Other Parts Discussed in Thread: TRF7964A
I am creating a design for the TRF7963a to read/write NTAG2016 tags in a very restricted environment. I am using the TRF7960EVM as my reference circuit but with the loop antenna replaced with a 1.5 uH through-hole ferrite drum inductor (Bourns RLB0913-1R5K). The inductor axis is always coaxial with the tag axis and the maximum reading distance from the top of the inductor is 10 mm. I used a VNWA to obtain the complex impedance of the inductor at 13.56 MHz and SimSmith to determine the values of the matching network (C21-C24 and R6 in the EVM schematic). I then (a) modified a copy of the EVM by deleting use of the loop antenna and adding a SMA connector and (b) created a PCB to test the antenna circuit. It works very well and provides a reading distance of 25+ mm.
Now comes the rub. The inductor has a tolerance of +/-10%. When I feed the min/max values into SimSmith the matching network does not do a good job which is expected. So I have three options of where to go from here. 1) Ignore the inductor tolerance and hope the worse case values will still provide a reading range of 10+ mm. 2) Replace C23 with a trimmer capacitor and manually tune each copy of the circuit in manufacturing test (UGH!). 3) Replace C23-C24 with a pair of cathode connected varactor diodes and use a DAC to tune the series capacitance value via the control firmware.
I prefer Option 1 but I need feedback from others to determine if this is reasonable. So I am leaning toward Option 3. To make this work I would vary the varactor control voltage to adjust the parallel capacitance and use the external signal level values in the RSSI register to determine when the tuning is optimal. The process would use a successive approximation algorithm. The tuned value would be saved in Flash ROM. Has anyone tired such an approach? If yes, does it work? In theory it should work but ....
