Simple way to AM modulate a function generator?

Discussion in 'General Electronics Chat' started by flat5, Feb 7, 2015.

  1. flat5

    Thread Starter Active Member

    Nov 13, 2008
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    Function generators that I have do not have provision to AM modulate.

    Can I put a fet across the output and modulate that to modulate the carrier?
    Can someone please show a practicle circuit to do this?
    see attached idea pic
    am modulator.png

    or use a photo resistor or what?
    Application will be to align an am radio IF at 465khz by ear.

    Found this thread but it is not simple.
    https://www.physicsforums.com/threads/amplitude-modulation-with-function-generators.83984/
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
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    That would probably work as long as the FET was kept in Triode mode (analogous to saturation in BJT's) Much more common is to modulate the collector (drain) voltage of an amplifying transistor with the emitter (source) of another transistory, using a 'totem pole' arrangement.
     
  3. flat5

    Thread Starter Active Member

    Nov 13, 2008
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    Thanks. I am trying not to modify the FG. Just vary the output amplitude.
    I guess a bjt pulldown circuit might work too but the details require more knowledge than I have.
    I could experiment but if someone put me on the right path I might actually complete the project and put it in a box. I expect the fet will be the simplest way to go. I have a few mpf102 jfet.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    The JFET should work but you need to couple the modulation signal through a capacitor and add a DC voltage to the JFET gate to bias it in the active region near the JFETs cutoff voltage (negative voltage for an N-JFET.
    You may also have to experiment with the resistor values to get a good modulation envelope.
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
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    The FET would probably be better in the feedback network of an op-amp.

    It takes a bit of searching, but there are examples/explanations on the inter-wotsit.
     
  6. bertus

    Administrator

    Apr 5, 2008
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  7. flat5

    Thread Starter Active Member

    Nov 13, 2008
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    Bertus, you are so good at finding links :)
    So you gentlemen are suggesting I couple in the rf to an op-amp and modulate it (the op-amp).
    One of my FGs will output a sine wave to 10 mhz so an op-amp good to that frequency would be desirable.
    I will play with Crutschow's idea first. It is simple. Maybe I need 50 ohm resistors on each side.
    Thank you for your input! I'm still listening :)
     
  8. andrewmm

    Active Member

    Feb 25, 2011
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    I did a hack a a few decades ago
    your comment reminded me

    resistor divider across output , one resistor being a light variable resistor
    If I remember a transistor with the back paint removed, yep that long ago

    the flicker of the lights made a nice AM tone on the FG output
    not efficient , not linear , but worked...
     
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  9. flat5

    Thread Starter Active Member

    Nov 13, 2008
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    Exactly what I need. I'm not broadcasting. Just aligning a cheap radio.
    I have light variable resistors. I'll try modulating an LED/lvr and see what happens.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Must pre-date 455kHz ceramic resonators, or it probably wouldn't need re-aligning.

    Over the years there have been a number of magazine projects for alignment aids using ceramic resonators, they're easy to incorporate into a simple oscillator.

    Amplitude modulation is simply a matter of feeding the oscillator via an emitter follower with the base biased from a voltage divider.

    Then all you have to do is couple the AF to the base with a DC blocking capacitor.

    From the dim and distant past, I vaguely remember something about stagger tuning the IFTs to get sufficient bandwidth to carry the audio superimposed on the IF.

    What you're supposed to do is sweep the IF and tune the IFTs to get a response curve with as flat top as possible and steep sides. I don't remember the bandwidth you should aim for - 5 or 9kHz are at the back of my mind, but I'm really not sure.
     
  11. flat5

    Thread Starter Active Member

    Nov 13, 2008
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    Yes, thank you, ian. I've aligned am, fm, & shortwave radios since 1959. Not every day :)
    The radio is
    http://www.banggood.com/Seven-AM-Radio-Electronic-DIY-Kit-Electronic-Learning-Kit-p-946167.html
    It came with a green mixer transformer instead of a red. This may be my problem. I believe the local oscillator is dead. I have audio and if I couple rf near the loop inductor I can tune to it and see it peak at the tuning cap using a scope.
    Somehow I have to get the LO to oscillate.
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    Before my time - back when a single valve cost a weeks wages and sets usually only had 1 IF stage, the bandwidth was done by depth of coupling in one of the 2 IF transformers.

    One of the coupling arrangements gave a double peak response with a dip in the middle - a normal peak on the next IFT resulted in a close enough approximation to a flat-topped response curve.
     
  13. flat5

    Thread Starter Active Member

    Nov 13, 2008
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    When I was a kid I had one of these.

    AN/ARC-5 BC-453 Command Radio Set had variable coupling. You pulled a fiber rod tab coming out of the top of the IF transformer case to seperate the primary & secondary windings. Worked great. IF frequency was 85khz.

    http://www.carc.org.uk/pdf/FavRx002.pdf
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    You'd probably like the nearly complete archive of Wireless World on americanradiohistory.com - every so often there's news snippets of the ongoing battle between unlicensed listeners and the GPO engineers.

    Its not hard to imagine how they detected regenerative sets, but screen-grid and non-regenerating reflex sets must've posed more of a challenge. So far, I've read up to early 30's and superhets are still outnumbered.
     
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