TRF7970A Antenna

Hi Darko,

There is really no requirement to shield the bottom side of the antenna unless this is desired.  You could add a ground plane for a antenna intended for the TRF79xx devices if desired and just be sure that you tune the antenna based on the measured inductance with the ground plane present.  If you don't use the ground plane under the antenna traces, you could even use the bottom side of the PCB for antenna coil traces.  Take a look at the TRF7960ATB reference design.  This uses 2 turns on the top and 2 turns on the bottom side to reach about 1uH.  Section 13 of the users guide has some good antenna tuning information as well.

TRF7960ATB Users Guide: http://www.ti.com/litv/pdf/slou297

Hi Eddie,

Thanks for answer. I don't plan turns on bottom. I have 4 turs on top layer, so it is enough according to calculation about 1.7uH. You have on this board on top and bottom layers cause size is small so to reach inductance.

I am just ask cause this my RFID module will be above main board so not to have any bad effect, cause module and antenna will be above components on main board not free space.

Darko

Hi Eddie,

Also what is recommended inductance. I have according to calcualtion 1.7uH, it is 58mm round antenna with 4 turns, 0.5mm width of PCB track?

Darko

Is there anyone who can help me about antenna ground shield under?

It is easiest to tune when the inductance is between 1-2uH.  Your calculation of 1.7uH sounds about right.  If you have a network analyzer available, you should measure the inductance on that @ 13.56MHz.   

How close is the circuit below the antenna?  Is this just the TRF79xx circuit below the antenna or is there some other circuits as well? 

If you do add a ground below the coil, this will reduce the inductance, so it should be measured to ensure proper tuning.

I have RFID module on top side is antenna track arround egde of PCB which is 58mm diameter, so track are about 5mm wide. On bottom side in the middle is TRF7970A and passive components. It is RFID module with header 2mm pitch connector and I plug that on main board with MCU and surround components, so RFID module PCB is about 10mm above main board.

Darko

Here is picture of PCB, top layer antenna tracks, bottom components and all that as module goes on main PCB with main components, interface, MCU....

What is your opinion according to pictures of PCB, should I ground bottom layer like shield or not?

The shield is not neccesarily required.  If you want to be sure that you are not interferring with your main microcontroller circuit, you should add an e-field shield.  Please see the attached image for an example.  FYI, this will require a 4 layer board. 

Hi Eddie,

So what you think about my design where I have 2-layer design, where turns are on top layer and shield is on bottom layer. I think that shield is useful cause on 5mm under I have MCU on 800MHz, OMAP and other logic, HDMI

Darko

Darko,

This looks good!

Hi Eddie,

Thanks. You think that's ok. On antenna layer, blue, I have ground plane you see inside module, on one side close to antenna traces, do you think I need to reduce that.

Darko

Darko,

This should be just fine as long as you are not shorting the coil.  It may affect your inductance slightly, but that is not a problem as long as you measure the inductance and tune accordingly. 

Hello Eddie,

Instead of using an antenna as suggested in 7970EVM, to reduce the PCB size, can I use a spiral antenna with 1-2uH inductance. I have gone thru some calculations in the attached document and the spiral antenna mentioned in the Page 11, can I use it?

My PCB is using MSP430 along with 3rd party Bluetooth module on it.

6710.RF Antenna Design.pdf

Sure, there is no problem with using a spiral antenna.  Just be sure to match it to 50 ohms for maximum transfer with the TRF7970A 50 ohm matching circuit as used in the EVM.  1-2uH will be perfectly acceptable for this. 

Well I am using the matching circuit in the EVM as it is, however, is there something in the antenna design to take care of to make it matching to 50 ohm? you have to bear with me here, as this is the first time I am working on an antenna.

Also, what other factors decide the read range and what would be the approx read range for this EVM?

How can I increase that read range?

Also, do I need to go for gold finish for such antenna or even a lead finish would do?

You will need to alter the antenna tuning components to match to 50 ohms.  This includes parallel capacitance, parallel resistance, and series capacitance.  Have a look at section 13 of the TRF7960ATB users guide.  This gives a good explanation of the antenna tuning process.  Ideally, you should use a network analyzer to measure the complex impedance and inductance of your antenna coil and then determine the tuning values based on this.  I also recommend using the Smith Chart simulation tool that is used in the users guide for coming up with the estimated tuning values.

http://www.ti.com/lit/ug/slou297/slou297.pdf

Smith Chart Tool: http://www.fritz.dellsperger.net/

The read range will depend on the size of the antenna, Q factor, and also the size and Q of the tag that you are reading.  The range can be increased slightly by increasing the Q, but the most effective way to increase the range is to increase the antenna size.  I have attached the read range with the EVM of some typical tags.

4786.TRF796x EVM Read Ranges.pdf   

Gold finish is not required.  Lead finish will work just fine. 

Why does the antenna on TRF7970A EVM or TRF7960ATB have a metallic finish such as lead? A solder mask (no colour, green or something else) wouldn't be enough?

From my calculus a solder mask with a normal thickness (10-30 um) above copper, and a er=3.5 to 4.8 will not modify inductance on an amount bigger than 0,5% for those frequencies.

In did a metallic finish will decrease the resistance that's all I can point.

Best regards,

Vitor Barbosa