Hey there

I am the new one and I am from Germany. Currently I am in the USA writing
my Bachelor Thesis.

I have the following issue:
I have an AM modulated signal with a carrier voltage about 100V to 500V and
a message voltage of about 250mV. This signal is out of a coil which is
part of an RFID reader (more MCID - magnetic coupling... that's why the
voltage is so high). The carrier has a frequency of 134.4khz and the
message 4khz.

My job is it to demodulate the signal digitally (if possible) and do some
work with it to get the information out.

My idea to get this signal into a DSP was just to take a transformer to put
the voltage down to 5V which makes the message really small. Therefore I
thought on taking a 16bit DSP and do some oversampling (I thought on 1 to
2MHz concerning the carrier frequency to improve the resolution and noise
ratio).

I read a ton of articles in this forum and the internet and it's a bit like
looking for a needle in a haystack.
I think the best way to do this is this
http://www.dspguru.com/dsp/tricks/magnitude-estimator way, isn't it?.
I am using a C5515 from TI so PLL would be possible. But I don't know if I
need PLL because I am producing the signal for the coil with the dsp (a
134.4khz rectangular signal works fine). So I know the frequency and I can
produce a internal signal with this timer, too. And a correct knowledge
about the carrier is a good thing I think.

Therefore I am looking for books to reference which are explaining this...
Otherwise I would have to invent it new... I cannot just use something out
of the internet without saying why and profing it with a source (book).
I hope someone can help me :)

Greetings

> I am from Germany.
de.sci.electronics would be an alternative.

> demodulate the signal digitally (if possible)
> ... get the information out.

Have look at the datasheet of the old
Telefunken / Atmel U2270B reader IC.

> take a transformer to put the voltage down to 5V 
Due to the high voltage a simple diode is easier,
its demodulated then. A 4pole Butterworth
/ Bessel LP-filter follows. Then a schmitt-trigger
to get it digital. The manchester decoder is
done in the 8 bit microcontroller.

MfG  JRD

AlanWrench wrote:
> Hey there
> 
> I am the new one and I am from Germany. Currently I am in the USA writing
> my Bachelor Thesis.
> 
> I have the following issue:
> I have an AM modulated signal with a carrier voltage about 100V to 500V and
> a message voltage of about 250mV. This signal is out of a coil which is
> part of an RFID reader (more MCID - magnetic coupling... that's why the
> voltage is so high). The carrier has a frequency of 134.4khz and the
> message 4khz.
> My job is it to demodulate the signal digitally (if possible) and do some
> work with it to get the information out.
> My idea to get this signal into a DSP was just to take a transformer to put

[...helpless bleat skipped...]

Microchip has a bunch of good appnotes on RFID system design. Start from 
there.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

>> I am from Germany.
>de.sci.electronics would be an alternative.
>
>> demodulate the signal digitally (if possible)
>> ... get the information out.
>
>Have look at the datasheet of the old
>Telefunken / Atmel U2270B reader IC.


I had a look at this IC. The U2270B could redundantize the DSP and
everything. But this is not what my requirements list is telling me for my
thesis. Therefore I don't think that I can use this IC. But I have an eye
on this.

Why not digitally? It seems that you will tell me that it will not work to
demodulate the signal inside a DSP 'cause you write nothing about it. If
so, tell me why not.

>
>> take a transformer to put the voltage down to 5V 
>Due to the high voltage a simple diode is easier,
>its demodulated then. A 4pole Butterworth
>/ Bessel LP-filter follows. Then a schmitt-trigger
>to get it digital. The manchester decoder is
>done in the 8 bit microcontroller.
>
>MfG  JRD
>

Yep, it is possible. Actually this is the actual version of the RFID
reader. But it should be digital with a DSP therefore I can not use a 8bit
microcontroller or a didoe envelope detector. By the way: it is biphase
coded.

And I can not see any problems with a transformer and 100V to 500V. At
least there are transformers for higher voltages. I could imagine a problem
with the small message signal, though.

AlanWrench wrote:


>>Have look at the datasheet of the old
>>Telefunken / Atmel U2270B reader IC.
> 
> 
> I had a look at this IC. The U2270B could redundantize the DSP and
> everything.
> Why not digitally? 

What is the point? Digital is going to be about 100 times more expensive 
and 10 times more complex.

> It seems that you will tell me that it will not work to
> demodulate the signal inside a DSP 'cause you write nothing about it. If
> so, tell me why not.

Sure you can make a right-angled toothbrush to clean teath from behind a 
corner. It could work but for some reason they don't make curved 
toothbrushes.

> And I can not see any problems with a transformer and 100V to 500V. At
> least there are transformers for higher voltages. I could imagine a problem
> with the small message signal, though.

You didn't try to accomplish anything yet; that's why you don't see any 
problems.

VLV

>
>
>AlanWrench wrote:
>
>
>>>Have look at the datasheet of the old
>>>Telefunken / Atmel U2270B reader IC.
>> 
>> 
>> I had a look at this IC. The U2270B could redundantize the DSP and
>> everything.
>> Why not digitally? 
>
>What is the point? Digital is going to be about 100 times more expensive 
>and 10 times more complex.

True. But to be honest it wasn't my idea, I just choose the project.


>> And I can not see any problems with a transformer and 100V to 500V. At
>> least there are transformers for higher voltages. I could imagine a
problem
>> with the small message signal, though.

>You didn't try to accomplish anything yet; that's why you don't see any 
>problems.

Yep I didn't. Unfortunately I don't have the time to test several ways. The
transformer would have been one thing but because of that I wrote in this
forum to get this idea ... kind of accomplished or destroyed ;)

I am looking at the microchip site and see if I can find something
helpfull. I also will see if I can use this IC or not.

Anyhow: Can someone tell me a book about the demodulation with complex
numbers? And can someone explain me the problem with the transformer?

Cheers

On 10/06/2010 07:37 AM, AlanWrench wrote:
>>> I am from Germany.
>> de.sci.electronics would be an alternative.
>>
>>> demodulate the signal digitally (if possible)
>>> ... get the information out.
>>
>> Have look at the datasheet of the old
>> Telefunken / Atmel U2270B reader IC.
>
>
> I had a look at this IC. The U2270B could redundantize the DSP and
> everything. But this is not what my requirements list is telling me for my
> thesis. Therefore I don't think that I can use this IC. But I have an eye
> on this.

If you have a solution that's well established in the analog world and 
you want to implement it in the DSP world, it is smart to fully 
understand the analog implementation of the solution.  With luck just 
simulating the analog implementation in DSP will be a pretty good 
solution, but even if that doesn't get you there, when you do the 
mathematical analysis of the analog solution you may find your "DSP 
answer" staring you in the face.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

On 10/06/2010 08:04 AM, Vladimir Vassilevsky wrote:
>
>
> AlanWrench wrote:
>
>
>>> Have look at the datasheet of the old
>>> Telefunken / Atmel U2270B reader IC.
>>
>>
>> I had a look at this IC. The U2270B could redundantize the DSP and
>> everything.
>> Why not digitally?
>
> What is the point? Digital is going to be about 100 times more expensive
> and 10 times more complex.

If the prof wants it done with a DSP then that's the "right" solution, 
regardless of how it may survive in the commercial world.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

On 10/06/2010 04:19 AM, AlanWrench wrote:
> Hey there
>
> I am the new one and I am from Germany. Currently I am in the USA writing
> my Bachelor Thesis.
>
> I have the following issue:
> I have an AM modulated signal with a carrier voltage about 100V to 500V and
> a message voltage of about 250mV. This signal is out of a coil which is
> part of an RFID reader (more MCID - magnetic coupling... that's why the
> voltage is so high). The carrier has a frequency of 134.4khz and the
> message 4khz.
>
> My job is it to demodulate the signal digitally (if possible) and do some
> work with it to get the information out.
>
> My idea to get this signal into a DSP was just to take a transformer to put
> the voltage down to 5V which makes the message really small. Therefore I
> thought on taking a 16bit DSP and do some oversampling (I thought on 1 to
> 2MHz concerning the carrier frequency to improve the resolution and noise
> ratio).
>
> I read a ton of articles in this forum and the internet and it's a bit like
> looking for a needle in a haystack.
> I think the best way to do this is this
> http://www.dspguru.com/dsp/tricks/magnitude-estimator way, isn't it?.
> I am using a C5515 from TI so PLL would be possible. But I don't know if I
> need PLL because I am producing the signal for the coil with the dsp (a
> 134.4khz rectangular signal works fine). So I know the frequency and I can
> produce a internal signal with this timer, too. And a correct knowledge
> about the carrier is a good thing I think.
>
> Therefore I am looking for books to reference which are explaining this...
> Otherwise I would have to invent it new... I cannot just use something out
> of the internet without saying why and profing it with a source (book).
> I hope someone can help me :)

Have you googled "synchronous AM"?  Have you read up on synchronous AM 
demodulation in the ARRL handbook (that's always the go-to place for any 
communications technology that may intersect with amateur radio)?

Monolithic ADCs often get noisier as their maximum sampling rate goes up 
-- do you gain anything using a really fast ADC sampled at 1MHz vs. a 
slower ADC sampled at 100kHz?

Your signal is, indeed, teeny compared to your carrier.  Is there any 
way that you can subtract out your carrier before the ADC step, or is 
that "not digital enough" for your prof?

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

 > By the way: it is biphase coded.

Manchester goes by many names ...

> The U2270B could redundantize the DSP and everything. 

Thats a version were the U2270 is replaced by standard
components:
http://www.embeddedFORTH.de/temp/r.pdf  ( text is in german )
The MC68HC908 8 bit controller generates the 125kHz via timer.
And samples the analog signal via 8 bit A/D. The schmitt
trigger and some ALC-typ adjusting of switching thesholds is
done in software.
The filter is analog, the MC68HC908 was not fast enough
for that. Could be software, but probably not a reason to
switch to DSP, rather another faster controller.

> But this is not what my requirements list is telling me for my
> thesis. 

In theory a fancy demodulator could be useful to extend the
range of the reader by detecting very weak signals. But these
RFIDs are powered by the reader. If they do not get enough energy
they will not send a weak signal but will be unable to send any
signal.
Adjusting the frequency to the LC-component tolerance of the RFID
can be usefull to improve the strengh of the signal, but isn?t
related to demodulation directly.

> And I can not see any problems with a transformer and 100V to 500V. At
> least there are transformers for higher voltages. I could imagine a problem
> with the small message signal, though.

A 200V signal with 1V load modulation is easily handled by the
si-diode circuit. 0,25V not so. Schottky isn?t that usefull at high
voltages, so 0,5V would be hard too.
So you could scale the signal down and feed it via A/D-converter
to a DSP.
8 bit would not be enough, probably 12 bit or more. Samplerate
2x125kHz = 250kHz, some analog circuit that makes sure it samples
at the peaks would be usefull.
Remains only the real world problem: would a marginaly
improved performance be relevant relative to the more complex
circuit ?

MfG  JRD

Okay - I think I will miss something to answer but I'll do my best.

I know that what a professor is telling me is not the best thing everytime.
What I wanted to say is that I will try to hold my requirements. That's it.
If there's another and better solution I'll do it. But I have to proof it.

That brings me to my next point, the analoge understanding: I understand
the envelope detector with a diode. That's what we're using right now. I
read a bunch of things about demodulation and sync. AM demod but I didn't
find a source that gives me a great understanding about it. That's why I
ask about a book. Especially a source which comes to the point to use
complex numbers. I will take a look at ARRL handbook - didn't know that
before.

I also know that making things complex just because you can mustn't be good
(like the "Hello World" example between a beginner, a semi pro and a pro).
But I should do the demodulation and filtering inside a DSP and I'm excited
to do this 'cause I'm new in this business and I will do my next grade on
this topic (I've decided during the last 3 semesters that digital signal
processing and embedded systems are more or less my world). So why not
using a DSP.

Sure, there are a bunch of ways to get rid of the carrier. But this is
actualy not what it is supposed to do.

>Remains only the real world problem: would a marginaly
>improved performance be relevant relative to the more complex
>circuit ?

What do you mean? Do you mean why I've chosen a 1MHz ADC?


Please let me know if I missed somthing.

I've heard today that they fixed the device. So I will go there and see if
I can run some tests with filters and a transformer to see what happens.

Tim Wescott <tim@seemywebsite.com> wrote:
(someone else wrote

>> What is the point? Digital is going to be about 100 times more expensive
>> and 10 times more complex.
 
> If the prof wants it done with a DSP then that's the "right" solution, 
> regardless of how it may survive in the commercial world.

But can't you at least subtract some of the carrier before
going digital?  Not all of it, such that the demodulation
is still in the digital side, but without such a huge 
background to hide the signal.

-- glen 

AlanWrench <stud.magraf@n_o_s_p_a_m.googlemail.com> wrote:
(snip)
 
> I also know that making things complex just because you can mustn't be good
> (like the "Hello World" example between a beginner, a semi pro and a pro).
> But I should do the demodulation and filtering inside a DSP and I'm excited
> to do this 'cause I'm new in this business and I will do my next grade on
> this topic (I've decided during the last 3 semesters that digital signal
> processing and embedded systems are more or less my world). So why not
> using a DSP.

(snip)

Well, often this works.  Digital electronics (assuming economy
of scale) is often enough cheaper than analog electronics that
it does make sense.

Still, you (usually) need to understand the analog problem and
solution.  

It seems to me that, at least in the beginning, you should
subtract some of the carrier before digitizing.  It isn't
that hard to do, and isn't really demodulation.  (That is,
you aren't cheating.)  Maybe later it will turn out to be
more economical to just digitize the 500V signal, but 
maybe not at first.

-- glen

On 10/06/2010 01:14 PM, glen herrmannsfeldt wrote:
> Tim Wescott<tim@seemywebsite.com>  wrote:
> (someone else wrote
>
>>> What is the point? Digital is going to be about 100 times more expensive
>>> and 10 times more complex.
>
>> If the prof wants it done with a DSP then that's the "right" solution,
>> regardless of how it may survive in the commercial world.
>
> But can't you at least subtract some of the carrier before
> going digital?  Not all of it, such that the demodulation
> is still in the digital side, but without such a huge
> background to hide the signal.
>
> -- glen

Certainly if you can use dropping transformers you ought to be able to 
do that.  Particularly if you have to do some digital magic to get 
things phased right.

If you do you'll probably have to filter the square wave from the 
processor, so you're just subtracting fundamental from fundamental.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

On 10/06/2010 01:21 PM, glen herrmannsfeldt wrote:
> AlanWrench<stud.magraf@n_o_s_p_a_m.googlemail.com>  wrote:
> (snip)
>
>> I also know that making things complex just because you can mustn't be good
>> (like the "Hello World" example between a beginner, a semi pro and a pro).
>> But I should do the demodulation and filtering inside a DSP and I'm excited
>> to do this 'cause I'm new in this business and I will do my next grade on
>> this topic (I've decided during the last 3 semesters that digital signal
>> processing and embedded systems are more or less my world). So why not
>> using a DSP.
>
> (snip)
>
> Well, often this works.  Digital electronics (assuming economy
> of scale) is often enough cheaper than analog electronics that
> it does make sense.
>
> Still, you (usually) need to understand the analog problem and
> solution.
>
> It seems to me that, at least in the beginning, you should
> subtract some of the carrier before digitizing.  It isn't
> that hard to do, and isn't really demodulation.  (That is,
> you aren't cheating.)  Maybe later it will turn out to be
> more economical to just digitize the 500V signal, but
> maybe not at first.

Besides, one always has to do _some_ signal conditioning to get a signal 
into and ADC.  The question is always "how much", the answer is almost 
always "whatever works well and minimizes the lifetime cost of the 
system", and the argument always starts when you try to pin down the answer.

I suspect that in your case subtracting out some of the carrier is a 
good candidate for the best answer, but it may not be at all -- so 
you've got to check (and run things by your prof).

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

What would be a good signal conditioning before going into the ADC?

We had some discussion today and we figured out two plans:
 1) I will wire a transformer just to see how the signal at the output
looks 
    like
 2) When I have my stuff I will use the diode envelope detector and go
after 
    that into a ADC and from there to the DSP. If the whole thing is
working 
    with this and I have time I will see if the transformer version works
or 
    not

I am still looking for something to read like a book about the analog thing
(envelope detector, other solutions) ... anyone has a good idea for that?

AlanWrench wrote:

> What would be a good signal conditioning before going into the ADC? 
> We had some discussion today and we figured out two plans:

>  1) I will wire a transformer just to see how the signal at the output
> looks 
>     like
>  2) When I have my stuff I will use the diode envelope detector and go
> after 
>     that into a ADC and from there to the DSP. If the whole thing is
> working 
>     with this and I have time I will see if the transformer version works
> or 
>     not
> 
> I am still looking for something to read like a book about the analog thing
> (envelope detector, other solutions) ... anyone has a good idea for that?

Idiot

>
>
>AlanWrench wrote:
>
>> What would be a good signal conditioning before going into the ADC? 
>> We had some discussion today and we figured out two plans:
>
>>  1) I will wire a transformer just to see how the signal at the output
>> looks 
>>     like
>>  2) When I have my stuff I will use the diode envelope detector and go
>> after 
>>     that into a ADC and from there to the DSP. If the whole thing is
>> working 
>>     with this and I have time I will see if the transformer version
works
>> or 
>>     not
>> 
>> I am still looking for something to read like a book about the analog
thing
>> (envelope detector, other solutions) ... anyone has a good idea for
that?
>
>Idiot
>
>

A very kind and smart way to greet new guys on the board ... Actually I
don't know why I should be a idiot or rather why you call me one.
Maybe because I use the time during waiting for my stuff to see why a
transformer is bad or I am a idiot 'cause I am looking for books to read
and inform myself (besides the ARRL Handbook and Understanding Digital
Signal Processing).

But okay. It's your opinion though.
Cheers and thanks for helping me so far.

AlanWrench <stud.magraf@n_o_s_p_a_m.googlemail.com> wrote:
(someone wrote)

>>Idiot
 
> A very kind and smart way to greet new guys on the board ... Actually I
> don't know why I should be a idiot or rather why you call me one.
> Maybe because I use the time during waiting for my stuff to see why a
> transformer is bad or I am a idiot 'cause I am looking for books to read
> and inform myself (besides the ARRL Handbook and Understanding Digital
> Signal Processing).

You get used to him after a while.  

-- glen

>AlanWrench <stud.magraf@n_o_s_p_a_m.googlemail.com> wrote:
>(someone wrote)
>
>>>Idiot
> 
>> A very kind and smart way to greet new guys on the board ... Actually I
>> don't know why I should be a idiot or rather why you call me one.
>> Maybe because I use the time during waiting for my stuff to see why a
>> transformer is bad or I am a idiot 'cause I am looking for books to
read
>> and inform myself (besides the ARRL Handbook and Understanding Digital
>> Signal Processing).
>
>You get used to him after a while.  
>
>-- glen
>

Doesn't make it better ... Everyone of use was in the situation to be the
newbee and no one would like a post like this ... 

> Okay - I think I will miss something to answer but I'll do my best.

Read more of the datasheets and applicationnotes and you will
get a clearer picture what really affects performance of the reader.
It might be advisable to breadboard a U2270 from scratch
http://www.embeddedFORTH.de/temp/marc4_4.pdf ( text german )
just to get a feel for it.
There are books too. The one i can recommend:
http://www.amazon.de/RFID-Handbuch-Klaus-Finkenzeller/dp/3446220712
has a translation by Wiley:
http://www.amazon.com/RFID-Handbook-Fundamentals-Identification-Communication/dp/0470695064/ref=sr_1_1?ie=UTF8&qid=1286432749&sr=8-1

> Sure, there are a bunch of ways to get rid of the carrier. 

Well again: suppose you scale down the signal by a resistor divider
or a another coil that monitors the field. And the signal
goes to a 12 bit flash-A/D that is triggered by some additional
analog circuitry so it samples the 125kHz carrier at the peaks.
250k samples/sec now. Since the signal will probably be not
symmetric one would process the positive and negative peaks,
( two 125k sample streams ) seperatly. The digital signal
consists of the large DC-part ( "200V" ) and a small ripple that
looks like the Manchester-coded signal. A highpass/bandpass-filter
would remove the DC and the schmitt-trigger convert it binary.
One can do that for both streams ( contain the same signal )
and combine the result.
The A/D is obviously not needed: sample & hold will do.
After filtering the 8 bit A/D of a slow 8 bit controller
would do rest of the job.
That would not be limited to a 1V signal like the si-diode but
could handle the 0,25 signal too.
But as already said: another approach is to get signal up
to 1V by adjusting the frequency/coupling.

MfG  JRD

On 07-10-2010 @ 07:58:23 AlanWrench  
<stud.magraf@n_o_s_p_a_m.googlemail.com> wrote:

> Doesn't make it better ... Everyone of use was in the situation to be the
> newbee
I wasn't. At least I'd never tried to break the wall with my head.
Insted I was looking for a hammer.

> and no one would like a post like this ...
I would like.
It's very motivating information
which I couldn't probably find by myself.

-- 
Mikolaj

In article <C7KdnZdeHa1TpDDRnZ2dnUVZ_g6dnZ2d@giganews.com>,
 Vladimir Vassilevsky <nospam@nowhere.com> wrote:

> AlanWrench wrote:
> 
> > What would be a good signal conditioning before going into the ADC? 
> > We had some discussion today and we figured out two plans:
> 
> >  1) I will wire a transformer just to see how the signal at the output
> > looks 
> >     like
> >  2) When I have my stuff I will use the diode envelope detector and go
> > after 
> >     that into a ADC and from there to the DSP. If the whole thing is
> > working 
> >     with this and I have time I will see if the transformer version works
> > or 
> >     not
> > 
> > I am still looking for something to read like a book about the analog thing
> > (envelope detector, other solutions) ... anyone has a good idea for that?
> 
> Idiot

Hey  Vlad:

Please turn back on your "Am I being a douchebag?" filter.

Cheers,

Ken Prager

On 10/06/2010 10:45 PM, AlanWrench wrote:
>>
>>
>> AlanWrench wrote:
>>
>>> What would be a good signal conditioning before going into the ADC?
>>> We had some discussion today and we figured out two plans:
>>
>>>   1) I will wire a transformer just to see how the signal at the output
>>> looks
>>>      like
>>>   2) When I have my stuff I will use the diode envelope detector and go
>>> after
>>>      that into a ADC and from there to the DSP. If the whole thing is
>>> working
>>>      with this and I have time I will see if the transformer version
> works
>>> or
>>>      not
>>>
>>> I am still looking for something to read like a book about the analog
> thing
>>> (envelope detector, other solutions) ... anyone has a good idea for
> that?
>>
>> Idiot
>>
>>
>
> A very kind and smart way to greet new guys on the board ... Actually I
> don't know why I should be a idiot or rather why you call me one.
> Maybe because I use the time during waiting for my stuff to see why a
> transformer is bad or I am a idiot 'cause I am looking for books to read
> and inform myself (besides the ARRL Handbook and Understanding Digital
> Signal Processing).
>
> But okay. It's your opinion though.
> Cheers and thanks for helping me so far.

Don't feel like you have to encourage him when he gets this way.  Either 
he's forgotten what it's like to be a student, or he was really good at 
engineering and forgets all of the things that he was really bad at.

Think of it as an opportunity to be noble.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

>> Okay - I think I will miss something to answer but I'll do my best.
>
>Read more of the datasheets and applicationnotes and you will
>get a clearer picture what really affects performance of the reader.
>It might be advisable to breadboard a U2270 from scratch
>http://www.embeddedFORTH.de/temp/marc4_4.pdf ( text german )
>just to get a feel for it.
>There are books too. The one i can recommend:
>http://www.amazon.de/RFID-Handbuch-Klaus-Finkenzeller/dp/3446220712
>has a translation by Wiley:
>http://www.amazon.com/RFID-Handbook-Fundamentals-Identification-Communication/dp/0470695064/ref=sr_1_1?ie=UTF8&qid=1286432749&sr=8-1
>
>> Sure, there are a bunch of ways to get rid of the carrier. 
>
>Well again: suppose you scale down the signal by a resistor divider
>or a another coil that monitors the field. And the signal
>goes to a 12 bit flash-A/D that is triggered by some additional
>analog circuitry so it samples the 125kHz carrier at the peaks.
>250k samples/sec now. Since the signal will probably be not
>symmetric one would process the positive and negative peaks,
>( two 125k sample streams ) seperatly. The digital signal
>consists of the large DC-part ( "200V" ) and a small ripple that
>looks like the Manchester-coded signal. A highpass/bandpass-filter
>would remove the DC and the schmitt-trigger convert it binary.
>One can do that for both streams ( contain the same signal )
>and combine the result.
>The A/D is obviously not needed: sample & hold will do.
>After filtering the 8 bit A/D of a slow 8 bit controller
>would do rest of the job.
>That would not be limited to a 1V signal like the si-diode but
>could handle the 0,25 signal too.
>But as already said: another approach is to get signal up
>to 1V by adjusting the frequency/coupling.
>
>MfG  JRD
>
>

I know it sounds dumb but I have to ask if I got this right:
Do you mean that I should take/need 2 ADCs to monitor the signal?

I added something to my original plan: A analog circuit for peak detection
after the diode envelope detector to get the peaks. But I have to do some
research on this, never heart/used something like this before but it sounds
like that it makes sense ;)

> Do you mean that I should take/need 2 ADCs to monitor the signal?
No, you have only one signal so one A/D with 250kHz samplerate will
do for the 125kHz carrier if you hit the peaks.

Thats a fairly typical industrial synchronous demodulator:
http://www.seekic.com/uploadfile/ic-circuit/200972452849422.gif
It uses a zero-crossing detector ( much easier then a peak-detector )
to generate a digital signal that controls the switch.
In this implementation the amplifier is switched inverting/
noninverting. The following RC-lowpass smooths the rectified
sinewave to DC.

But depending of the way the carrier is generated i would not
take it for granted that the sinewave is symmetric/clean.
Therefore combining both peaks like in the synchronous demodulator
may be counterproductive.

MfG  JRD

>> Do you mean that I should take/need 2 ADCs to monitor the signal?
>No, you have only one signal so one A/D with 250kHz samplerate will
>do for the 125kHz carrier if you hit the peaks.
>
>Thats a fairly typical industrial synchronous demodulator:
>http://www.seekic.com/uploadfile/ic-circuit/200972452849422.gif
>It uses a zero-crossing detector ( much easier then a peak-detector )
>to generate a digital signal that controls the switch.
>In this implementation the amplifier is switched inverting/
>noninverting. The following RC-lowpass smooths the rectified
>sinewave to DC.
>
>But depending of the way the carrier is generated i would not
>take it for granted that the sinewave is symmetric/clean.
>Therefore combining both peaks like in the synchronous demodulator
>may be counterproductive.
>
>MfG  JRD
>

Not directly an answer for my question but interesting to see how a
demodulator analog works. Why the switched capacitor?

And is the idea to digitalize after the envelope detector and a
lowpassfilter inside the dsp dump or not? I'm not sure right now 'cause I
need something like a Schmitt-Trigger after the filter to have clear 1 and
0 values ... Therefore I could just use a microcontroller ... Dump
requirements -.-

>> Thats a fairly typical industrial synchronous demodulator

> Not directly an answer for my question 

If your problem is "can?t use a diode because 1V signal is
needed and the spec says 0,25V is available" then the obvious
next step is a synchronous demodulator that replaces the diode
by an analog switch.

> Why the switched capacitor?

There is no switched capacitor. Its a switched amplifier gain
+1 or -1. Works as a full-wave rectifier.
The advantage of analog circuits is that they have standard
symbols and are easier to grasp then flowcharts for software.
Implementation is most often possible analog or digital.

> Therefore I could just use a microcontroller ... Dump
> requirements -.-

If this is your Bachelor Thesis what do you intend to do
afterwards ? If you go into industry the obvious step would
be to join a company that builds this type of circuits. But
then it should better work, be technically reasonable.
If on the other hand you go for academia a DSP will not
do, thats old 80ies technology. You will have to throw
in at least a FPGA and a Kalman filter.

MfG  JRD