I'm implementing the suggestion from AN11314, i.e. to have multiple antennas circuits grounded with the help of mosfets. But antennas are deported with the help of transmission line (100 ohms characteristic impedance - twisted pairs cable). The load of the antennas are matched to the transmission line, so basically i have a effective 100 ohm impedance load and i need to convert it to 20 ohms as required by the PN5180 datasheet through a matching circuit and the EMC filter.
The problem is the following :
AN11314 says "Keep the capacitance values of the series capacitors (C3, C4, C7, and C8 in Figure 6) low if possible so they do not influence the EMC resonance a great deal."
But in my case, because of the matching 20 Ohms<--100 Ohms, the serie capacitor is bigger than the C0 EMC capacitor. So i can't even put 2 antennas together...
Do you have any suggestion ? Because having multiple antenna is interesting, but less is you have to put all of them at the same place...
Hello bastiaens,
About the transmission line + NFC antennas, there is a "golden" rule which says: The device operates at 13.56 MHz and the wavelength of 13.56 MHz is 22 metres in free space and about 15 metres on a typical PCB. So, the golden rule is that if your tracks are longer than about one-tenth the wavelength then you should start to consider using transmission line calculations. Are the tracks likely to be close to anything like 1.5 metres long is the question you should consider. If not then don't worry about matching because it won't make a difference practically.
So what should be the length of the twisted pair you would like to you? If it is something like 10-30cm you can just replace it with an LR model -> a twisted pair will increase the resistance and inductance of your antenna.
I would recommend taking your NFC antenna + twisted pair and measuring its parameters using a VNA. This will give you an LR model of the NFC Antenna + twisted pair. This I would use as input for your simulation. See below:
The parallel capacitor can be estimated since it is not easy to measure it, use e.g. 3pF-5pF
See an example below: (simulated in ADS but you can do it in RFSIM99 as well)
BR
Tomas
