An NFC device, or a label or tag, as it is called, contains a series of specific subblocks
that allow the tag to generate its own magnetic field and thus actively modulate the
antenna loop. It is composed of a reference clock, an extractor of the external clock
(carrier frequency of the reader) in order to recover it and a frequency-phase tracker
that performs tracking operation of the frequencies and phases of both clocks. In this
way, the synchronous or asynchronous operation is obtained. Both phase mismatch and
drift largely depend on the transfer of data from the tag (protocol and the duration of
the data transmission). The structure of a tag that actively modulates the field and
transmits data can become extremely complex; therefore, research work to synchronize
the frequency and phase of ALM modulation with a reader’s carrier is key for achieving
compliance with existing and future standards which address an active transmission.
The challenge is to prepare the system for ALM modulation in a way that would
comply with current standards and future requirements, where a higher speed of communication and increased reliability is expected. The problem with ALM modulation
is the inability to completely synchronize the tag’s locally modulated signal with the
reader carrier. This is because the carrier signal of the reader is obscured by the signal
of the tag’s active modulation and thus it is difficult to observe the carrier frequency
of the reader during the modulation and data transmitting procedure. Consequently,
the reader receives the signal with a phase shift and, for longer transmission sequences,
also with a phase drift, the result of which is a reduction of the transmitted data
amplitude on the reader antenna or even zero amplitude. The transmission of the tag
data becomes asynchronous, as it is almost impossible to completely and permanently
synchronize the frequency of the local oscillator and the reader carrier.
We investigated ALM solutions for the synchronization problem with a weak reader
carrier in the background of the strongly modulated tag carrier frequency. A possible
solution would be partial synchronization of carrier frequencies within tolerances and
full phase synchronization at the beginning of the transmission sequence. Phase synchronization can be performed continuously with a parallel digital loop, which reduces
the delay that would otherwise be present in stable analog loops. Digital implementations
are based on the development of digital phase interpolators (PI), time to digital
converters (TDC), phase selectors (PS), etc. The key sub-blocks for accurate frequency
and phase tracking are phases and frequency lock loops composed of appropriate detectors,
frequency dividers, voltage-controlled low noise oscillators, phase selectors, phase
interpolators, etc.
We looked for solutions in high-speed tracking of the frequency and phase, in the
stability of the selected frequency and phase, and in the fast setting of the correct
phase during data transmission. We have introduced parallel digital processing of phase
tracking and an interference measurement of the phase difference between the received
weak non-modulated reader carrier and the transmitted phase-modulated carrier. We
introduced phase interpolation and differential modulation of an active transmission.
These measures partially correct the transmission frequency spectrum, reduce the effect
of phase drift during transmission, and further reduce noise by means of digital filtering.
Simulations and measurements have shown that homodyne demodulation is a suitable
way to demodulate the interference signal and calculate the phase drift on the test
device even at a low ratio of the signal of the tested device and the reader. With
the homodyne technique, the phase can be controlled and calculated, as the technique
demodulates the signal by signal processing procedures.
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