I recently watched, on You Tube, a NOVA program about the British school masters and students who were the first to publish the news that the Soviets had started launching satellites from a new launch site back in 1958. (School Boys Who Cracked the Soviet Secret). Early in the program (about 8:35, if you watch it), one schoolmaster asks another what he would need in order to receive and record the transmissions from the early Soviet satellites. He was told three items were needed: a receiver, a tape recorder, and a signal generator.
Why would you need a signal generator? The program is short on technical information.
Although interested in radio, I am not an expert, so please excuse me if I have asked a dumb question.