More RFID questions!

Discussion in 'Wireless & RF Design' started by geoffers, Mar 27, 2014.

  1. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    Hi All,
    Yet another question I'm afraid. I've made quite a few rfid readers now, using the u2270b for one type of transponder and the tms3705 for others.

    This works ok but I have to use two coils, the tms has to come from the US and costs around £10 a time, blah blah blah!

    My knowledge of rf electronics is limited, I've picked it up as I go along so maybe missing some basics! :confused: . Attached is a schematic of the basic building block for using the tms3705, a coil is attached to the conector forming a lc tank driven by the chip. Across the coil at the moment I am seeing 40v peak to peak, I have seen much more with different coils.

    The data sheet states the maximum voltage at sense can be 0.3v above supply. The sense pin is the input to a rf amplifier, which is according to the data sheet a op amp with a fixed internal refrence, this feeds back to sense via sfb through a 150k resistor.

    At sense which is 'across' the coil via a 47k resistor I am seeing about 5 volts peak to peak, my question is how can this happen? I would like use a op amp and comparators to replace the tms3705 but have a feeling there must be more than the rf amp attached to the sense pin inside?

    Is there a op amp that could stick up to 180v peak to peak? (I've seen that much!)

    TMS3705.jpg

    Ps. this type of transponder charges then transmits when the field is turned off, it runs at 134khz

    Cheers Geoff
     
  2. tgdrew

    New Member

    May 4, 2014
    2
    0
    The tms wants to see a bit more than 5v pk-pk across the coil otherwise it thinks that the rf side isn't working correctly, not at resonance, or just plain dead.
    I have found that about 7v+ pk-pk seems to be enough to wake it up and start decoding.

    My first rfid receiver was built on a 10x20cm pcb and used millions of comparators, opamps, hc ttl chips and a microcontroller (and sweat,blood etc).

    It worked, but then I found the tms3705 doing everything except the crc in a surface mount 16 pin . No contest.
     
  3. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    Hi ,
    Thanks for the reply, your right the TMS is a handy chip, however I would like to better understand what's going on? It may or may not be practical to do my own reader front end but I'd still like to have a stab at it, if you still have it could you post a schematic of what you made before you used the tms3705?
    Cheers Geoff
     
  4. tgdrew

    New Member

    May 4, 2014
    2
    0
    I'll see what I can do. I laid the schematic out using Easy-PC on a Pentium 2 PC running DOS only and outputting to a dotmatrix printer!
    I'll see if I can scan the faded printout.
    I can also tell you all the things that turned out to be wrong with the design (with large amounts of rearview mirror vision.)
    The main problem was way too much gain in the front end to compensate for some fairly fatal (electronically speaking) antenna design issues.

    A bit like burying a microphone in the middle of a mattress and then cranking the pre-amp gain up to x1 000 000 to compensate....
     
  5. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    That would be great if you could, always better to hear from some one who has done the job! :), sorry for being such a long time replying, damn ol grass harvest gets in the way of everything.
    Cheers Geoff
     
Loading...