The student can get in touch with modulation and radio frequencies and have fun with experimenting.

The next image shows a spectrum of a typical AM modulation:

Here, the spectrum shown is based on the purple trace. The RadioBox allows to have both RF output lines connected to the same internal signal, also.

FM modulation

Let’s look into FM modulation. Due to the modulation signal the carrier is deviated around the middle frequency what the connected scope would show us, indeed.

The current implementation of the RadioBox transmitter contains a FIR-filter for signal conditioning. See how such a filter could look like. More is possible for future enhancements.

The world of high frequncies is at your finger tips. Let’s play and have fun!


Ulrich Habel, DF4IAH
Walter Sandel, DD8UU

Who is Ulrich Habel?

Ulrich Habel is radio-communication system engineer. He works as a RF laboratory assistant at Mannheim high school, helping students get familiar to magic of high frequencies.

Subscribe to our newsletter

Related Posts