*2.3. Frequency Discrimination Using Envelope Detection for Proposed FSK Radar*

The discrimination of phase-difference signals at two different operating frequencies is the most important process for implementing the FSK radar. However, the signals at two frequencies are not continuous by the FSK operation, and the sampling points at each frequency are not identical. An envelope detection method can discriminate two nonoverlapped signals with different DC offsets at each frequency, because the offset characteristics of the radar components depend on the operating frequency [16]. This method is implemented using spline interpolation with not-a-knot conditions over local maxima separated by 10 samples. The baseband signals can be discriminated in each frequency by using the envelope detection method, but it cannot be known whether the discriminated signal shows anything of the two frequencies. The control signal for frequency switching in the signal generator is used to store each data-set of *I*/*Q* channel signals with each discriminated frequency in four data-sets. In the initial state, the stored data-set is regarded as the signals at *f* <sup>1</sup> when the control signal is high, and the data-set is regarded as the signals at *f* <sup>2</sup> when the control signal is low. The frequency is adjusted to ensure that the distance obtained from each data-set is positive. Figure 3a,b show the baseband *I*/*Q* signals for each frequency extracted from the raw data by the envelope detection method.

**Figure 3.** Proposed envelope detection method for discriminating the baseband signals at each operating frequency of the FSK radar. The frequency-control signal of the signal generator, which is indicated by red and blue shaders in the waveform, is used for synchronous data acquisition: (**a**) In-phase channel signals; (**b**) quadrature channel signals.
