NFC with power amplifier to increase reading range.

NFC chip with power amplifier design

hello,

I am trying to attach a power amplifier to the NFC chip so I could read out from distance (~30cm).
I though it would be quite simple but it does not work and drive me crazy..
Here is the circuit and block diagram I designed.
Would anyone could comment what I am doing wrong?

For now, I use CR95HF as a reader and it sends data to the tag but does not receive.
Output power is high enough: ~8W.
I can see perfect sinusoidal waveform at the antenna but when it was attached to RX pin, it is a little distorted.

Thank you in advance.

Re: NFC chip with power amplifier design

Adding an external amplifier to a transceiver is not so simple. You need an external T/R switch between the power amp output and the receiver input. Without one, you have likely already damaged the RX circuitry.

Also, the chip seems to want to detect the antenna current during transmit:

Datasheet section 5.6.5 said:

During the tag detection sequence, the CR95HF regularly emits RF bursts and measures the current in the antenna driver IDRIVE using the internal 6-bit DAC. When a tag enters the CR95HF antenna RF operating volume, it modifies the antenna loading characteristics and induces a change in IDRIVE, and consequently, the DAC data register reports a new value.

Click to expand...

If you use an external amplifier, then the chip won't be able to measure the antenna current. I'm not sure if this can be made to work.

Re: NFC chip with power amplifier design

Thank you mtwieg,
There are resistor between receiver and antenna which limits current to rx pin. I monitored voltage there and it was lower than its maximum value.
I also tried to isolated rx and antenna using transistor but it still it does not work.
I believe that though CR95HF modifies loading characteristics, electrical environment for CR95HF is remained: TX amplitude was amplified and RX value is reduced from the view of CR95HF as well.
Except that, would the schematic look fine? there are TX and RX grounds there but there are no explanation from the datasheet so I am quite confused.

Re: NFC chip with power amplifier design

The datasheet expects a differential connection of matching network and reader coil. The asymmetrical connection shouldn't be a problem in case of TX as long as the amplifier is achieving sufficient field strength. In case of the RX pins I'm not sure if it's O.K. to connect it to ground level. The development kit schematic suggests a floating RX circuit, apparently the RX pins are self-biased.

Regarding tag detection, it's used to allow low power operation of the reader. But missing tag detection shouldn't block regular operation in active mode.

The basic requirements for reader operation with passive tags are:
- sufficient field strength at tag location (1 - 5 A/m for ISO14443 and NFC)
- received load modulation above the CR95RF RX sensitive threshold of 8 mV

Correct setup and commands for the used tag type are presumed. You should verify sufficient field strength first, then check if the tag is responding, can be e.g. sensed with a pick-up coil near the tag.

Communication with a passive tag (by load modulation) doesn't allow T/R switches, the reader is sending unmodulated carrier while receiving

How to isolate RF power?

Hello,

It might be very easy question for you but as I don't have sufficient knowledge and understanding of RF, it is a little hard to figure out for me.
The purpose is quite simple: I want to add a wire to the loop 13.56Hz antenna to receive its signal. As this antenna sends power and read signal by AM, I should not directly attach a wire or other components as it greatly changes Q factor of the antenna.
So I want to isolate the antenna and reader system by some methods.
Would you please advise me how to do that?
Though it was not tested yet, would bidirectional or directional coupler help?

Many thanks in advance.

Re: NFC chip with power amplifier design

You are of course talking about 13.56 Mhz.

A directional coupler presumes matched load impedance, all reflected power is bounced back to the receiver in any case. Although you can theoretically match the loss resistance of a NFC coupler to the transmitter impedance, it's at best roughly matched in a real system, so a directional coupler (for 13.56 MHz typically implemented as transformer or LC hybrid circuit) would still return a lot of transmitter signal.

That's why 13.56 MHz reader receiver channels are usually designed to tolerate the full transmitter signal. This brings up of course a problem if you are trying to use a reader chip beyond it's intended range. It might be helpful to reduce the direct transmitter signal at the receiver input by means of compensation.

Probably the best method to separate transmitter and receiver path is to use a second coil for the receiver with sufficient separation from the transmitter coil.

As previously suggested, it's helpful during tests to place a "sniffer" coil near the tag position to check if the tag is responding at all.

Hello,

I am using TRF7970EVM and commercialized external power amplifier to increase reading range.

As shown in schematic attached, I inserted inductor and two capacitors for 45 degree phase shift and those were connected to RX pins.

The waveform at RX pins showed perfect 45 degree shift and its amplitude was adjusted to be 3V.

Surprisingly, when output power is less than 1W, it worked but it does not work anymore when I increase a power a little bit.

The magnetic field is strong enough: transmitting power is confirmed.

Could anyone guide me to right way? I am not RF expert so I think there are terrible mistakes especially in the inductor and variable resistor.

Thank you so much in advance.

Have you considered receiver overload?

Did you try checking if you have modulated signal on Rx Ports? If I understand it correctly modulated Rx signal is generated by modulating Tx Load: tag antenna is periodically shorted and this changes impedance of reader antenna. When you make tag operating at longer distance from reader antenna are you still changing reader antenna impedance sufficiently to produce detectable signal?
At that frequency you should be able to see modulated input to Rx with scope.
?

When you make tag operating at longer distance from reader antenna are you still changing reader antenna impedance sufficiently to produce detectable signal?

Click to expand...

There's a more basic problem before the reciver won't be able to perform load modulation. It's that the received voltage is simply not sufficient to power the receiver chip. If the sufficient voltage is achieved, some amount of load modulation should be possible.

The effect is however almost the same, checking tag response with a nearby pick-up coil should answer the question.

FvM, I am not sure if I entirely agree with your description. For reference principle of data transfer from passive tag to reader is described in the following document:
www.nxp.com/documents/application_note/78010.pdf
page 9. From this follows that modulated voltage is result of changing load to reader IC output (Tx). Accordingly I would attempt measuring modulation on terminals of reader antenna or directly on Rx terminals of reader IC. Introducing additional inductance between tag and reader (your suggested pick up coil) only obscures the coupling mechanism. I hope this helps.

May be I misunderstood your previous post. My point is that we should check first if the tag is actually responding to the reader request. A pick-up coil would be a passive probe that doesn't disturb the magnetic field.

The other possibility is that the tag is responding but the signal caused by the tag load modulation at the reader is too small to be detected. That's in fact most likely what's happening here. If the reader and tag coil size and distance between both are given, there's no other option than increasing the receiver sensitivity.

FvM, I agree with your last statement. Likewise I can imagine situation when reader transmits lots of energy (it is now amplified), tag powers up and tag does it's thing: shorts and opens tag's antenna. But since it is now far away from reader antenna the change in reader antenna impedance (=Tx Load) is so small that it results in no useful modulating signal. If this is the case amplifying signal applied to Rx inputs should help. This have to be done intelligently though: another condition when receiver stops working is when input is too strong. This results in desensitized area in close proximity to the reader. This condition is explained somewhere in app. notes for these circuits.

- - - Updated - - -

Curiously, the solution - amplifying input signal - becomes self-evident if we think about this problem from system perspective: adding power amp for reader output increases link budget for uplink. To increase range link budget for downlink must be increased as well.
Interesting.

NFC has been designed for proximity range and so are the respective reader ICs. 30 cm is beyond the range that standard readers are designed for.

Experimental setups investigating the security of ISO14443 RFID systems (same reader and tag hardware as NFC passive mode) were able to eavesdrop an existing reader tag communication up to about 2m distance with a 0.25 m coil and optimized receiver hardware. So with a certain simplification this distance can be considered as noise limited range for the tag to reader communication. Range of tag activation and reader-to-tag communication is limited by the reader coil size and current.