Urgent Spice simulation

Discussion in 'General Electronics Chat' started by wilson_ssc, Feb 28, 2006.

  1. wilson_ssc

    Thread Starter New Member

    Feb 23, 2006
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    I've tried to simulate FM transmitter according to the attached file. The simulation does not get the correct output signal that FM is supposed to get. Is there anything wrong with the transistors that i replaced with 2N3904 because the original transistors cannot be found in the library.

    Is there anything wrong with the settings that I've set? maybe you can suggest a circuit that can simulate FM that can be simulated with FM output using Orcad Release 9.2. Thank you.
     
  2. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
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    2N3904 is not a high frequency transistor. Find another transistor with higher GBW.
     
  3. Papabravo

    Expert

    Feb 24, 2006
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    Wilson
    From the datasheet which I told you how to find at www.ushasemi.com the Gain Bandwidth Product of the 2SC9018 is 1,100 MHz. The Gain Bandwidth Product for the 2N3904 is just 300 MHz. In simple terms this number is the product of the maximum gain available at a given frequency and that frequency. So a 2N3904 has a maximum gain of 1 at 300MHz. The 2SC9018 has a maximum gain of 1 at 1,100 Mhz. See the difference?
     
  4. wilson_ssc

    Thread Starter New Member

    Feb 23, 2006
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    0

    Papabravo

    I know the Gain Bandwidth Product of 2N3904 is 300MHz, I try change the LC tank below that 300MHz also get same result even I using Orcad or Multisim.

    I try some simple FM Tx circuit from internet either one stage or two stage using transistors like 2N2222, 2N3904, C547 and C945 also can't get the results.

    Are you have any FM Tx circuit or ideal can run the simulation as long as can get the FM signal?
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    I have no FM Tx circuits in my back pocket. Just changing the tank circuit is probably not enough to make the circuit operate at a lower frequency. In the design of RF circuits impeadance matching between the elements is critical. The avaiable gain at the frequency of interest may not be sufficient, and finally general purpose simulators, may have no upper bound on frequency, but they also may not model the behavior of real devices at RF frequencies. Finally the available models may not correctly model items of interest like inter-electrod capacitence correctly or incident and reflected power.

    If the approach of finding a transistor, with an available model, has any chance at all then it needs to be as close a match as possible. It may just be the case that you'll need a simulator that deals with RF circuits.
     
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