RFID PASSIVE TAG QUESTIONS

[crenyen]

Dear all
I am new in RFID field and begin to study the principle of RFID by reading many books and papers.
Here, I have a couple of questions about RFID tag.
1. If a passive tag contains no oscillator, how can it transmit signals at frequency 125kHz or 13.56MHz?
2. I have no idea about the terms "backscatter"and "load modulation", are they the modulation methods of the passive tag?

Need your help.
Thanks

 

[pisoiu]

Hi,
The answer is somehow common. They do not transmit anything, they rather modulate the received field, and it is the job of the reader to detect the modulation. Any tag has its receiving circuit (coil+internal capacitors) tuned on the working frequency (125khz or 13,56mhz). Some cards even contains some adjustable capacitors (by software means), they allow adjustments in the resonant frequency of the tag circuit. A brute example of load modulation would be: take a regular power transformer, plug it into mains and put in the secondary circuit two loads: one permanent and one through a switch. Measure all the time the current consumption in the primary circuit. When you will switch the second load on and off, you will notice that on the current indication in the primary circuit. The same principle is used in RFID, the tag and the reader are inductively coupled. In this case the permnent load would be the electronics inside the tag, it must be powered somehow, and the second switchable load is a simple switch which is controlled by the tag's electronics.

/pisoiu

 

[startboy]

my dear:

I answer you those two question,


1. what's passive define? it's passive active by active device,

so tag need to get power by active system that's call reader.

tag need to get clock signal by reader, when 100% ASK modulation,

tag haven't clock signal in this time interval, we need to solve by

digital techinical.

2. Load modulation is the way that is tag reply data to reader, the tag

haven't product any signal, it's use bypass current to effect induce

energy that is change antenna point voltage amplitude to make a ASK

modulation signal, this is simply RFID communication principle.




c.c.huang

Added after 2 minutes:

my dear:

I answer you those two question,


1. what's passive define? it's passive active by active device,

so tag need to get power by active system that's call reader.

tag need to get clock signal by reader, when 100% ASK modulation,

tag haven't clock signal in this time interval, we need to solve by

digital techinical.

2. Load modulation is the way that is tag reply data to reader, the tag

haven't product any signal, it's use bypass current to effect induce

energy that is change antenna point voltage amplitude to make a ASK

modulation signal, this is simply RFID communication principle.




c.c.huang

 

[raduga_in]

1. If a passive tag contains no oscillator, how can it transmit signals at frequency 125kHz or 13.56MHz?

PASSIVE tag need no oscillator as it transmitt back
the incident wave in such a fashion that the amplitude
of the reflected wave gets modulated by the data
on the tag.

2. I have no idea about the terms "backscatter"and "load modulation", are they the modulation methods of the passive tag

Here simply i would like to tell you

Backscatter : Scattering/ Reflecton of the incident
wave from the tag to reader

Loadmodulation : Variation of the load which the
antena looks as according to the data inside Tag.
By this way we can chage the amplitude of the
reflected/scattered wave as it will have different
coff. of reflection for different Databits ...

If you need any more explaination plz let me know ..

 

[szekit]

a simple analogy would be the barcode and barcode reader you see in the supermarket.

 

[JohnJ]

Is that all anyone is using?

A, now defunct, company I worked for had developed RFID using frequency division. They actually had a measurable output, and we had a seperate receiver for the response from the tag. (Little in nature divides frequency).

The little metal strips in that popular (in the US) anti-shoplifting plastic tabs actually frequency divide. The one or two shiney strips are of specially treated met-glass that spits out half the frequency it is excited with, within the frequency range determined by it's physical size. (Yes it does divide, not just resonate; you can vary the freq. and it will track). The other strip is a magnet that is required for biasing the met-glass into operation. That also provides a way to cancel the tag at the register.

We also used the frequency division idea for RFID by clocking a CMOS shift-register containing the ID number which modulated the return signal. Those, like our regular tags, used seperate receive and transmit coils.

In all those cases the power came from the reader transmitter and returned through a receiver.

 

[PachinkoC]

Here are some related documents for your reference

 

[tdf]

1. If a passive tag contains no oscillator, how can it transmit signals at frequency 125kHz or 13.56MHz?

PASSIVE tag need no oscillator as it transmitt back
the incident wave in such a fashion that the amplitude
of the reflected wave gets modulated by the data
on the tag.
.........
If you need any more explaination plz let me know ..

Hi,

In case the tags is using OOK protocol, it seems that no need to use osc in passive tags.

How about the PSK/FSK? Does it need osc?
How the tags can generate a clk from the Reader's signal?

Rgs,
tdf.