Forum: DSP Which approach can I use to demodulate this signal?

von Daniel F. (Company: UFMG) (danbakana)

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I'm trying to read a LF(Low Frequency) transponder, it works in the 
following manner: It uses load modulation, the receiver generate the 
carrier and the transponder modulates it using magnetic coupling(like a 
electrical transformer). The tag being read switch a load (resistance) 
and it changes the carrier's amplitude like can be seen in the figure 

The information being modulated, as can be seen, is very weak compared 
to carrier's amplitude. The carrier frequency has 125kHz and the base 
band signal has a 20kHz bandwidth.

The figure modulation_2 the signals were sketched in frequency domain.

I have only a dsp to process this signal and a conditioner circuit to 
put the signal received in an appropriate voltage level. I assume that 
amplitude demodulation would be a better approach to solve the problem.

After the sampled signal go through the filter I perform a amplitude 
level comparison to decide in wich logical level to translate in bits( I 
determine this level experimentally ). But, this is not being good 
enough, the amplitude level after filtering is very low and there is a 
DC( constant value) component present after filtering. The level of 
amplitude can vary to diferent tags being read and the comparison to 
detect the logic level may not be a robust approach.

To design the low pass filter I used a windowed sinc with window 
function Kaiser-Bessel. Frequency sample of 900 kHz, -3dB frequency of 
30kHz and 12 taps(coeficients).I'm using  dsPIC33EP it's sampling rate 
can reach up to 1MHz and the DSP core can reach up to 70 MIPS.

Please, I don't know what to do anymore, so, whatever tip will be very 

von Karl (Guest)

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Try to measure the load current.

von Manfred M. (bittbeisser)

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I am not really an expert but the problem sounds interesting. But I have 
some questions about this. And please apologize my simple english.

1) Why do you use such a high samplerate? This would make it harder to 
design a good lowpass because the corner frequency is narrower to the 
edges of the passband.

2) What is the amplitude range of the signal and how many bits are used?

3) What kind of signal is that? Is that something like a Manchester 
code? Is it free of DC bias? In this case you can use a simple moving 
average to get a decision threshold.

4) And why you are using only a 12 tap FIR? Is this a hardware 
limitation? If so I think you should also try to calculate a 14 tap 
filter and use a Von-Han, Lanczos or a Cosine window. These windows will 
make the first and the last coefficient to be zero and so I think they 
don't need to be realized in hardware.

In the last point I may be completely wrong, just an idea.

: Edited by User
von Daniel F. (Company: UFMG) (danbakana)

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ok, I'm not an expert either, far from it. I'll apologize for my bad 
english, it is not good too. Anyway, thank you in advance for your 

1)  I used a high sample rate, it was at the begining,  because when I 
used I thought it would be better, but when I used a lower sample rate, 
but still respecting the Nyquist sampling, I didn't observed any 
considerable improvement. I used a 500kHz sampling rate either, and it 
was better, but not so much. I really would like to slower the sample 
rate and have a better performance of the filter.

2) It is a LF RFID, the amplitude is not fixed,  can vary, it depends on 
the tag being read, but it doesn't exceeds 3.3V.

3) Well, the coding used will be dependant of the tag being read, it can 
be Manchester, NRZ maybe others I don't know exactly. But, about the DC 
bias, I can say that after perform the filtering the resulting signal 
has a DC component, it it is being a very bad problem because the DC 
component is not fixed. At the begining, before transmission and carrier 
generation, it has a fixed value, but when comunication starts with the 
tag present, the DC offset changes.  So it depends of the tag being read 
and it's phisical placement in the device. So, I cant remove only 
measuring the DC offset at the begining and subtracting it from the 
samples. It is bad because the modulated amplitude is very low compared 
to the carrier's amplitude and after sampling it still remains low and, 
because of the DC compenet I can't apply a gain.

4) I dindn't followed any criterion to determine the amount of taps, I 
just observed that it made difference until 8 taps after that, 
increasing the number of taps did not make any better, I even got to use 
32 taps but it did not become better. After the pass filter I tried to 
use a DC blocker filter, and the many taps on the low pass filter 
decreased the performance, so I decided to use only 12.

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