The LIDAR electronic hardware firmware architecture is
The LIDAR electronic hardware / firmware architecture is shown in Fig. 3. The firmware has been implemented in VHDL by using ALTERA Quartus II Macrofunctions.
The FPGA firmware is composed of the following blocks:
A PC, connected as shown in Fig. 2, is used to configure the FPGA board and to acquire the elaborated signals (such as the FFT result). The connection PC-FPGA board is realized by using the USB blaster device and the Altera software tools.
In the following, some considerations about the frequency limitations related to the digital implementation of the detection algorithm are reported. As shown in Fig. 1 (see curve R(t)), it is possible to generate and sample signals with instantaneous frequency up to Fs/2 = 75 MHz. Moreover, aliasing phenomena are possible in the high frequency components CIH and CQH of the product signal C(t).
The LIDAR resolution can be improved by increasing the sweep bandwidth and the chirp rate fr. For this reason and considering the hardware constraint, in the first version of the LIDAR electronic realization the frequency of R(t) has been swept from 0 MHz to 75 MHz. The low pass FIR decimator (decimation factor = 8; cut-off frequency 0.35⋅Fs/8 < ½⋅Fs/decimation_factor) is needed to remove most of the and components. However, being CIH and CQH two chirp signals, during the chirp cox 2 inhibitors there are two short time intervals where residual signal are present, not removed by the filters (see yellow triangles in Fig. 1). The reduction of the FIR cut-off frequency decreases the duration of CIH and CQH residuals, improving the signal to noise ratio (SNR), but on the other hand reduces the maximum measurable range.
Taking into account the decimation factor and to avoid discontinuity of the CL components (due to an FFT window longer than the R(t), with the resulting presence of two or more sequences R(t) in the window) the number of samples for the FFT window is limited to 2048. The chirp laser modulation period has been set to 16,384 samples (corresponding to 109 µs). The characteristics of the used electronic board limit the resolution, because it depends on many parameters (start_freq, stop_freq, sampling frequency Fs etc.). In our first implementation setting the start_freq = 0 and stop_freq = Fs/2 we obtain
On the other hand, it is possible to extend the measurement range by increasing the sweep_duration or reducing the decimation_factor, or by using a more sophisticated laser modulation based on phase-shift keying . The maximum detectable frequency is limited by the cut-off frequency of the FIR decimation filter; in our case, Fs × 0.35 / (decimation_factor ×0.5) is the available output bandwidth. As a consequence, the theoretical range obtained in the developed system is
Table 1 summarizes the main parameters related to the first version of the Fully Digital LIDAR system.
Table 2 reports the Main firmware functions of the CW–IM algorithm. In particular the resource utilization is related to the first version of the LIDAR electronics (FPGA Altera 2S60).
Upgrade of the LIDAR electronics end relative tests The CW-IM digital LIDAR electronics has been recently upgraded to a new version with a more performant proto board. It is based on: Considering the increase of the sampling frequency of the A/D converters (overclocked to 165 Msps), the new architecture does not need the interleaving features. Moreover, while the 2 meter distance resolution has been conserved, minor improvements in the algorithm implementation (start_freq, stop_freq, sweep_duration) have been made. In particular, by increasing the out-of-band rejection of the low-pass FIR and the start_freq, and reducing the stop_freq, the residuals have been drastically reduced. The result is a decrease of the signal noise floor (see Fig. 10) and a consequent improvement of the Signal to Noise Ratio (SNR), particularly in the case of high level echo signals (see Fig. 11). The reduction of the noise level increases the ability to detect small amplitudes echoes, while the increase of SNR allows the detection of simultaneous electrical echoes having very different amplitudes.