Advice needed for envelope detector.

Discussion in 'General Electronics Chat' started by Th.Mihov, Nov 7, 2017.

  1. Th.Mihov

    Thread Starter New Member

    Feb 2, 2017
    4
    0
    Hi everyone.
    I'm trying to build my own 125kHz RFID reader.
    I'm driving a series RLC resonant circuit consisting of 100 ohm resistor,10nF cap and 345uH coil (not pretty sure about the coil inductance,because i've bought it from ebay long ago and i didn't measure it) with 5V 50% duty cycle 125kHz square wave form Atmega328.In the mid point between the cap and the coil,i'm getting nice sinewave without distortion with 4Vpp amplitude.Since the signal from the RFID tag is AM modulated,to extract the data from the carrier frequency it's logical to use envelope detector.And here comes my question -what should be the cut-off frequency of the envelope detector,formed from the RC group after the diode?
    I've made some calculations:
    According to the EM4100 protocol description,the RFID tag sends 64 bit to represent it's number.There are few options about each bit's length-16,32 or 64 carrier frequency periods per bit.I have a bunch of the 64-bit version so i'll use them for teting.
    So, Tcarrier=1/125kHz=8uS
    Tbit=64*8=512uS => f=1/512=~2kHz
    I'f I am rgiht,the cut-off frequency should be a bit above 2kHz or i'm wrong?
    About choosing the resistor value -i think that the resistor should be chosen so that it's resistance is much greater than the capacitive reactance of the RLC tank capacitor ,since the envelope detector is in parallel with him ,so Renv>>Xc.
    For my case,i'm using 10nF cap in the tank circuit which have Xc=127ohm at 125kHz,so a resistor of about 22-33k should work or not?
    Thanks for any help
    Best regards
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,173
    401
    Time Constant needs to be smaller than 1/highest message frequency and larger than 1/IF. This is the classic formula for the largest capacitor you can use before "diagonal clipping" sets in.
     
  3. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,173
    401
    Diagonal Clipping is referred to as "failure to follow" clipping in some texts. It's the main reason passive envelope detection has gradually faded in favor of product detection.
     
  4. Th.Mihov

    Thread Starter New Member

    Feb 2, 2017
    4
    0
    In other words Tmessage>>tau>>Tcarrier ?
     
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