A sampling FM wide-band demodulator useful for laser Doppler velocimeters

A sampling FM demodulator of large bandwidth, consisting of a swept filter and an oscilloscope, is described. The sampled FM function is reconstructed and displayed on an oscilloscope. For a detection bandwidth of 100 MHz and a spectral resolution of 3 MHz (3 percent) the temporal resolution is as high as dv/dt = 1014s-2= 100 MHz/µs in the case of a periodic modulation signal (e.g., flow oscillations). Measurements at an SNR as low as -40 dB, referred to the detection bandwidth, are still possible. Transient FM's can be investigated at a reduced temporal resolution of dv/dt = 3 MHz/µs. The system transfer characteristic is linear to better than 0.5 percent over the whole range and is not affected by the random AM of the Doppler signal. A theoretical analysis and experimental investigations of the system and its application are given.