Joined: 30 April 2008
Location: 222
Posts: 1

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Dear technicians, Due to a quite wide range of tag-loads the RFID-antenna shows impedances that vary from 50 Ohms; therefore I cannot guarantee the RX-input-limits. I tried to use the envelope detector given in your MLX90121-cookbook, but it didn´t work. First I can see that the DC-input ist much higher than that 2-3 V so I increased R3 (RX-VDD-connection) to appr. 70kOhms which lowers the DC-voltage at the RX-pin to about 2.5V. Even though the subcarrier-voltage has a peak-level of more than 1 V within the RF-signal the DOUT-pin is constant HIGH. Now I use simple resistor network given in the cookbook but with 340Ohms instead of 680Ohms to get useful signals. Any ideas?

Joined: 1 February 2006
Location: CH
Posts: 86

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Dear,

Increasing the resistor R3 of the external detector will reduce the DC level but will also increase the time constant of C3 and R3. This is not be suitable for a correct reception (the the stabilization time may be too big).

Anyhow, it seems that in you application, the RFID reader has to receive a quite large back modulation from the TAG (sub-carrier of about 1V peak). I am afraid that with such a big modulation, the MLX90121 enters in saturation, even if the external an external diode detector is used. The only solution is to use an attenuation cirucit like you did.

Best Regards
Denis