Hi guys,

I am venturing into the realm o RF, more specifically I am doing my thesis on emulating an RFID card with 13.56MHz carrier frequency.

I wold appreciate if you could point me to some books etc. concerning antenna design (probably closed loop) and RF power transfer.


Also, I had this idea. Usually the RFID antenna circuit is a parallel LC resonator. But since the reader and card coils are in a configuration similar to a transformer, why does the receiver need to be in resonance, and can´t simply use just the inductor? Is it because without the capacitor there wouldn´t be enough voltage amplitude to power anything?

 

Really, no one? Not even any good books on RF?

 

Wish I could help guy, but I don't know much. I have read articles on specialized chips for RFID, where they feed a really large binary number back.

I have also disassembled badges (sorry, no pictures) to see what made them tick. They had a coil inside the plastic edges that feed to a small chip and capacitor. I assume the chip had a diode, and charged the cap to power the chip.

As I understand it, and I could be wrong, is the chip / coil absorb the signal in a digital sort of way, imposing modulation on the carrier wave.

The same carrier wave frequency is also used at very high power levels for other industrial jobs, as it was set aside for by the FCC in the USA. Your country may differ. We use it for plasma ashers at 500W routinely in the clean room.

 

Actually, I got some publications about the data modulation and other stuff relating to RFID in specific, so that is not a big problem.

Sadly my background in RF is very weak, so what I need is some publication on basic RF stuff, like EM filed in general, antenna calculations, air-coupled transformers and anything like that.

 

If you are looking for a book I would recommend the following:
http://www.amazon.com/Complete-Wireless-Design-Cotter-Sayre/dp/0071370161

I think it may be able to help with any RF questions. I don't recall that it goes into great detail about RFID but from what I have read (4 chapters) it does get into subjects just as antennas, transformers, modulation, and amplification.

 

...

Missing link?

 

Hi guys,


Also, I had this idea. Usually the RFID antenna circuit is a parallel LC resonator. But since the reader and card coils are in a configuration similar to a transformer, why does the receiver need to be in resonance, and can´t simply use just the inductor? Is it because without the capacitor there wouldn´t be enough voltage amplitude to power anything?

Usually a resonant tank is used for a couple of reasons. It gives frequency selectivity so that other strong RF fields don't power up the RFID tag. Second, many RFID tags are passive - meaning that they do not have an internal power source, and usually do not generate an RF signal in response to being queried by the reader. Instead, they change the resonant point of the tank circuit in order to reply to the reader. The tank circuit will absorb the reader's RF signal when at resonance, and will reflect the reader's RF signal when tuned off resonance. The tag will modulate the tank's resonance with a "pattern" which constitutes the tag's response. The reader will then receive this modulated reflection as the tag's response.

 

The tag´s response modulation is achieved by shorting LC tank the with a transistor, which loads the reader´s tank and can be read that way. I don´t think that loading the tank should change its resosnant freqency, am I right?
Also is this really loading the card reader´s field, or is it creating some reflection?

 

The tag´s response modulation is achieved by shorting LC tank the with a transistor, which loads the reader´s tank and can be read that way. I don´t think that loading the tank should change its resosnant freqency, am I right?
Also is this really loading the card reader´s field, or is it creating some reflection?

Look up Parallel RLC circuit and explain to me how loading doesn't change the resonant frequency...

What they're probably doing is this:

The RFID device is either active or passive. Active implying an internal power source, passive implying that it leeches off the RF signal to run for a short time.

Upon reception of an initial signal, it either wakes up via battery power or the energy-harvesting portion of the tag stores energy to "speak" with.

At some point, the RFID tag then begins to respond. This is probably done via on-off keying and with the transmitting device listening for an echo.

The echo is either a burst of RF from the resonant circuit (ON) or no echo at all (OFF, transistor shunts the resonant circuit and the device is silent).

The nitty-gritty details of all this are heavily dependent on your project, types of tags, distances, power, frequency, and bandwidth, along with what intelligible data you're trying to gather. Hopefully this helped a bit, anyway.

 

Look up Parallel RLC circuit and explain to me how loading doesn't change the resonant frequency...

http://www.electronics-tutorials.ws/accircuits/parallel-resonance.html
fr=1/(2Pi.√(LC)) no matter what load you have there.

I know how the tags work, but I have not enough RF background to really understand the behavior of the two coupled circuits. I dont really know what you mean by echo, but I guess that you mean the reponse from the tag. That is done by shorting the tag´s coil, which loads the readers coil and it senses a change in current ampliude on it´s own LC circuit.

I still don´t understand where reflections are coming into this.

 

Actually, PaulEE, I disagree. Adding resistance (or shorting in this case) of a LC circuit changes the Q of the circuit, not the resonant frequency, as the LC components do not change. Any frequency change is very slight, and can be considered negligible for this exercise IMO. Shorting a LC circuit would add to the loading of the RFID transmitter, which would be measurable. If it were modulated, it would be even more so.

I am about to step out, but it reminds me of a really simple radar detector using 2 Shottky's, a feed horn setup, and an audio square wave generator. I'll sketch it up when I get back.

 

try these for some "light" reading on the topic >>> http://ww1.microchip.com/downloads/en/devicedoc/21299e.pdf

http://ww1.microchip.com/downloads/en/appnotes/00710c.pdf

 

Thanks, those look great.

 

Bill,
You're right. I have no idea why I said that. Please disregard.

Shorting a LC circuit would add to the loading of the RFID transmitter, which would be measurable. If it were modulated, it would be even more so.

I don't know about that statement. Why do you say this?

 

kubeek,
Looky here:

That's all I was saying.