RP2040-based RF sweep/alignment generator

This uses a Raspberry Pi Pico (or other RP2040 board) to generate a configurable-frequency crystal-locked RF signal


Prerequisistes:
Python installed on host computer 
MicroPython "mpremote" tool on host 
Pi Pico with MicroPython 

1) Install MicroPython on the Pico (see https://www.raspberrypi.com/documentation/microcontrollers/micropython.html)
2) Ensure python 3 is installed in your host computer's command line (https://www.python.org/)
3) Install the MicroPython "mpremote" tool (see https://docs.micropython.org/en/latest/reference/mpremote.html)
Note: to run mpremote on my system I needed to type "python -m mpremote" instead of just "mpremote" after running pip


Installing the code on the Pico:
1) Copy the code file: mpremote cp classygen1.py :boot.py
2) Reset the Pico: mpremote reset


Hardware connections:
GPIO pin 16 is RF signal out. This is pin 21 on the Pico (bottom right when viewed from above)
GPIO pin 17 is a sync output when performing sweeps. This is pin 22 on the Pico (next to the previous pin)
The third pin from either edge on either of the Pico's connector is a valid ground conection


To use:
1) Access REPL on Pi: either run "mpremote" without options or connect to the virtual COM port using PuTTY or another terminal emulator program.
2) Output a fixed frequency (specify in kHz, in this case 4.5MHz):
outputClock(4500)
Some frequencies are not possible due to PLL limitations, these will result in errors.
3) Specify a frequency sweep: here center frequency 24MHz, width 8MHz, step of 250kHz
gen.freqList = freqRange(24000, 8000, 250)
4) Start the sweep:
gen.start()
Every 1ms the frequency will update to the next in the list. When the first frequency is output the sync pin will output a high voltage
If a frequency in the sweep cannot be output it will automatically be removed from the list.
5) Verify the frequencies in use:
gen.freqList
6) Stop the frequency scan before outputting a single frequency:
gen.stop()


Known shortcomings:
Frequencies above 100MHz will likely crash, consider making use of the square wave's harmonics if necessary
Low frequencies are not available with the current code. I tihnk 313kHz is probably the limit
The output is temporarialy disabled when stepping between frequencies.
The raw output neither impedance matched nor attenuated. An external circuit may help here.