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AM modulator with MC1496 project problem

Discussion in 'General Electronics Discussion' started by tzitzikas, May 9, 2010.

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  1. tzitzikas

    tzitzikas

    16
    0
    May 9, 2010
    Hello, i have done this project. It is an experimental oscillator - modulator for mw frequencies. The schematics are here:
    oscillator:
    http://tzitzikas.webs.com/oscillator-1.JPG
    modulator:
    http://tzitzikas.webs.com/modulator-1.jpg

    The problem is that the designer told me to use an 1kohm variable series resistance between oscillator and modulator (after

    c9=22n capacitor of oscillator) to adjust the carrier level. I saw that the 1kohm variable series resistance is not enought

    because with 1kohm series resistance the audio was't very good. I think that the carrier level was high. So i change this

    resistance with another 4.7kohm. Now i am using about 2kohm of this series resistance. The modulation is very good But i

    think that the audio signal in the radio is not at the center of the frequency but a little above.
    (The operation frequency is 1000khz but i listen the audio with quality and strong voice at 1003khz.)
    which is the problem? how i can fix it, to listen the audio very well at center frequency and not at sideband?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    Have you confirmed that your carrier is actually 1MHz? On the other hand, are you sure your receiver is accurate?

    Those would be the first things I'd look at.

    edit: You're using a VCO. It's amazing you're within 0.3% of the frequency you're after without some form of feedback (PLL?). I would be less than surprised if the frequency doesn't wander around with temperature, humidity, and possibly even phase of the moon.
     
    Last edited: May 9, 2010
  3. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    Considering the problems I pointed out with the other circuit you posted from this source, I suspect the problem is a poor design. I would look at things like not taking component tolerances and other real life issues into account.

    To answer your question any further, I would need to read up on the chips in question since I am not familiar with them. Like I suggested in your other thread, I would go back to the website you got the circuit from and ask a few questions there.

    ---55p
     
  4. tzitzikas

    tzitzikas

    16
    0
    May 9, 2010
    55pilot :the circuit is from a Greek website.
    (*steve*): i controll the vco , by using a pll with SAA1057 and pic16f628 microcontroller.
    Do you have any idea why the modulation is not at the center frequency? Maybe improper armour?
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    Armour? Shielding? I doubt it. That's more likely to introduce noise than a systematic frequency error.

    Have you independently measured the carrier frequency?
     
  6. tzitzikas

    tzitzikas

    16
    0
    May 9, 2010
    yes i have measured the carrier frequency. It is ok. i work the vco with pll (SAA1057 and pic16f628 microcontroller)
    i was meaning good sield, sorry.
     
  7. davenn

    davenn Moderator

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    Sep 5, 2009
    yes because you appear to have the MC1496 wired as a DSB (double sideband) suppressed carrier modulator. So therefore you are not going to see a signal on the main carrier freq as that freq is suppressed. Rather you are going to see a signal on the 2 sideband freqs.

    Dave
    VK2TDN
     
  8. tzitzikas

    tzitzikas

    16
    0
    May 9, 2010
    do you know what modifications i must do at this circuit http://tzitzikas.webs.com/modulator-1.jpg to work as am modulator? the designer claims that this schematic is for am modulation.
     
  9. davenn

    davenn Moderator

    13,669
    1,891
    Sep 5, 2009
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