Hi,
We have an application with TMS37157 that works good in a range of 3 to 7cm between basestation and tag. But I can't figure out why it doesn't work below 3cm distance.
For the tag, we use a custom coil, diameter 24mm, 2.66mH. To isolate the problem, we connected this coil to the ez430-TMS37157 target board: same problem.
Below is a plot at 3cm, where it still works. You can see the decaying FSK response of the tag after the read page 3 command.
(Green: VCC, yellow: RF1, blue: EOB)
Below: same plot but now at 2.5 cm. We verified the timings, they seem ok. Also, the EOB signal is the same, so I would think the TMS37157 should correctly decode the 'read page 3 command'.?
However, there is no FSK response from the TMS37157. Instead, VCC is pulled to ground.
I reduced the voltage of the class D driver on the basestation from 5V to 3V, but that has no influence: still works above 3cm, but not below 3cm.
Any ideas?
Thanks,
Johan