CR95HF VPS_TX current vs matching impedance

[davidgutierrez9]

Posted on November 17, 2016 at 14:54

Hello,

I would like to design a system with a controlled consumption in the RF side of the CR95HF. I have read different application notes and made some calculations using your Excel matching file, but still have some doubts. 1. The current can be calculated as I_tx = (VPS_TX) / (Zout + Zmatching)? 2. The datasheet says its value is between 70mA and 100mA for 3V output voltage operation. It means an impedance between 30 and 42 ohms, but the recommended is 27. Where does this 100mA values come from? 3. What is the maximum current I can drive to the antenna? 4. Should I set the operating voltage (3V/5V) via register or it is automatically set with the voltage level at VPS_TX? Thank you #cr95hf

[Peter Boringer]

Posted on November 25, 2016 at 17:25

hello,

Not sure to have all the answer, but equation  is correct I_tx = (VPS_TX) / (Zout + Zmatching) I don't know mutch about the current but for sure the Zmatching impedanc should be higher than 27 ohms not below. And you will not be able to change via register the output power it is linked to the  voltage level at VPS_TX Hope it will help Regards

[Peter Boringer · Accepted solution]

Posted on December 08, 2016 at 11:25

To add info on:

1)The antenna circuit connected between TX1 and TX2 (including the tuning circuit + RX circuit + the antenna) receives a square voltage between VPS_TX* Zload/(Zload+Zout) and – VPS_TX*Zload/’Zload+Zout).

This signal is AC and no more DC. So when looking at voltage and current values from antenna circuit side, all calculation must be done considering AC signals.  

At VPS_TX=3V, Zout = 27ohm which leads to a square voltage between +1.5V and -1.5V.

This signal is a square signal at 13.56MHz. The magnitude of the sinusoidal 13.56MHz component of this signal is given by 1.5V*4/pi=1.9V.

As a consequence, the AC current at 13.56MHz flowing into Zload is ITX1-TX2 = 1.9V/27=70mA. For RF calculations on antenna side, only signal part at 13.56MHz are considered.

2)

VPS_TX, DC current consumption includes all currents sink on TX1/TX2 (at 13.56MHz and all other harmonics) in addition to all additional current consumption due to the analog circuit itself. Table 31 of the datasheet shows that the typical measured DC current consumption on VPS_TX is 70mA while we guarantee that it will never exceed 100 mA  (taking measurement variation and a guard band).

3)

The current driven to antenna depends on VPS_TX, the input impedance of the antenna matching circuit (“antenna tuning�) and the antenna impedance itself.

Current in antenna is obtained by dividing the antenna voltage by the antenna impedance. Since the antenna impedance is inductive, current calculated in 1) and current in antenna are not in phase and completely different.

The CR95HF EMI filter calculation sheet (

CR95HF EMI FILTER CALCULATION.xlsm)

provides a voltage graph showing the voltages calculated at different location of the antenna matching circuit. Antenna voltage is shown. Current in antenna can be calculated by dividing this voltage by the antenna impedance.

For instance, if you enter the CR95HF demo board component values and antenna parameters, voltage on antenna is around 8V peak, which lead to a current Iantenna= 8/(424nH*2*pi*13.56e6)= 221mA.

Regards