Quick introduction to Secondary Surveillance Radar and how it works. After explaining the absolute basics, I am showing how one can simulate transponder responses using off-the-shelf test equipment. RTL1090 is used to verify the generated signals as valid.
If you want the arb waveform file for the 7777 Mode A response, here it is:
https://baltic-lab.com/wp-content/uploads/2015/12/SQUAWK_7777_IDENT.zip