Colpitts oscillator help

Discussion in 'Analog & Mixed-Signal Design' started by latino, Oct 4, 2017.

  1. latino

    Thread Starter New Member

    Oct 4, 2017
    1
    0
    Hello!

    I have to make a Colpitts oscillator working on 1.1 GHz, and another Colpitts oscillator on 1.5-2 GHz.
    Here is the circuit:
    upload_2017-10-4_14-40-40.png
    Now, the C1 and C2 capacitors are 1 pF, L1= 5 nH, Q1=BFR92P
    The frequency is about 880 MHz, and i've already tried a lot of different values for L1, C1,C2 but i can't increase the frequency to 1.1 GHz. My professor said that there is no problem with the circuit, so let's try again and again, but i've no more ideas. I've already calculate(Thomson formula) that if C1=C2=1 pF and L1=5 nH, the circuit has to work on 3,1 GHz(??).

    So, what do you think? What is wrong, what should i do?

    Thank you
     
  2. RichardO

    Late Member

    May 4, 2013
    2,273
    894
    Did you look at the data sheet for the BFR92P? It shows junction capacitances of a good fraction of a picofarad.
    These capacitances are large relative to your 1pF capacitors.

    I suggest you simulate the circuit with a model for the actual BFR92P and see what you get.

    I am guessing that another problem is that you have a significant amount of stray capacitance in your oscillator circuit. Again, significant, is just a fraction of 1 pf.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    9,709
    2,345
    Have you allowed for the inductance of the connecting wires?
    Is the circuit built on a breadboard?
     
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