AM modulation with colpitts oscillator ??

Discussion in 'Wireless & RF Design' started by Walidj, Jun 30, 2011.

  1. Walidj

    Thread Starter New Member

    Jun 10, 2011
    I want to generate an AM modulated signal with colpitts oscillator :

    can this be performed by connecting an audio signal between Rb2 and the ground ? if not then what is a good way to get AM signal wth that oscillator ?

  2. KL7AJ

    Senior Member

    Nov 4, 2008

    Yes, you can certainly modulate it that way. You have to be careful to control the modulation, however, since you may cause the oscillation to quit at high negative modulation. Also, be aware that directly modulating an oscillator almost always results in some frequency modulation as well. Depending on the application, this may not be a big problem.

  3. PeeSeeBee


    Jun 17, 2011
    A better way of modulating it would be to add a buffer stage & modulate that, rather than modulate the oscillator directly. You will then get considerably less FM.
  4. skeptic

    Active Member

    Mar 7, 2010
    I agree with PeeSeeBee. It is best to leave the oscillator alone.

    In 1981 when I got my first computer, without monitor, so I decided to use my television as the monitor. This meant I had to build my own RF modulator. To do that I added a video modulator stage onto a crystal oscillator.

    The oscillator signal was coupled to the emitter of the stage by using a series capacitor between the oscillator and the emitter and an inductor from emitter to ground. The capacitor and inductor were series resonant at the oscillator frequency. The video was added at the base and adjusted to keep the output amplitude to a level that avoided clipping.

    It worked quite well. Even with the bandwidth limitations of the TV, the only ASCII character that it couldn't completely resolve was the " which looked like a small square