uSDR-pico

This Git repository contains a Micro-SDR implementation, based on a RP2040 Pi Pico. The project is highly experimental, foremost intended to investigate how the Pico HW and SDK work with an application like this. Also it is a platform to experiment with digital signal processing techniques. The repo contains the code for an experimental implementation of the control and signal processing for a QSD/QSE based transceiver. Furthermore, the repository contains the electronic design of some modules that cover the mixing, filtering and RF amplification.

The ZIP files contain a consistent package, but the latest code is in the files in the main directory.
Please refer to the doc folder for a full description.

The platform used is a Pi Pico module with an RP2040 processor. This processor has dual cores, running default at 125MHz each, and a very configurable I/O which eases the HW design. The software is distributed over the two cores: core0 takes care of all user I/O and control functions, while core1 performs all of the signal processing. The core1 functionality consists of a TX-branch and an RX-branch, each called from a function that waits for inter-core FIFO words popping out. This happens every 16usec, because on core0 a 16usec timer callback ISR pushes the RX/TX status into that FIFO. Hence the signal processing rythm on core1 effectively is 62.5kHz.
On core1 the three ADC channels are continuously sampled at maximum speed in round-robin mode. Samples are therefore taken every 6usec for each channel, maximum jitter between I and Q channels is 4usec, which has a negligible effect in the audio domain.

The TX-branch

• takes latest audio audio sample input from ADC2 (rate = 62.5 kHz),
• applies a low-pass filter at Fc=3kHz,
• reduces sampling by 4 to get better low frequency response Hilbert xform (rate = 15.625 kHz),
• splits into an I-channel 7 sample delay line and a Q-channel 15-tap Discrete Hilbert Transform
• scales, filters and outputs I and Q samples on PWM based DACs, towards QSE output

The RX-branch

• takes latest Q and I samples from QSD on ADC0 and ADC1 (rate = 62.5 kHz)
• applies a low-pass filter at Fc=3kHz,
• reduces sampling by 4 to get better low frequency response Hilbert xform (rate = 15.625 kHz),
• demodulates, e.g. SSB: -- applies 15-tap DHT on Q channel and 7 sample delay on I channel -- subtracts I and Q samples
• scales, filters and outputs audio on an PWM based DAC, towards audio output

On core0 the main loop takes care of user I/O, all other controls and the monitor port. There is also a LED flashing timer callback functioning as a heartbeat.

The Pico controls an Si5351A clock module to obtain the switching clock for the QSE and QSD. The module outputs two synchronous square wave clocks on ch 0 and 1, whith selectable phase difference (0, 90, 180 or 270 degrees). The clock on ch2 is free to be used for other goals. The module is controlled over the i2c0 channel. The display is a standard 16x2 LCD, but with an I2C interface. The display is connected through the i2c1 channel, as well as the bus expanders for controlling the various relays.

Open issues:

• take care of processing cycles, by moving signal processing parts to the second core
• add some more filtering
• implement the user I/O bit: LCD+Rotary encoder+buttons
• implement AGC
• implement LSB
• implement AM
• SW based VOX
• implement RSSI
• design a set of PCBs
• sort out the new HW modules
• improve speed: better dual-core management for memory and timer
• improve speed: overclock processor 2x or so
• add control for new HW: BPF and pre-amp/attenuator switching

Installing and using the SDK for Windows:

Please refer to https://github.com/ndabas/pico-setup-windows/releases where the latest installer can be downloaded (e.g. pico-setup-windows-0.3-x64.exe).
Execute the installer to set up the SDK environment, e.g. in ~/Documents/Pico (let's call this folder $PICO).
You can upgrade the SDK to the latest version by replacing the complete $PICO/pico-sdk folder with the newer version. The latest version is on Github: https://github.com/raspberrypi/pico-sdk (download code as zip, extract the pico-sdk-master folder from it, rename it to pico-sdk and use it to replace the original)

Building uSDR-pico:

Clone/copy the uSDR-pico code files into a subdirectory: $PICO/uSDR-pico
Create the build folder: $PICO/uSDR-pico/build

Before the first build you need to check and adapt the file $PICO/uSDR-pico/CMakeLists.txt, using your favourite editor, to make sure it reflects your own directory structure. Also in this file, select whether you want stdio to use the UART on pins 1 and 2 or the USB serial port. The monitor terminal is on stdio. This is needed because CMakeLists.txt directs CMake in the creation of your make environment. In fact, every time you change something in CMakeLists.txt (like adding another source file to the build) you will have to delete the build folder and re-issue CMake.
In $PICO/ you will find a command to start a Developer Command Prompt for Pico (DCP, like a "DOS box"), make sure to use this one instead of any other DOS box. Within this DCP all environment settings have been properly set to enable the building process.
In the DCP window, chdir to the build folder and execute: cmake -G "NMake Makefiles" .. (do not forget the trailing dots, it points to the folder containing CMakeLists.txt).
Now you have initialized the make environment (for nmake) and by executing nmake in that same build folder, all SDK libraries and finally the Pi Pico loadable file uSDR.uf2 will be created.
Rebooting the Pico while the bootsel button is pressed will open a file explorer window with the Pico shown as a Mass Storage Device (e.g. drive E:). Moving the binary to the Pico is as easy as dragging and dropping this uf2 file into that MSD.

Releases:

Stable packages are archived in zip files. The source files in the root folder are newest and could be used to replace files from the zip archive. There are pre-built UF2 files for three display types, which could be tried. However, there are too many differnt types and addresses, so it is better to build a fresh one for your own implementation.
The PCB files have been made with Eagle 5.11, and can be modified or otherwise re-used when needed. The CAM files for each board are packaged in separate zips, these can be used as-is to order PCBs.

Background

The folder $PICO/docs also contains some manuals, of which the C-SDK description, the RP2040 datasheet and the Pico Pinout are absolute must-reads when you start writing software.
For calculating filters I have used the free software from Iowa Hills (http://www.iowahills.com/8DownloadPage.html)
I also used the online FIR filter calculator T-Filter (http://t-filter.engineerjs.com/)

Copyright notice

The code and electronic designs as well as the implementations presented in this repository can be copied and modified freely, for non-commercial use. Use for commercial purposes is allowed as well, as long as a reference to this repository is included in the product.

See also my Wave Form Generator project.