I connected the SPI pins on Wayne Holder (10, 11, 12, 13) to the SPI pins in UNO (10, 11, 12, 13). Pin 2 and 9 on Wayne Holder were also connected to pin 2 and 9 in the UNO shield as well. The +5v supply of Wayne holder was also taken from the shield - as shown in the figure below.
I came across the fact that the main difference between UNO and Pro Mini is the operating clock.
I always measure 3.3 volts at the Decawave's SPI pins (SS, MOSI, CLK) even when no 5v is connected to the input of the LLC.
For one of another board, I measure a square wave of 10Hz and 3.3v as well without connecting anything to the input of the LLC.
About the square wave that I get on the second board I soldered - which is another Wayne board, I get something like a square wave at SS and MOSI pins when the input of the LLC is floating - not connected to anything. But when I connect the LLC input to GND the output goes to GND as well.
SS and MOSI are on the inputs of the corresponding LLCs. I don't understand what exactly you measured.
Is there a chance that I damaged the DWM1000 module while soldering it and that's why it behaves funny?