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modulator circuit

Discussion in 'General Electronics Discussion' started by xpower, Apr 1, 2012.

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  1. xpower

    xpower

    4
    0
    Apr 1, 2012
    hi guys.
    am new here and this is my first thread :) hopefully am posting in the correct section :D

    Ive been trying to understand the circuit of a cheep Chinese wireless door bill. Ive done the circuit in the Proteus simulator. but i couldn't get the oscillation at 315MHz. the circuit consists of encoder that produce pulses and oscillator to modulate the pulses. but in the simulator i couldnt get any oscillation hence couldn't guess if it is FM or AM.
    C9018 transistors were used in the circuit, but i used 2n2222a cuz the first is not available in the simulator. please help
    the circuit is attached.
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,833
    2,442
    Nov 17, 2011
    Try to find a high frequency transistor in your simulator. The C9018 has a transition frequency of 800 MHz (typ), whereas the 2N2222a has only 300 MHz. Since the gain of a transistor is 1 at the transition frequency and falls below unity at frequencies higher than the transition frequency, your circuit can't oscillate.


    Harald
     
  3. GonzoEngineer

    GonzoEngineer

    321
    2
    Dec 2, 2011
    Excellent!:D
     
  4. xpower

    xpower

    4
    0
    Apr 1, 2012
    thanks Harlad :) but Ive modified the circuit to oscillate at 30MHz then ended up with the same result. it seems that this simulator has a bug or shortage. according to my understanding, the circuit should oscillate on each high pulse from the encoder and this is an AM modulation. any idea how can i increase the range. ive added one stage (attached) but doesnt work. :mad: any clue?
     

    Attached Files:

  5. Harald Kapp

    Harald Kapp Moderator Moderator

    10,833
    2,442
    Nov 17, 2011
    I'm not very versed in HF technology. Could it be that some of the inductors should be coupled?

    Harald
     
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