How RFID works and what is inside RFID tags or cards which you use every day? More importantly, we learn why RFID tags don’t need a battery or any power source which is one of the main reasons that made RFID so popular.
RFID is used in many applications like your key for the door, for checking products, libraries, shops, supply chain management and many more.
RFID stands for Radio Frequency Identification but what made RFID so popular?
RFID could solve one of the most challenging things in electronics devices which is a power source.
You see every electronic device needs power and if it is a portable device like your laptop or your cell phone, you need to have a battery to provide the energy for the operation of the device.
These batteries can be bulky and heavy and you will need to recharge them or change them now and then.
But what if we can send the power to the device whenever it needs?
It is possible yes.
If we put two cords beside each other and pass the AC current through one of them, then it will induce some current in the other coil through the magnetic field coupling.
The device can use this current to charge a capacitor and work without a battery.
Of course, this power will not be high enough to turn on a screen or move a motor but it is sufficient enough for reading some data from the device.
That is what made RFID so popular that we don’t need to have a battery on the device anymore.
Well this is passive RFID and we also have an active RFID switch, they do have a battery but I think these passive firefighters made them very popular.
So an RFID system has a reader and a tag.
Now let’s see what is inside this tag and how it is designed.
The tags have a coil or antenna and a low-power chip that can save some ideal data and send them back to the reader whenever needed.
RFID tag, the antenna sits on the outside and the low-power chip is on the inside.
Now let’s see how to design this chip.
Which blocks do you think we need to have for this chip?
Of course, we need to have the power to rectify and regulate the RF signal sent to the coil.
What else do we need?
We need a controller unit with some memory to record the data and ID.
This memory can be simple but in many cases, we need to be able to write new data like you add some money to your canteen key.
Right, that’s why we need to have an EPR.
What else do we need?
We need the transmitter.
You need it course to send the data back to the reader and as we discussed we also need to write some data to the chip so we need to receive a block as well.
So these are the blocks that we need to implement an RFID chip.
Let’s start with the transmitter block because there is a very intelligent idea there that I want to share.
We mentioned that the power we sent to the tag is good enough for transmitting some data.
So you may think that perhaps we should have a transmitter to generate the RF signal and send it to the reader, however, the RF transmission can be quite power-hungry.
You need to have oscillators to generate the carrier frequency and you need to modulate the signal to derive the antenna.
This antenna is usually a very low impedance load and if you want to have a transmitter, it will immediately aim at the capacitor.
So they came with this idea and said what if we do not transmit any RF signal then how do we transmit the data?
Here’s the idea.
The reader is sending the RF signal to the tag.
Right, we can actually put a switch across the coil in the tag.
Whenever we have binary bound data we close the switch.
And when we have a zero we open it.
So if two coins have a coupled magnetic field, when we short one of them it changes the inductance in the other cord.
If you have an RF signal on the first coil it will attenuate the signal there and the reader can use this information to see when the coil in the tag was shorted.
This way we can send the information without actually sending any RF signal from the tag to the reader.
They call it Backscatter modulation.
However the change in the signal of the reader would be very low at the range of 1 – 1000 of the actual RF signal, so this change is very susceptible to noise.
That’s why they use some other modulation like frequency shift keying or phase-shift keying to improve the noise rejection in these modulations.
We still change the amplitude of the source signal by switching the coil but now for example, for frequency shifting, we say if we keep 4 cycles off and 4 cycles on then the data is 0.
And if we keep 5 cycles off and 5 cycles on then the data is 1.
So this way we are coding the 0 & 1 inside the frequency.
In the case of phase-shift keying, we say when we change the phase of the signal it can be interpreted as 1 or 0.
Now let’s move to the design of the power unit.
As we showed we need to have a rectifier and this rectifier saved the charge on the capacitor.
Sometimes they also include a multiplier circuit to increase the voltage level along with the regulator to regulate the voltage.
So let’s go to the design of the controller.
It can be easily designed using logic gates but one thing I want to mention here is that we need to have an accurate clock especially if we want to do phase shift keying or frequency shift keying for the modulation.
Having an accurate clock can be achieved by a crystal oscillator and using PLLs.
However, this is also a power-hungry plot so they came up with another idea.
They said let’s use a lot of oscillators, for example, a ring oscillator and then we sync it with the incoming RF signal because that incoming RF signal is generated by the reader and has an accurate frequency.
So again they use the signal from the reader to achieve what they want in this case accurate clock signal.
Now let’s move to the last block on this tag IC which is the receiver.
The data which is sent by the reader can simply have an amplitude shift keying modulation because we can change the amplitude from the reader as much as we want.
It is not highly affected by the noise any more than in the tag.
We use an envelope detector to find these ups and downs in the RF signal and extract the data.
So that was a rather long explanation of the design of the RFID tag chip.
Here you can learn more about RFID technology and how it works.
At this point, I want to discuss RFID frequencies.
RFID devices can have different frequencies.
- Low Frequency
- High Frequency
- Ultra High Frequency
- Super High Frequency
The most common frequency band is 13.56 MHz and the majority of RFID tags work in this frequency.
The other thing we’ll learn from this data is that higher frequencies provide a larger distance range for example UHF is mainly common for the applications where a large distance is required.
RFID devices are not interchangeable and the frequencies are not adjustable that is because each frequency range requires its own antenna.
You can actually figure out the RFI, the frequency of the device by looking at the antenna.
The low-frequency antenna has a thin copper wire that is coiled by hundreds of turns.
Why the high-frequency antenna will have a few turns?
So the higher the frequency range, the shorter the antenna will be.
Now let’s talk a bit about the readers.
As we discussed before, RFID readers are the units that transmit the power through the RFID tag and read or write the data to the tags.
Right. There are usually stationary units therefore they can have their own power source and we have more flexibility for design in the reader.
For the reader, we also have an antenna and the modulator and demodulator.
There are single-chip solutions available for example M FRC 5 to 2 from n XP is one of the examples for 13.56 MHz RFID.
We do in this typical schematic.
It modulates the signal and transmits it to the antenna and the same signal goes back to the receiver input to detect the signal change due to the backscatter modulation.
Low-cost RFID reader ports are based on a chip and are using a PCB antenna.
You can connect RFID Readers with Arduino and design lots of interesting projects.
Here in this post what I just wanted to explain the main concept of RFID and how RFID actually works.
Here you can explore RFID Tags and Hardware.
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