Meeting with real-world signals
The next figure dissects one anti-collision segment (REQA-ATQA) captured at a 500 MHz sampling rate using the handmade antenna shown in part 1. The lower part shows the corresponding Fast Fourier Transform FFT for the PCD-to-PICC REQA command and the PICC-to-PCD ATQA response. One thing to note on this figure is the relatively high level obtained for the backscatter signal (load modulation). It was possible by adjusting the antennas arrangement for best results.
For estimating the signal’s bandwidth we can use the results from the FFT. The SciLab script snipped below calculates and displays the FFT for the PICC-to-PCD segment shown in the above figure. The variable segREQA.PICCtoPCD.Data was previously loaded with the appropriate data vector. The data vector was extracted from the data set downloaded from the DSO.
Another useful tool for estimating the time-frequency behavior of signals is baudline. This free and open source Linux tool uses digital signal processing techniques for signals spectrogram calculation and visualization. See below the calculated spectrogram for the signal segment shown above. It was calculated using the Kaiser Window and a transform size of 4,096 points. Several harmonics are clearly visible too.
Air interface signal demodulation
One clear advantage we have in NFC-A air interface signals processing is all the information comes in the carrier’s amplitude. For extracting the PCD-to-PICC baseband signal and the PICC-to-PCD sub-carrier we can simply use the classical ASK demodulation approach: Rectification + Band-pass filtering + Thresholding .
See below for some results. Note the usage of a leaky pick detector with independent attack and decay constants for generating the binarization threshold. These constants can be fine adjusted to reduce the effect introduced by the overshoot peak.
A similar procedure (High-pass filter + Rectification + Low-pass filtering + Thresholding ) can be used for extracting the PICC-to-PCD signal when encoded in Manchester OOK like depicted below. The encoded bit stream is shown in red color. Note this method needs some modifications for higher bit rates (fc/64, and above) where BPSK/NRZ-L encoding is used. Looking at “Figure 2: Example PICC to PCD communication signals for Type A and Type B interfaces ” in ISO/IEC-14443-2 you will surely find the solution.