Help With an AM Modulation Project

Discussion in 'The Projects Forum' started by MS&T, Jun 17, 2010.

  1. MS&T

    Thread Starter New Member

    Jun 17, 2010
    Hello all.
    I am working on an AM Modulation project. The plan is to combine a 100 Hz square wave with a 240kHz signal. I have attached some pictures of my setup.

    The problem is the output. ( I very roughly drew it, srry). It doesn't look like it should. It never goes below zero, and modulates on the low parts of the square wave instead of the high parts. I double-checked my modulation (multiplier/mixer) ckt, as shown below, but perhaps I'm doing something wrong. I didn't use the exact resistors they had (instead of 10k, I used 8k, etc.), but they were close.

    Do you know where I should start looking for problems?

    I have attached 3 pics, one is the oscillator ckt I made at ~240kHz (this is the carrier signal, and yes, I know its not a nice sine wave), the next is the modulation ckt, and the last is the output I'm getting. The modulation signal is a 100Hz square wave.

    Thank you all very much for your help. Please explain things in simple terms, as I am new to most of this.
  2. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    The top of page 6 of the attached file shows an AM modulator. It is different than the one you posted. Maybe the capacitive coupling output makes a difference. Also, make sure the input signal isn't overdriving the modulator.
  3. MS&T

    Thread Starter New Member

    Jun 17, 2010
    Well, capacitors do block DC, perhaps I'll give this a try today, and let you know if it works.
    Thanks for the reply!