Help with AM modulator

Discussion in 'Wireless & RF Design' started by PRS, May 16, 2014.

  1. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    Last edited: May 16, 2014
  2. Art

    Distinguished Member

    Sep 10, 2007
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    Hi :)
    Well I can't hotlink those pics either, you need a complete link to the images with extension in the link I think.

    When you modulate an AM carrier, is the clear carrier the highest level,
    and the modulation subtracts from it or is it the other way round?
    I always thought the modulation of audio strengthens the carrier.

    Sorry to answer your first question with another question! :D
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Below the Reply box click 'Go Advanced'. Then scroll down and click 'Manage Attachments'. That gives you a pop-up box with a 'Browse' function.
     
  4. Wendy

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    Allow me to show the images in question (I used PrtScn to copy them)...

    [​IMG][​IMG]

    If you were to rectify that signal it would become an AM signal, as AM is created by nonlinear reactions. Then feed it through a bandpass to remove objectionable harmonics.
     
    Last edited: May 18, 2014
  5. alfacliff

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    definatly not am modulation, only a dc shift of carrier. am actually modulates or change the output level of the carrier signal.
     
  6. PRS

    Thread Starter Well-Known Member

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    Testing, testing... Thanks!

    And does this work?

    [​IMG]
     
    Last edited: May 19, 2014
  7. PRS

    Thread Starter Well-Known Member

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    See Alec's directions below. It works!


    The carrier input is small but is amplified by the transistor. The audio is usually larger than the carrier. The result is an envelope consisting of the carrier, the carrier plus the audio, and the carrier minus the audio. They are supposed to mix nonlinearly and on an oscilloscope the upper side band shapes the amplitude of the carrier's top portion while the LSB forms its bottom portion. The picture on my scope is similar to an audio mixer where the signals are just added together.
     
  8. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    Bill, how did you put the pics right on the page like that? Alec only gave a means to attach them as thumbnails.

    As for feeding it through a rectifier (like a diode bridge?) it seems that would only wipe out the rf signal. I don't follow you.
     
  9. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    I realize that. I got the concept from a book, but the result is disappointing. I tried the base insertion method but it did the same thing. I'll try the collector insertion method next. To be honest I'm confused about how AM modulation actually comes about. I'm going to have to find a good source online.
     
  10. Art

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    Press the quote button on his post and you can see the IMG tags.
     
  11. alfacliff

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    use the modulation to vary the gain,. the carrier should be at 50% with modulation from 0 to 100%
     
  12. Wendy

    Moderator

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    Can he do that? As moderator I have a lot abilities other folks don't have, such as editing other people posts. Because of that I don't always know (or remember) what a Sr. Member can or can not do.

    Like I said, rectify the existing signal with a simple high speed diode, then look at the signal on the scope. That signal will be loaded with unpleasant harmonics, which will need eliminated with a filter such as a band pass or low pass.

    Nonlinear mixing is a classic way to modulate an AM signal. It creates the needed side bands.

    Indeed, a variable gain amplifier can also AM modulate a signal. Most electronics have several ways of looking at things, all of which are valid. That is one of them.
     
    Last edited: May 19, 2014
  13. PRS

    Thread Starter Well-Known Member

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    I can't see any IMG tags.
     
  14. PRS

    Thread Starter Well-Known Member

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    I've encountered different forums using different techniques. The most friendly let you just copy a photo from your pictures folder and paste them to the post you're writing.

    The image on the oscilloscope I gave above in not nonlinear mixing; it is linear as per an audio mixer. This can be accomplished just by joining two resistors at one end then applying two different signals to the open ends. At the joined end they mix. The photo is that of a 1kHZ sine wave and a 1MHz sine wave. A diode detector circuit with an LP filter to recover the audio does not work since the upper part of the "envelope" and its lower part cancel each other for zero volts at the output.

    Yes, nonlinear mixing is the desired result. Do you know how to fix the circuit I bread boarded so that it does indeed mix nonlinearly? An oscilloscope should show the upper part getting fatter at the same time the lower part is getting fatter, not just giving the rf a roller coaster ride. ;)
     
  15. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    That would be the case if the waveform was properly shaped. But here we have linear mixing, not the desired nonlinear mixing.

    [​IMG]
    [​IMG]
     
    Last edited by a moderator: May 19, 2014
  16. Art

    Distinguished Member

    Sep 10, 2007
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    Works for me, and I have the same member status (Safari on a Mac).
    Problem is if you try to show someone in a forum post it will just link the image :O

    [​IMG]

    Paul, I just made a dial marker for analogue VFOs and it worked :)
    I got a 100 KHz crystal out of another radio probably used for the same thing.
    It's 5th harmonic is the first I hear on AM broadcast radio at 600 kHz.

    So next I would like to find a 10MHz crystal and divide that by 10,
    and modulate the resulting 1 MHz signal with audio, so I'm keen to see your results!
     
  17. Wendy

    Moderator

    Mar 24, 2008
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    Paul, I added the images to one of your posts so you could see how it is done. Use the edit function on this post and the [​IMG] button. You do not need to use an image you've uploaded, just enter the IP address of the image between the IMG tags.

    Like I said, linear mixing does not modulate. Feed two signals that are linearly mixed into a simple rectifier and all of a sudden you have heterodyning, another word for AM modulation.

    Heterodyning is the sum and difference of two signals. In other words, sidebands. A 1Mhz signal modulated with 1 Khz will produce a 0.999 Mhz and 1.001 Mhz signal. If you were to linearly mix a 1Mhz signal and 1.001Mhz signal you would have a single sideband 1Khz modulated 1Mhz carrier, and it would look like an AM signal on an oscope.

    It is also the mainstay for converting one frequency into another.

    Changing the amplitude of a carrier wave is one way of looking at it, this is another.

    Do me a favor, just add a diode / load resistor to your circuit and see what your oscope shows. Add a filter after that and the AM signal cleans right up. If you need me to draw it I can, but you will be embarrassed how easy what I am talking about is.
     
    Last edited: May 19, 2014
  18. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    Thanks. I'll play around with it later when I've got time. Gotta mow my lawn.

    I understand all of this.

    I put a diode at the output, fed it into a 1K resistor to ground and, you're right, I got nonlinear AM modulation at a much smaller amplitude than either of the two input signals. But it is there, nevertheless!

    So, I had an idea: What if I abandoned the amplifier and just combined both signals, each through its own 1K resistor, then fed their junctions at the other ends to the diode and its series resistor? Sure enough, it worked. You're right -- this is simple!

    But now I'm wondering how this works. One thing: if you were to shift the phase of the LSB by 180 degrees with respect to the USB of a linearly mixed signal, you'd get the nonlinear modulation as used in AM radio. I realize the diode is a nonlinear device, but I don't see that as an explanation. Can you explain it?
     
    Last edited: May 20, 2014
  19. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    Bill's right. See my latest post to him.
     
  20. Wendy

    Moderator

    Mar 24, 2008
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    The diode method works, but it is a kludge. It gets the point across well though.

    Any AGC circuit (Automatic Gain Control) can be an AM modulator though. There are mixers that will suppress the two input carriers and come out with with just the sidebands that are pretty simple, not to mention chips (the 1496 comes to mind) to do the same thing.

    When I get a chance I think I can explain it to your satisfaction. About to step out to go to a job fair, wish me luck.
     
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