Spectrum transform the process shown in Figure 3,F said that analogue frequency,the actual frequency signal,digital frequency f said that the sampling frequency of the normalized frequency.The center frequency of 70 MHz,10 MHz bandwidth of the analog signal (Figure 3 (a) below) sampling frequency of 80 MHz,amplitude and frequency characteristics are Figure 3 (b) shows the signal,the signal from the sampling signal prior to 80 MHz be a cycle extension,the map was drawn spectrum of the signal cycle.Figure 3 (c) of the digital downconversion charted after the signal,the left 10 MHz frequency spectrum.The low-pass filter after only reservation near zero-frequency signals in Figure 3 (d) shows.To ensure the signal from four times after Aliasing-Free,Low-pass filter must be installed digital bandwidth of 1 / 16.4 times the signal from Figure 3 (e) shown,we can see that the figures signal a wide spectrum four times.Noting the sampling of the signal,the signal is frequency-domain spectrum in reverse,the spectrum will be taken after a further reversal and restore the OFDM baseband spectrum,in Figure 3 (f) as shown.
Receiver address some of the core DSP and FPGA receive M Baud rate reduced to 20 of the base band signal,the signal synchronization,channel estimation,balanced and demodulation,thus completing the receiver functions.
Digital Downconversion technology is the construction of programmable digital radio receiver structure of a key technology.It allows broadband signals in the IF bandpass sampling of the practical applications,systems concern of the spectrum signal can be accurately sampled signals from the isolated and under to the baseband frequency.