Help (MI-Module Index)

[Harald Welte]

On Tue, Apr 08, 2008 at 04:17:14AM -0300, Diogo B. Franco wrote:
>   Hello Harald !

Hi Diogo!
>                       I'm writing to ask you some help! I'm student of 
> Electrical Engineering in Brazil and I'm studying contactless cards and 
> trying to develop some system based on this technology.

great.

> Here is unusual this services and it's been very 
> hard to advance on it. Anyway, I really need some help. Nowadays I'm 
> reading the Smart Card Handbook to improve my knowledge. I have to do 
> some tests with cards that were sent to my university but I need to use 
> some reader that allow me to change de MI(module index) in the process. 
> For example, to test the typeB proctocol I need to do some tests with 
> 8%MI and with 14%MI, just to chek the limit that the cards accept. To do 
> this, I need some component that allow me to change the MI(module index). 
> The carrier that I'm using is 13.56MHz and subcarrier up to 847.5Khz.

I completely understand.  By the way, it's called "modulation index" :)

> Normally the readers have the MI fixed based on  the type of protocol
> and then it's been very hard to find something that really provide me
> what I want. Have you some idea to help me? Do you know some component
> witch one I can use in the reader? I really appreciate your help and
> for sure you'll be helping us to grow!!! Thanks a lot!

The modulation index is usually software-programmed, i.e. the reader
hardware is able to provide different modulation index, but the software
(device firmware, driver, protocol stack) is proably not exporting this
feature.

With our open source OpenPCD and librfid design, it is possible to
change the source code and thus to change the modulation index.  For the
NXP CL RC632 reader ASIC (which we use in OpenPCD) there are the
mod_conductance registers, which determine the bias voltage of the
output driver stage that drives the antenna output.

You should probably attach a loop antenna to your oscilloscope and look
at the waveform, then try different mod_conductance values until you get
the desired modulation index.

-- 
- Harald Welte <laforge at openpcd.org>          	         http://openpcd.org/
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