Frequency modulation–continuous wave method to generate and detect sound beams at 35 GHz

We report on successful application of frequency modulation–continuous wave (FM-CW) technique in sound-echo experiments at microwave frequencies. As an electromagnetic source, a reflex klystron with only 100 mW CW power or even less is sufficient to produce outstanding spectral echo signals in the kHz frequency range using homodyne detection. In contrast to the common pulse-echo technique in the GHz range, there is no need of a high power source like a magnetron and no need of a separate local oscillator that is essential in heterodyne detection. The FM-CW technique seems to be a very promising method for sound experiments at still higher frequencies.