Measuring RFID Tag Voltages (DC)

Hi folks,

I am planning on performing tests on an RFID passive tag antenna (spiral coil), without a Tag IC connected. I will have the tag at various distances from an RFID reader antenna.

How can I reliably measure the DC voltage induced across the tag coil? I've found a possible setup from Texas Instruments(See Link Below)

See Page 10
**broken link removed**

Is there anyway to measure the power/voltage induced on the tag wirelessly?

I assume if any probes are connected to the coil then the results won't be accurate.


Any advice is much appreciated


Kind Regards

Cat[/b]

To measure the voltage in this case is quite difficult as the voltage level is quite low in the tag.

However why don't you try to calculate it using the approximations and other mathematical models ? It can turn out to be close enough for the application if you do that properly.

Other way could be to have a dummy tag and couple it to another circuit where the voltage can be measured using magnetic coupling, but again this might disturb the original circuit, however this method can be applied if some tolerance in error is accepted.

Hope this helps.

Thank you


Can anyone suggest approximations or mathematical models that will help out in this situation?

Measuring the voltage across the coil with the tag chip removed is quite irelevant. The tag chip contains tuning capacitors, to bring the circuit to resonance.

/pisoiu

I'm going to attach tuning capacitors in parallel with the coil terminals. That will be ok surely?

If you are working at 13.56 MHz, it is easy. First, put good RF variable capacitor across the coil. Next connect a small signal diode in series with a parallel RC circuit. Select the RC according to the input impedance of the RFID tag chip you plan to use. Put the diode+RC across the capacitor. Now, use a DC voltmeter to measure the voltage across the RC circuit. Tune the variable capacitor for maximum voltage.

Calculating the voltage that should be seen can be done easily using Sonnet. I have posted a pdf + appropriate Sonnet files for how to do this elsewhere on EDABoard (either here or in the EM section).

Some other observations: you can test the presence of the field by placing the osc probe in the front of the reader, with the ground wire connected to the tip probe. Just set a low v/div at vertical amp. Some readers (especially those powered by batteries) do not transmit the field continuously, the field is activated by an optical sensor when the tag is approached, take care at this aspect. Most probably you will see a nasty waveform without a tag, the pure sine waveform is present only in the presence of a tuned tag in the reading field.

/pisoiu

rautio said:

Next connect a small signal diode in series with a parallel RC circuit

Click to expand...

Would it be better to use a full wave bridge rectfier instead, a single diode will only give a half rectified voltage, or does it not really matter?

rautio said:

Select the RC according to the input impedance of the RFID tag chip you plan to use

Click to expand...

At the moment, it is unclear what the impedance of the RFID tag chip will be, are they usually 50 ohms?


How can i use the impednace to calculate the values for a simple RC circuit?


Apologies for the stupid questions

Cat

Easy questions are always welcome, no problem.

For measuring the voltage, one diode will be plenty. If you actually want to get some power out of the circuit (like the RFID chip does), two diodes would be a good idea.

Make sure it is a "small signal" diode. These diodes have less capacitance and are often used in mixers, etc., in receivers. A power rectifier usually has more capacitance and does not work so well at RF.

My pdf on how to do RFID using Sonnet is attached. I also attached a MicroChip tutorial on RFID. This discusses how to analyze the RFID tag in detail. This is a couple years old, there might be something more recent on the MicroChip web site by now.

Since the RFID chip needs maximum voltage to operate at the longest range, they do not use 50 Ohm input impedance. Rather, they use a much higher input impedance so the voltage can rise high enough to operate the chip. If you do not know what the input impedance is, just make sure you have a high value of resistance and make sure the time constant is much longer than 1/f.

i have one problem. when you say"Make sure it is a "small signal" diode. These diodes have less capacitance and are often used in mixers, etc., in receivers. A power rectifier usually has more capacitance and does not work so well at RF",i used agilent hsms286c. can it use instead of the diode?
it's spec is CJ0 =0.18pF,RS =6.0 Ω.

Try it and see.

rautio said:

For measuring the voltage, one diode will be plenty. If you actually want to get some power out of the circuit (like the RFID chip does), two diodes would be a good idea.

Click to expand...

I assume you mean two dioes in parallel

One diode will do it. Just imagine you are doing a half-wave rectifier for a power supply. Only now, it is RF. Two rectifiers will do a full wave rectifier. Four will do a full wave bridge. A quick google yielded the following tutorial, it is an easy read;

www.allaboutcircuits.com/vol_3/chpt_3/4.html

Time for another silly question

For calculating the impedance I have seen equations involving something like 'jωL' . What does 'j' represent?

<See Attached Image Below>

If someone could give me an example calculation showing how to represent 'j' it would be a big help.

Regards

Cat

rautio said:

If you do not know what the input impedance is, just make sure you have a high value of resistance and make sure the time constant is much longer than 1/f.

Click to expand...

For 13.56MHz:
1/f = 1/(13.56*10^6) = I have calculated this to be around 0.7ns, is that correct?

Is it better to have a very small capacitance i.e around 100 nF ?

For example, If I choose 1MΩ resistor and a 300nF capacitor then the time constant will be 0.3 seconds, which is more than enough. does this sound reasonable to use?


Kind Regards

Cat

j is sqrt(-1). The impedance is complex.

What R and C will work best depends on the diode you use. I would think 300 nF is too small. Seems like you would want the C to be significantly larger than the off-state C of your diode. Get a hand full of parts and try them out. Of course, you could also do a non-linear analysis of your circuit, if you have a good non-linear model of your diode and a good non-linear analysis. But at 13.56 MHz, I would just get a soldering iron and play.

I don't think a diode is going to do it. The rf voltage to detect will be too low. I would hook up a log amplifier, like one from Analog Devices, to it. You could hook that up to a voltmeter with a large display face and read it "wirelessly". If you get out too far get some binoculars to improve the "wireless" range.

If there is a lot of local interference, you might also need a bandpass filter before the log amp.

A diode will do it once the peak RF voltage is above about 0.5 to 0.7 volts, depending on the diode. The peak voltage depends on the loaded Q of the LC circuit. The higher the Q, the higher the voltage. In fact, if you could get infinite Q (which is impossible), you could get infinite voltage. A single diode (formed by a cat-wisker (small wire) touching the surface of a crystal of galena, with the output connected to a set of headphones is exactly how very early receivers worked. Needless to say, they were not as sensitive as today's receivers, but they did in fact work.

RFID chip needs about 3-4 volts for normal operation. Worth to look at their specs where it is stated.
The iso14443 says that :
"The PICC shall continue to function normally after testing in accordance with the test methods described in ISO/IEC 10373 where the angle of rotation a equals 15°."

if you are about proximity - that is one to test . Vicinity needs separate investigation.

so here ISO/IEC 10373 goes (i have only part 6)

Have a look to its content :
Contents Page
Foreword.....................................................................................................................................................................v
1 Scope .............................................................................................................................................................1
2 Normative references ...................................................................................................................................1
3 Terms and definitions, abbreviations and symbols...................................................................................2
3.1 Terms and definitions ..................................................................................................................................2
3.2 Abbreviations and symbols.........................................................................................................................2
4 Default items applicable to the test methods .............................................................................................3
4.1 Test environment..........................................................................................................................................3
4.2 Pre-conditioning ...........................................................................................................................................3
4.3 Default tolerance...........................................................................................................................................3
4.4 Spurious Inductance ....................................................................................................................................3
4.5 Total measurement uncertainty ...................................................................................................................3
5 Static electricity test.....................................................................................................................................3
5.1 Apparatus ......................................................................................................................................................3
5.2 Procedure ......................................................................................................................................................4
5.3 Test report .....................................................................................................................................................4
6 Test apparatus and test circuits...................................................................................................................5
6.1 Calibration coil ..............................................................................................................................................5
6.1.1 Size of the Calibration coil card ...................................................................................................................5
6.1.2 Thickness and material of the Calibration coil card ..................................................................................5
6.1.3 Coil characteristics.......................................................................................................................................5
6.2 Test PCD assembly.......................................................................................................................................6
6.2.1 Test PCD antenna .........................................................................................................................................6
6.2.2 Sense coils ....................................................................................................................................................6
6.2.3 Assembly of Test PCD .................................................................................................................................7
6.3 Reference PICCs...........................................................................................................................................7
6.3.1 Reference PICC for Hmin, Hmax and PCD power ...........................................................................................7
6.3.2 Reference PICC for load modulation test....................................................................................................7
6.3.3 Dimensions of the Reference PICCs............................................................................................................8
6.3.4 Thickness of the Reference PICCs board ...................................................................................................8
6.3.5 Coil characteristics.......................................................................................................................................8
6.4 Digital sampling oscilloscope ......................................................................................................................8
7 Functional test - PICC ..................................................................................................................................8
7.1 Purpose..........................................................................................................................................................8
7.2 Test procedure..............................................................................................................................................8
7.3 Test report .....................................................................................................................................................9
8 Functional test - PCD ...................................................................................................................................9
8.1 PCD field strength.........................................................................................................................................9
8.1.1 Purpose..........................................................................................................................................................9
8.1.2 Test procedure..............................................................................................................................................9
8.1.3 Test report ...................................................................................................................................................10
8.2 Power transfer PCD to PICC.......................................................................................................................10
8.2.1 Purpose........................................................................................................................................................10
8.2.2 Test procedure............................................................................................................................................10
8.2.3 Test report ...................................................................................................................................................10
8.3 Modulation index and waveform................................................................................................................10
8.3.1 Purpose........................................................................................................................................................10
8.3.2 Test procedure............................................................................................................................................10
8.3.3 Test report ...................................................................................................................................................10
8.4 Load modulation reception (informative only) .........................................................................................11
8.4.1 Purpose........................................................................................................................................................11
8.4.2 Test procedure ............................................................................................................................................11
Annex A (normative) Test PCD Antenna.................................................................................................................12
A.1 Test PCD Antenna layout including impedance matching network.......................................................12
A.2 Impedance matching network ....................................................................................................................14
Annex B (informative) Test PCD Antenna tuning...................................................................................................15
Annex C (normative) Sense coil ..............................................................................................................................17
C.1 Sense coil layout.........................................................................................................................................17
C.2 Sense coil assembly...................................................................................................................................18
Annex D (normative) Reference PICC for field and power measurements .........................................................19
Annex E (informative) Reference PICC for load modulation test .........................................................................20
Annex F (informative) Program for the evaluation of the spectrum.....................................................................21

I have part 7

ISO/IEC FCD 10373-7

Identification cards — Test methods — Part 7: Vicinity cards

Am i correct to think this is the better choice for 13.56MHz RFID?

I think part 6 is for proximity (close coupling) RFID.