VHBR timing requirements can be difficult to reach. If the antenna is not able to fulfill these requirements, the maximum datarate will be impossible to achieve. Very High Baud Rate (VHBR) technology allows the exchange of large amounts of data between a contactless smart card and a reader. These data are stored on the smart card itself, and the exchange is initiated by a simple swipe of the card within the proximity of the reader. Faster data rates create potential for new applications of NFC. For example, e-Passports and health care cards can carry high-quality images, that can be quickly shared between a tag and a reader.
In addition, within production facilities, card personalization time can be drastically reduced when downloading data using the VHBR technology.
VHBR ASK technology specification allows:
- Communication speeds up to 6.8Mbits/s (instead of 848kbits/s),
- Improved communication speed when combined to frames size of 4kBytes
Increasing the communication data rate implies reducing the rise and fall times to respect PCD ASK modulation (defined in 14443-2:2016).
What are the challenges of these new timing requirements?
Figure 1. Timing requirements at 6.8 Mbits/s
If the antenna bandwidth and quality factor (Q-factor) are not compliant with VHBR requirement, the highest data rates will be impossible to reach:
Figure 2. VHBR modulation waveform
Figure 3. Modulation waveform timing paremeters for a bit rate of fc/4
On an antenna with a high quality factor, the rising and falling times are typically too long to reach the required duration of the VHBR rising and falling edges.
Figure 2 shows a rising and falling edges measurement at a 106kbits/s data rate on a High-Q antenna. The signal sent to the antenna has been generated with the fastest slope available. The falling and rising edge duration is entirely due to the antenna.
This VHBR specific issue means the following items are important to test on any VHBR compliant device:
- PCD capability to accurately generate VHBR signals
- PICC capability to correctly decode such signals, which implies that a test tool should be able to generate these*
How is this implemented?
When changing the impedance matching network, the antenna bandwidth and Q factor will be modified.
By reducing the Q factor and thus increasing the bandwidth, the falling and rising edges can be significantly reduced. Figure 3 shows the same measurement as Figure 2 but on a modified antenna.
Figure 4. Relationship between Q Factor and Bandwidth
Antenna Q factor and bandwidth can be checked by measuring the antenna S11 parameter:
Figure 5. High-Q antenna S11 measurement using the MP500 TCL3
On a High-Q antenna, the bandwidth is lower. It allows reliable communication at low data rates:
- Better signal quality
- Better noise immunity
- Less energy required to generate the same RF field level
On Low-Q antenna, bandwidth is higher (up to 6.8MHz). A bandwidth around 3.3MHz is a good compromise, as such an antenna will be able to communicate using data rates from 106kbits/s to 6.8Mbits/s:
Figure 6. Low-Q antenna S11 measurement using the MP500 TCL3
Micropross can help design optimized antennas for any form factor that are able to communicate efficiently at high data rates.
Figure 7. Example of VHBR specific antenna design
Pre-equalization of test tool signal
In order to reduce its rising and falling times and boost signal modulation, the MP500 TCL3 output signal can be pre-equalized.
The signal output by the MP500 TCL3 can be modified point by point, with a timing resolution of 1/10fc. Once the signal is generated to the antenna, it is immediately acquired by the same MP500 TCL3 on its antenna (the same or another one). Then the characteristics of the new signal are printed, and output waveform shape is adjusted accordingly:
Figure 8. MP500 TCL3 signal pre-equalization window
Pre-equalization consists of changing the waveform shape to match the desired characteristics.
Signal characteristics can be:
- Edge monotonicity
- Rising and falling slopes
Figure 9. High-Q antenna modified to improve high data rate Figure 10. High-Q antenna modified to improve high data rate, with pre-equalization
To know how Micropross can help you with your VHBR projects, please contact us !