Rfid tx

Discussion in 'General Electronics Chat' started by eng_alaakaa, Dec 5, 2011.

  1. eng_alaakaa

    Thread Starter New Member

    Sep 21, 2011
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    Hi everybody.....
    this schematic is for RFID TX . I have these qustions :
    1- what's the purpose of these inverters (1,2,..6) and
    why 2,3 and 4 cascaded ?,also 5 and 6 in parallel :eek:?!.
    2- why these capacitors at antenna in parallel?.
    3- are these 22pF cap (C2,C4 and C28) at crystal used to stabilize the clock :rolleyes:?
    4- why there are 10MΩ (R20) feedback resistance?
    5- L1 and L2 used for .....:confused:?
    6- why this 10kΩ(R6) at +5V ?
    7- what's the type of this amplifier ?
    finally, can I simulate this ckt using multisim11 (where can I find the oscillator and antenna ???
    please I want to know every thing about this ckt, and I need your experiance .......:):):)
     
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  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Here are my guesses to a few of your questions.

    1. Maybe the three inverters in series introduce a desirable propagation delay, or maybe the designer just used them because he had them in the package. Maybe having the last two in parallel increased the gate drive to the mosfet, or maybe the designer used them because he had them.
    2. Maybe to tune the antenna to a particular frequency.
    3. Very often, you will see a couple of 20 or 30 pF caps on each side of a crystal to ground; apparently it helps to ensure the crystals will oscillate.
    4. No guess.
    5. Again, maybe antenna tuning.
    6. Maybe it's a pull-up resistor to ensure that the gate of the mosfet is low unless the output from U1A is low.
    7. No guess. Don't know anything about multi-sim.


     
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  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    I may be repeating some of what Tracecom said...

    The first inverter is part of the crystal oscillator. If you want to learn about them read the following application notes:

    http://www.onsemi.com/pub_link/Collateral/AND8053-D.PDF
    http://www.icbase.com/pdf/add/ti/AN-101-00039en.pdf

    C28 in series with the output looks different, I don't know if this could work.

    It's not a common practice to put inverter stages in parallel, their propagation delays etc do not 100% match. It was surely intended to give a boost to the gate of the transistor.

    L3 is supposed to be a loop antenna I suppose. The caps in parallel are supposed to tune the antenna circuit to the oscillator frequency. [​IMG]

    R6 is part of the High pass filter R6 C6.

    Even if this worked as a RFID transmitter, the only thing you have is the RF, no ID, the carrier is not modulated to provide any form of identification....

    I also have a question: Where is this schematic from?
     
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  4. eng_alaakaa

    Thread Starter New Member

    Sep 21, 2011
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  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    For future posts: It always helps to provide as much information as possible right from the beginning.;)

    Ok, so it seems the READER is really only transmitting a carrier signal. Bringing a receiver coil near the reader will detune the antenna circuit, therefore changing the the rectified antenna voltage via D6. The following OPAMP stages recover the modulation signal to bring it to a level the PIC can work with.

    U8:E is part of the crystal oscillator (output buffer). It seems to me that the circuit would also work if U8A and U8C were omitted, but since there ARE spare gates of IC8 they must have put them for a reason. They obviously wanted the oscillator input pin of the PIC kept separated from the rest.

    I don't see a simple way to simulate the antenna circuit in MULTISIM, especially not because we don't know how much will be the received modulation voltage level.
     
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  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    For an antenna in multisim, link 2 inductors with a very low coupling coefficient and low signal, so the input is approximately what you'd expect.

    Create a new component, basically like a transformer, but air core and not a very good one. There's a few threads on coupling inductors and simulating antennas on NI's forum if you search.


    L1 and C26 form a low pass filter for the power, cutoff around 1.5Mhz. L2 C6 make a notch filter at about 6Mhz

    The rest of the capacitors and inductors make the tank circuit for transmitting.
     
    Last edited: Dec 6, 2011
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  7. eng_alaakaa

    Thread Starter New Member

    Sep 21, 2011
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    please can any one provide me with the proper function of this transistor......???
     
  8. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    What do you mean?
     
  9. eng_alaakaa

    Thread Starter New Member

    Sep 21, 2011
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    I built this circuit and test it, the signal is pretty good sinusoidal 13.56MHz before it inter this transistor, the output signal's voltage from this transistor is attenuated and the frequency increased to 20MHz.....?? what's the function of this transistor, can it be a current gain...??
     
  10. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Two things:

    You need more power than a logic gate output can provide to drive your coil/antenna. The transistor provides it.

    The input to the FET is a square wave. You don't want to send a square wave to your coil , it's full of harmonics. You need only the fundamental frequency. The following LC's do that for you.

    How did you build this circuit? (Layout) Can you post a picture of it?
     
  11. eng_alaakaa

    Thread Starter New Member

    Sep 21, 2011
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    this is my circuit.....:)
     
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