TRF7960A Antenna Impedance

Mike H Meyer

Expert 1560 points

I'm working on a design with a trace antenna. is there an optimal complex impedance to target for the trace antenna?

over 9 years ago

0 Eddie LaCost over 9 years ago

TI__Mastermind 40270 points

Hi Mike,

Generally, you would like to see about 1uH - 1.5uH inductance. Have a look at section 10 of the users guide below which walks through tuning an antenna with XL = 0.8 + j80.8(~1uH).

http://www.ti.com/lit/pdf/slou372

0 Mike H Meyer over 9 years ago in reply to Eddie LaCost

Expert 1560 points

We had a handful of good and bad RFID PCBAs that I was analyzing in attempt to determine what is causing the bad PCBAs to be bad. All the good RFID PCBAs had an inductance of ~ 1.5uH and the bad ones only had ~0.5uH. I figured this was the cause of the issue, but I had to figure out what would drive this change in inductance. We had failure PCBs that were from panels that had mostly good PCBs, so we couldn’t say that it was just a bad batch. We proceeded to do cross-sectional analysis of the PCB to see if there was a lot variance in the PCB geometry’s that could cause this delta in the inductance for the antenna. The following is the variance of a batch of 4 good and 4 bad PCBs.

Trace widths – 8.6 to 9.8 mils
Trace thickness – 2 to 2.6 mils
Solder mask thickness – 0.6 to 1 mils
Overall thickness – 27.9 to 28.4 mils

Would this variance cause the inductances deltas we are seeing?

0 Eddie LaCost over 9 years ago in reply to Mike H Meyer

TI__Mastermind 40270 points

Hi Mike,

I don't think that these variances would cause such a large difference in inductance. See the calculation below for a trace width of 0.3mm and 0.3254mm, which is a 1 mil difference in width. The inductance varies from 1.36uH to 1.33uH.

I think what could be more likely happening is that there is a short between two traces of the antenna coil. I have seen this happen before when the trace spacing is not adequate for the PCB manufacturer. This will essentially reduce one turn from your antenna coil, which will significantly reduce the inductance. Try to see If you can try to have a look all the way around the antenna coil trace under a microscope and see if you see any small bit of copper shorting the coil. It may be very difficult to see as it may only be a very small piece of copper.