How to match an antenna for a proper current comsumption

[davidgutierrez9]

Posted on January 03, 2017 at 17:53

Hello,

I have a CR95HF chip and I would like to set the TVDD power consumption to set a certain battery lifetime in my system. I used the excel tool to set a proper impedance and the formula to calculate current drained from VPS_TX. The problem is that they do not match.

I measured the impedance of my own antenna with a miniVNA-pro net analyzer. I used the Excel tool to calculate L0, C0, C1 and C2 values for a 27ohm impedance and I assembled tem. The problem is the final measured impedance of the matching+antenna set was ~51ohm. Also, the current measured at VPS_TX was about 37mA, value that corresponds to a impedance of aprox 78ohm.

Since anything is giving the values expected, what should I take as a valid result? How can I measure the final matching+antenna impedance?

Thanks

#antenna #current #impedance

[Henry Crane · Accepted solution]

Posted on January 09, 2017 at 19:03

Hello David,

current total consumption on VPS-TX corresponds to the internal current consumption on RF drivers delivering the 13.56MHz square signals on TX1 and TX2, as well a current consumption of the antenna circuit connected between TX1 and TX2 pins.Equivalent output impedance of the voltage source providing signal between TX1 and TX2 is around 27 Ohm at VPS_TX=3V and 13 Ohm at 5V.In order to help you to figure out things, I made some VPS_TX current measurements on CR95HF RF transceiver board, simply removing the L01 and L02 ferrites as well as the two 330 Ohm resistors in RX path and connecting TX1 and TX2 by 26 Ohm and 13 Ohm.  On the board used for these tests:- at 3V, current consumption on VPS_TX is 56 mA (Rload = 26 Ohms) --> VPS_TX/I=56 Ohms- at 5V, current consumption on VPS_TX is 150mA (Rload=13 Ohms) --> VPS_TX/I=20 Ohms

These results are in line with your measurements even if using resistors (pure real loads) can lead to slightly different results compared to using antenna circuit which is a frequency dependent impedance. Anyway, things are coherent.

the key factor is the input impedance of the antenna circuit. Antenna calculation tool provide theoretical calculation that do take into account parasitics on the board, as well as precision of components. Loop Antenna  impedance measurement is also an important parameter that can influence the result. that's why antenna tuning methodology recommends to adjust the component values by measurement of antenna circuit impedance in the system. To do so, you can connect the mini VNA (after calibration) on TX1 and TX2 with all components mounted on the board. Do not power the board so that all pins remain floating. You should measure the same impedance curve as shown in impedance calculation tool, with maybe a frequency shift or impedance magnitude shift compare to simulation. Using the calculation tool, you can adjust the components to move the measurement curve at the desired value.

best regards,

 NFC/RFID ST support team.