NFC Controller Clock Alignment Using Selective PLL Field Locking
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Solution Overview
Problem
NFC controllers operating in active load modulation (ALM) card mode face challenges in ensuring phase alignment of the local clock with the field clock during transmission, as using the same antenna for both transmission and reception is impractical, and relying on a dedicated accurate reference frequency is costly or unavailable.
Innovation Solution
A clock alignment module that includes a phase locked loop (PLL) with a digitally controlled oscillator and frequency divider, which selectively locks the local controller clock signal to the field clock signal only during longer gaps between transmit bursts, using a transmit envelope unit to determine when the antenna is sufficiently damped to sense the field signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a voltage controlled oscillator in a phase locked loop is left free running during the whole transmission, then the device complexity is reduced, but the phase stability deteriorates significantly
Solution Approach 1:
The phase locked loop dynamically switches between free-running mode and locked mode based on the transmit envelope detection. During transmit bursts, the PLL is decoupled from the antenna to allow free running; during receive periods, it locks to the field signal. This dynamic adaptation resolves the contradiction by allowing phase instability only when necessary for transmission, while maintaining stability when receiving.
Solution Approach 2:
The system periodically locks the PLL to the field signal during receive periods interspersed between transmit bursts. This periodic locking action maintains phase alignment without requiring continuous locking, thus achieving phase stability while allowing the oscillator to run freely during transmission periods.
2Reliability
If a dedicated external highly accurate reference frequency is introduced, then the phase stability is improved, but the manufacturing cost increases
Solution Approach 1:
The system uses the field signal itself as the reference for phase locking during receive periods. Instead of requiring an external highly accurate reference frequency, the NFC controller leverages the existing field signal to maintain phase alignment, thereby achieving phase stability without additional costly reference components.
Solution Approach 2:
The same antenna is used for both transmitting and receiving functions. During receive periods between transmit bursts, the antenna picks up the field signal which serves dual purposes: it provides the reference for phase locking while also being the signal that would be received in normal operation. This eliminates the need for separate reference frequency sources.
3Device complexity
If the same antenna is used for both transmitting and receiving, then the device complexity is reduced, but the measurement precision deteriorates due to inability to determine field clock during transmission
Solution Approach 1:
The system alternates between transmit bursts and receive periods. During the receive periods between transmit bursts, the antenna is used to detect the field signal and determine the field clock for phase alignment. This periodic switching allows accurate field clock measurement without requiring separate transmit and receive antennas.
Solution Approach 2:
The system performs field signal detection and phase alignment preparation during the receive periods before the next transmit burst begins. This preliminary action ensures that the phase locked loop is properly synchronized and ready for transmission, achieving accurate field clock determination without interfering with the transmit function.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution allows for phase alignment without the need for a dedicated accurate reference frequency, reducing material costs and maintaining transmitter performance by selectively updating the phase information during longer bursts, thus ensuring accurate phase alignment during transmission.
Implementation Method 1
a phase locked loop, PLL, configured to selectively lock the phase of the local controller clock signal to the phase of the field clock signal
Implementation Method 2
a transmit envelop unit configured to determine whether a time since an end of a latest transmit burst exceeds a threshold, Tdelay
Data Source
AI summary
Disclosed as a clock alignment module for a near field communication, NFC, controller operable in active load modulation, ALM, card mode, the module being operable during a transmit mode comprising transmit bursts and comprising: an input for receiving a field clock signal (CLK_FIELD); an output for outputting a local controller clock signal (CLK_FB); a transmit envelop unit configured to determine whether a time since an end of a latest transmit burst exceeds a threshold, Tdelay; and a phase locked loop, PLL, configured to selectively lock the phase of the local controller clock signal to the phase of the field clock signal, in response to the time exceeding the threshold and a next transmit burst not having started. Associated NFC controllers, integrated circuits and methods are also disclosed.