RX chain for LPCD mode MFRC63003

[jeanbrosset]

Hello all,

for MFRC63003

I would like to know if registers used to performe reception of RFID data, could have an impact for LPCD function?

example: Rcv registers, Rx threshold, or others…...

In my production I have disparities in distance of detection. for less than 20%, detection is very bad (2-3mm) and for the others, mean is around 15mm.

these impacts are only for LPCD function, because reading is better ( mean 30mm +/- 1mm ) and very stable even if LPCD is bad 

thanks a lot 

Jean

[xieshengguang]

Is it convenient to share your code? I have a problem writing EEPROM through MFRC63003. I want to refer to your code, thank you very much！

[jeanbrosset]

Hi Kan,

I took time to answer you, because I juste rent a probe with low capacitance (TAP1500 from TEKTRONIX). for me the result is the same with my probe of 3.9pF. I'm below the limit of 1.65V for the reading and for LPCD pulse.

Have a great day

Jean

[Kan_Li]

Hi Jean,

Thanks for the information! The Q is good - the higher the better for the LPCD. However, with such a small reader antenna and such small tags, I would not expect a huge detection range. you know , the antenna sizes determines the read range, and the read range is larger than LPCD detection range. This is, because the tag already start to operate, when it does not yet detune the PCD antenna. This is a hardware limitation of the LPCD, which we have to live with.

We usually try the following ways to extend the detection range, but there is limitation:
1) we decrease the threshold to 0:
that inculdes the risk to wake up due to LSBit-toggling -> can only be handled with special treatment like the filter options.
2) We increase the RF field (enable charge pump): but that has also limited effect. Enabling the charge pump might shift the phase, and then bring a "good" phase into "bad" and vice versa.

Please kindly check on your side if either meets your case.

Hope that helps,

Have a great day,
Kan

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[jeanbrosset]

Hi Kan,

In all cases, Rx level seems be less than 1.65V. ; I say " seems " because I use a scope probe with 3.9pF. 

for the Q factor; When I match antenna, I reach for a Q of 40. Is it a good way?

We don't have specifications  about tags, we buy them in china. but the diameter of them is near 27mm. ( a little less )

for the antenna of my reader; it's a symetrical pcb antenna with 4 turns. the smaller diameter is 26mm, and bigger is 32mm.

Have a great day

 Jean

[Kan_Li]

Hi Jean,

Thanks for the information! but is it possible to have a low capacitance probe(<2pF) to measure the RX level on RXN pin? You know, different measuring result might lead to a different conclusion.

Thanks for your patience! 

Have a great day,
Kan

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[jeanbrosset]

Hi Kan

these are my results: 

In the case off bad detection without charge pump ( less than 20%) I=29 Q=33 threshold=+0/-0

In the case off great detection without charge pump ( more than 80%) I=29 Q=33 threshold=+0/-0

In the case off bad detection with charge pump ( less than 20%) I=34 Q=30 threshold=+0/-0

In the case off great detection with charge pump ( more than 80%) I=34 Q=30 threshold=+0/-0

for Rx level, I will check again for these readers and according the AN11019 and let you know about it.

but during developpment I had checked that Umax and  Up-p doesn't exceed 1.7V.

Is it the same rules that reading chain? because in all cases reading distance is great and very stable.

thanks a lot

Jean

[Kan_Li]

Hi Jean,

Thanks for the information!

The I and Q levels look good if these values are decimal. The threshold are quite low, it might cause the device to wake up without tag in some conditions. Referring to AN11783, the recommended threshold is TH=1.

For RX level, please make sure that the overall level shall not exceed 1.7Vp. Normally the signal is biased at VMID, and the peak level shall not exceed 1.7V. Better is less than 1.65V.

With larger levels the Rx might clip: that does not destroy anything, but reduces performance.

These rules hold for the LPCD, too. Anyway you would not change the Rx coupling from normal Rx operation to LPCD operation.

so it looks like the antenna Q factor is really low.You know, the LPCD requires a detuning to detect the card.

Which tags are used?
What is the PCD antenna size, and what is the PICC antenna size?

Have a great day,
Kan

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[jeanbrosset]

Hi Kan,

I don't use the charge pump function. should I do?  I would like to be sure that if I use this function, distances of detection will be more regular.

thanks a lot

Jean

[Kan_Li]

Hi Jean,

Yes, with this feature, the detection range will be longer but the current consumption will be higher than before.

Have a great day,
Kan

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[jeanbrosset]

Hi Kan,

We made the test with charge pump function on the same products:

products with bad detection (2-3mm) become better( between 10-15mm), but products with best performance( 15mm) are being degrade to 5-6mm.

How we can explain these results?

however, distance of reading don't change.

could you help me more?

Have a nice day

Jean

[Kan_Li]

Hi Jean,

The LPCD detects detuning of the antenna, so the tag must detune the antenna. For the operation that is different, i.e. if the Q is low, the antenna is very "stable", and the detuning is very low. Then the LPCD is difficult.
I would like to know more details of your issue, with or without the charge pump, 
1. What is the threshold setting?
2. What are the related LPCD I and Q levels?
3. Is the Rx level according to the spec?

Have a great day,
Kan

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[Kan_Li]

Hi Jean,

There is no impact on LPCD detection range except the charge pump, so did you enable the charge pump?

Have a great day,
Kan

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