Joined: 30 April 2008
Location: AT
Posts: 4

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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: 96

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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

Joined: 30 April 2008
Location: AT
Posts: 4

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First of all thanks for your answer.

For details about my still existing problems I posted a few oscilloscope-pictures on my webspace. Pls. go to http://www.wissenwasser.at/mlx90121

First a few more details about my system: It has a power-consumption of nearly 40mW (yes, milli-watts). The reader- and tag-antenna have a diameter of appr. 16mm and work within a distance of 3-4mm with a coupling-factor >60%. The communication is only one-way from tag to reader. Because the tag has an µC with its own 1MHz-Oscillator I am not able to use ISO-conform subcarrier-frequencies but f_sc=250kHz.
As you may now guess it is a completely selfmade-tag with a few functions (tag.tif).

Something on your posting:
First I have to say that the R3-C3-time constant shouldn´t make problems due to the fact that communication only starts when the system is in some kind of a "static" state (appr. 300ms after Power-On).

External Envelope detector
==========================
There is one important question about the external envelope-detector: If I am right the current source at the RX-input has appr. 37uA. Therefore a 27kOhm-resistor causes an effective DC-level of appr. 4V. Isn´t that too high? The datasheet tells me an RX-VSS-voltage of max. 3V. Or is that only the AC-part? I tried to use this but in that case the DOUT-pin is permanently HIGH.

Then a question about R2-C2: The time constant will be appr. 100pF*(22kOhm||27kOhm)~1.2us. Isn´t that too slow? Even with a 212kHz-subcarrier the envelope-signal can´t relax enough. I would say that the time constant should be about the 3rd to 5th part of the half sub-carrier-period (i.e. 800ns for f_sc=212kHz - I know, that´s only 10x more than the 13.56MHz-period)


Majority Voting
===============
Furthermore I found out that using the Majority voting seems to have an influence on the data quality: Without MV activated all data can be well detected (shown on the oscilloscope; see PRINT_03.TIF). But if MV is active nearly the half of all data that should be interpreted as LOW is returned as HIGH (no picture available).

Please take a look to Picture PRINT_02.TIF. It has been taken using the resistor-network for RX given in the cookbook but with 340Ohms instead of 680Ohms.
Because the received signal quality is sometimes really bad I used 68us-syncronisation bits followed by 36us space and then 36us for each data bit. The upper part shows DOUT without MV. Btw. the lower signal is the ANT-pin with nearly 1V peak. As you said the receiver is in saturation in that case. Is that right?

PRINT_03.TIF shows one sync-bit. The CK-pin is LOW until my reader-uC detects HIGH at DOUT for at least 68us.

If you are still with me I would like to know your opinion about the schematic (schematic.tif) of a possible solution for both AC-levels (regular signal and subcarrier-signal):
As I wrote the tag-reader-coil-distance is within 3-4 mm variing about 1mm which causes big differences in the signal strength. Therefore I was thinking about a possibility for a signal levelling.
The first and last part of the schematic are like your external envelope detector given in the MLX90121-cookbook. In the middle there is a Schmitt-Trigger. The AC-input is raised to the half supply-voltage by R9 and R10. The low-pass filter R2-C2 has a big time constant so that the input signal is damped. The Schmitt-Trigger-levels are set to +/-25mV of the AC-input. At the output the rectangular signal is reduced to appr. 100mV peak-level and AC-coupled to the RX-pin.

I know that´s a little bit extreme but it should solve the problem with the input-signal-saturation.


But at least the most important things compressed:
*) DC-input range of RX-pin?
*) AC-input range of RX-pin for 13.56MHz and the subcarrier?
*) What about the problems found with Majority voting?

Joined: 1 February 2006
Location: CH
Posts: 96

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Dear,

As I can see, the MLX90121 plays its role by integrating the sub-carrier frequency to provide the base band signal on DOUT (the data rate is a multiple of the sub-carrier frequency).

If the communication is made without any sub-carrier frequency (data rate at 250kbps), I would suggest you having a look to the MLX90121 cookbook (available on our web-site) to the chapter "Support of different modulation modes". A specific register’s configuration must be programmed for such specific communications.

I guess the problem with the majority voting (MV) is also linked to the register's setting. The MV is time related and therefore must be started by rising-up the CK pin at the beginning of the reception (synchronization). Moreover, the time DecTimeRef must be programmed according to the data rate to receive.

Best Regards
Denis