Variable filter (tuner)

The filter has a window whose center frequency can be changed continuously. The bandwidth of the filter is critical: if the bandwidth is very narrow, there are many signals rejections and to build a good probability curve it is necessary to stay for a long time on each frequency; if the band is broadened, the precision of the measure decreases. This measuring system is very effective, but unfortunately highly inaccurate and relatively slow. It can be considered an excellent tool for tuning and diagnosis of the initial motion, but this is not the system to be chosen for routine measurements.

4.3.7.2 Frequency tracker

This measurement system was the first to be sold in complete systems anemometers. The working principle is illustrated in the block diagram of Figure 4.21: the input signal is mixed with the signal from an oscillator

whose frequency is a function of voltage applied to its input (voltage controlled oscillator, VCO). The two overlapping signals pass through the narrow filter and are directed to a frequency or phase discriminator, which works by comparing the phases of the two signals and that, through an integrator, generates an output signal in the form of a voltage, proportional to the difference in frequency, that is sent to the VCO. The output frequency of the VCO is then adjusted to remove this difference and is therefore a measure of the Doppler frequency.

This is a slave filter, which adapts automatically to the frequency of the signal. To use the frequency tracker properly, the signal should be continuous (very large concentration of particles). The problem that the signals are discrete is solved by a simple form of memory that allows, when the signal is not present, the oscillation frequency of the VCO to be set on the last reading. The typical problem of this system is that it fails to follow the fluctuations of turbulence beyond a certain limit value, the theoretical maximum permissible intensity of turbulence for a frequency tracker is 0.33.