resources for the digital side of RFID

[Hawaslsh]

Hello all,

Thanks in advance for any and all help!
I was looking into creating a UHF RFID system and I wanted to reach out with some questions and some possible resources to learn from.
My knowledge base lies more in the analog front end for a system like this. Below is a block diagram for a RFID front end system (stolen from a great analog devices tech document) describing where most of my questions lie. In everything I have read so far very little detail is given about what type of signals are generated for the DAC.

Up till now I have read about and have a very perfunctory understanding of how the actual modulation scheme for this type of communion is performed.
1) It seems Amplitude shift keying is how most RFID ICs communicate. Is this true? The reader transmits a high power continuous wave output to power the RFID IC. The IC will reply with modulated backscatter?
2) Follow up to 1: if true how does one write to a tag considering the tag is powered by a CW source?
3) Is there a standard RFID commutation protocol? I see most RFICs will reply with a unique identifying binary number. Is that all, or is more added to the reply, ie: parity bit? I assume there is a timing diagram that everyone refers to as some standard?
4) If I need a system to only read tags, the transmit portion would only be a CW signal? No need for any DACs in that case?
5) It seems most tag antennas are dipoles, but dipoles of a special case? why is the RFID IC shorted, see below?

Sorry if these questions seem basic, but gotta start somewhere,
Thanks,
Sami

 

[betwixt]

1. It isn't normally a continuous transmission, to save power most RFID transmitters pulse their output from anywhere between a few mS up to about 1S. If a tag responds it then changes to continuous output while it is being interrogated.

2. The transmission carrying data TO the tag is normally ASK. Yes, the tag is powered from the transmission and it carries information back to the reader by modulating the loading on the EM field.

3. There are several protocols in common use, MIFARE and HITAG are the most common but there are variants of both of those.

4. No, the tag contains memory and you have to send a command sequence to it to read it back. That includes the memory holding the tag ID.

5. In my experience, most antennas are loops, varying from single turn to as many as 25 or more for LF tags. I would guess the 'short' is part of some impedance matching but bear in mind that most RFID readers are designed for close proximity (a few cm) and may have radiation patterns optimized for specific needs.

Brian.

 

[FvM]

You are specifically asking about UHF RFID which achieves larger operation distances than the mentioned 13.56 MHz proximity methods. UHF tag antennas are usually electrically small dipoles as the shown one. Due to it's small radiation resistance and mostly capacitive impedance, they need a parallel inductive impedance matching element.

Regarding point 2 and 4, no RFID tag (LF, HF or UHF) will answer on a pure CW signal. It has to be activated by a specific sequence, also the multiple tags in the range of the reader are identified and selected one by one. You should review the protocol specification of different RFID systems to see how it' achieved.

--- Updated Sep 5, 2020 ---

See protocl specification of EPC RFID standard https://www.gs1.org/sites/default/files/docs/epc/uhfc1g2_1_2_0-standard-20080511.pdf

--- Updated Sep 5, 2020 ---

Also newer Gen 2 specification https://www.gs1.org/sites/default/f...-gen2v2-uhf-airinterface_i21_r_2018-09-04.pdf

 

[Hawaslsh]

--- Updated Sep 5, 2020 ---

Also newer Gen 2 specification https://www.gs1.org/sites/default/f...-gen2v2-uhf-airinterface_i21_r_2018-09-04.pdf

Thanks, that was exactly what i was looking for.
Follow up question if you dont mind. More related to the RFID IC itself.
I found an interesting paper that took a UHF RFID IC, and modified the antenna to operate at 2.4 GHz. Apart from the antenna provided to the RFID IC, is there circuitry with in the RFID IC itself that is also band limited? Below is an excerpt from an NXP datasheet for a RFID IC. The datasheet does have a frequency limit, but in theory, if that rectifying diode operates well at 2.4 GHz, shouldn't the chip still work?

Thanks,
Sami