ASK Demodulator

Discussion in 'The Projects Forum' started by shaikss, Jun 26, 2013.

  1. shaikss

    Thread Starter New Member

    Oct 31, 2011
    7
    0
    Hi,

    I am designing an ASK demodulator for RFID Tags. I have attached the circuit diagram. This is the ckt from IEEE paper.
    If you look at the ckt, the first stage is the inverter kind of thing. I didn't get why second NMOS is used for Stage I. Similarly, the second stage is also inverter with two NMOS. Again, the question is why second NMOS. But when I checked the output of that stage, it is acting like a latch. The last stage is buffer.

    I generated ASK modulated signal and fed as input to the demodulator. I expected that whenever modulated signal is present, demod output is '1' and when no signal is present, it is '0'. To my great surprise, it is always '1'. I tried to give the input by delaying it. So, for the very first time, because of the delay, I could see a logic '0`' at the output. After the delay, there is signal and so the output is logic '1'. Afterwards, though there is no signal, I don't see logic 0 at the output. Its always high. I assumed that the second NMOS at second stage is acting like a latch. Please let me know how can I redesign it? Or Is my understanding wrong? I have attached the simulated results.
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,137
    1,786
    That is not much of a circuit design with no passive components or values. What makes you thing such a thing will work? What frequency do you think a Wi-Fi signal operates on?
     
  3. shaikss

    Thread Starter New Member

    Oct 31, 2011
    7
    0
    Part '3'....I have connected the drains to the gates. I believe the connection is missing in the figure. Yaa, what u said about the operation of ckt is right. But I don't understand whenever signal is there, output should be logic '1' and when no signal present, logic '0' should be at output. I don't see such behavior here. Once the ckt provides logic '1', the output stays on logic '1' though there is no signal at the input. What should be the work around on this ckt, if I want to see my expected output?
     
  4. shaikss

    Thread Starter New Member

    Oct 31, 2011
    7
    0
    Wifi signal is opearting at 865MHZ
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,137
    1,786
    That sounds like a cellular telephone band which is not the same thing as Wi-Fi. In any case you still don't have anything that looks like a practical circuit. Maybe there is information in the drawing or the text that would help you construct this circuit on a substrate, but I don't even understand the symbols so I'm not going to be much help.

    According to the wiki, ASK (Amplitude shift Keying) uses different amplitudes to represent bits. The key to any modulation scheme like this is the ability to distinguish one amplitude for another. You need to explain how the circuit does that.
     
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