Joe W3JDR asked on the IO Groups BITX20 list for CAD details for a board layout for the Blue Pill” STM32F103 processor. Members quickly discovered that Joe has a blog with details of his hacks to the µBITx that would be of wider interest to constructors.
Check out https://w3jdr.wordpress.com/ for a range of mods including:
- audio preamp (Q70) dynamic range improvement by substitution of a resistor
- a digital sampling S Meter
- a slide rule type colour display for frequency
- an amazing front panel for his µBITx
- a high resolution optical encoder
“I’ve been busy behind the scenes doing a lot of things with the control system. I now have the entire app running on an STM32F103 “Blue Pill” board, which is only about $2.50 on Ebay. This gives me a 70 mHz 32 bit controller with 128K bytes of program memory.
“My software S-meter is now working quite well and only requires 2 resistors and a cap, plus some code. I’m using a separate ILI7735 display for the S-meter, both displays on the same SPI bus. The S-meter is derived from a 10kHz sample-rate of the pre-amp audio, with software peak detection. The TFT meter display has a max-hold pointer that resets every few seconds, while the main pointer is real-time. There are digital readouts on it for peak signal level in uV, dBm and S-units.
“The measurements from the S-meter will drive a digipot after the audio pre-amp to effect a feedforward AGC. Feedforward, with software calibration, should make for a very fast acting AGC without the overshoot/undershoot artifacts of feedback systems.
“I’m also using a 400 ppr optical encoder for frequency control with interrupt processing; it tunes beautifully smooth, with 1 hZ steps and software acceleration. . All of this puts a real strain on the little Nano u-controller, hence the move to the Blue Pill.
“I intend to layout a new Raduino that accepts the ‘pill’, with extra connectors for SPI and I2C busses. I developed my own tinier version of the Adafruit Si5351 board that will mount on the Raduino. I might even use a separate Si5351 for the main VFO in order to eliminate the crosstalk spurs generated in the single-chip approach. This was all moving smoothly while I was house-bound during Winter, but will slow down while the weather’s nice outside.”