Discussion of precise time and frequency measurement
The PTS250 manual is available from mediafire as either a 55MB standard pdf file or a 41MB OCR pdf file.....
https://www.mediafire.com/file/v8vd4o33d7ta1rd/PTS250.pdf
https://www.mediafire.com/file/5hp2hj1la7wws2s/PTS250%20OCR.pdf