Rf preamp

Discussion in 'Wireless & RF Design' started by geoffers, Nov 14, 2011.

  1. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    Hi all,
    I've a problem using the Atmel u2270b, its a RFID front end chip that interfaces with a PIC to read FDX transponders (they load the field generated by the chip and coil) I've got it reading ok but recently found a aplication note on how to increase read range by putting a op amp infront of the u2270b. My knowledge of opamps is very little but I set about it and built up a board to test things out. To start with nothing happend, that is until I touched the legs of c3 with my finger, hey presto it reads?? The only difference in my setup is the data rate of the transponders is f/32 rather than f/64 which is described in the application note (which hopefully is attached) Does anyone have any ideas about how to calculate the right capacitors to use? I guess I'm changing the capacitance by touching the legs? Any help would me welcome.
    Thanks Geoff
    Ps I have c3 and c4 on a breadboard and all the rest on a homemade doubleside pcb.
     
  2. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    Hi all,

    I've been playing around with this cicuit a bit and have got it to read! I think the problem was the lopass filter c1/r1, as my signal is faster, I halved c1 and it works. However read range is not improved! A previous circuit I made used diode feedback? While I don't realy understand how the diode feedback works, it does give a greater read range. But this was with a 2.3mH coil, the coil I used for the op-amp circuit was a little less. Does anyone know or have any thoughts on optimising range? I guess there must be a equation that would show the optimum, l/c ratio for range? Also the transponders I am using are quite small, 3mm diameter so the coil is even smaller! I may already be close to the limit of range? I have it reading at 25mm from coil to tip of the transponder with a 46mm dia reader coil. This is using diode feedback.
    Any thoughts on this would be welcomed!
    Thanks Geoff
     
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