Beat Frequency Oscillator gave license!

In 1980s, I used to listen to the conversations of Amateur Radio Operators (HAMs) as a Short Wave Listener (SWL), after tuning in 40 meters band of my transistor radio receiver. While I was able to hear most of the operators, I could not get the voices of some which were heard only as an unintelligible squabble! Later I was informed by an operator that it is a type of radio transmission called Single Side Band (SSB) and that many HAMs can be accommodated in the band by using it. He also gave me an idea to hear that SSB transmission also by using another pocket short wave band transistor radio near that main radio. It worked well and used it with wonder. Next, the operator gave me a circuit diagram for assembling a special device for that. The device’s name is Beat Frequency Oscillator (BFO). I assembled the BFO myself, though in a crude form! With this BFO wonder, I listened to HAMs, and it paved the way for me to take up the examination for getting the Amateur Radio Operator’s License and pass in it!