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SSB Carrier Supression

Discussion in 'General Electronics Chat' started by Space Varmint, Dec 3, 2008.

  1. Sceadwian

    Sceadwian Banned

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    I don't think it's cause he's a brit Space Varmint =)
     
  2. flat5

    flat5 Member

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    Last edited: Apr 14, 2009
  3. Mikebits

    Mikebits Well-Known Member

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  4. dave

    Dave New Member

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  5. Space Varmint

    Space Varmint New Member

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    I think your putting too much into the wrong emphasis unless your one of those analytical engineers. I'm in applications myself. You know, make it work and move on.

    But if you must know, you use the two formulas I gave you. These will give you the plug in values for the grand daddy:


    f=1/ (2pi * square root of LC)

    more simply put, when the inductive reactance is equal to the capacitive reactance, this will be your frequency because they are 180 degrees out of phase thereby cancelling the reactive impedance to that frequency. In effect, you have zero resistance at one frequency.

    Now, what I was saying about application. You got a variable capacitor, this will vary the capacitive reactance. So just get the oscillator to start up at power on. You have won 95% of the battle right there. Now check your range by varying the capacitor. If you need to go higher in frequency, just remove a turn or two on the coil.

    Next, start worrying about the mixer and crystal filter. Here is the real work. Oh, one more thing on the oscillator. Try to make it clean as possible. You want good symmetry in your sinusoidal wave form. Unless you have allot of expensive equipment,
    you won't be able to measure things like phase noise and so forth. With a scope, make sure you got no humps in your sign wave. These are harmonic energy and will cause you to pick up out of band signals. Keep it clean!
     
    Last edited: Apr 16, 2009
  6. Space Varmint

    Space Varmint New Member

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    I was taking the easy way out ;)
     
  7. flat5

    flat5 Member

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    SV, I know the generalities. The only specific thing I'm asking is a good ac resistance to shoot for for the coil. I could look at a lot of circuits and work backwards to see if it exists.

    I'll check the pdf Mike suggests. Perhaps it is in there.
    It will be different for fet vs bi-polar and I would think it will be different if the coil in the tuned circuit is in the emitter/source or base/gate or collector/drain sub-circuit. It just seems a good idea to me to know what reactance to design for.
     
  8. Mikebits

    Mikebits Well-Known Member

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    Let me see if I can clarify for you. Look at attached image. Circuit is a 1MHz colpits osc. Notice the tuned tank consist of 100uh inductor and split caps at 500pf each which means cap value is 250pf.

    Capacitors in Series

    Now if you plug these numbers into the formula Fo= 1/(2pi√LC)
    You get a value of 1MHz. This result should give an indication that above formula works.

    So you should start with two known values, say Frequency and capacitance.
    Transpose above formula to solve for L. L= 1/(4pi²CF²)

    If you don't want to work out the math, try this online resonance calculator.

    Resonant Frequency Calculator

    This should get you close to the ballpark figure for the inductor. Most LC oscillators I have seen use a variable inductor. The kind that you adjust the core with little plastic screwdriver.

    There is really a lot of ugly math involved, but above method should get you in the ballpark.
    Hope that helps.

    Oh, this app note also has a few good tips.
    http://www.electro-tech-online.com/custompdfs/2009/04/lonsosc.pdf
     

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    Last edited: Apr 18, 2009
  9. Space Varmint

    Space Varmint New Member

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    If you think application, than what you are stuck with is a variable capacitor as one half of your frequency determining components. Sure the impedance is a factor but to a lesser degree than the actual X of L and X of C values. And so, with most of the variable capacitors I've seen, they are always a very low value. You want to work with that because if you start adding parallel capacitance to the tank resonant circuit, you will significantly reduce the oscillator bandwidth. Meaning, try not to pad the oscillator. Use only the variable cap if possible. We are talking VFO (variable frequency oscillator).

    Therefore your inductor value will be contingent upon the capacitor more than anything else. So your usual formulas will apply.

    f = 1/ (2pi * square root of LC)

    So start with Xc for a given mid frequency. So your cap is static within it's assigned range. Take a mid-range value of approximately 150pf. Then use the formula Xc= 1/ (2pi * fC)

    example:

    frequency = 15 MHz
    Capacitance = 150pf

    then XC = 70.7 ohms

    Next you want to match the inductive reactance of the coil because:

    f= [2pi*fL = 1 / (2pi*fC)]

    When the inductive and capacitive reactance are equal and using the formula directly above, it can be broken down to:

    f = 1 / (2pi * square root of LC)

    So:

    XL = 2pi*fL

    solving for L we get:

    L = XL / 2pi*f

    70.7 ohms / 6.28 * 15 times 10 to the 6th = .75 uH

    That should put you in the ball park except for one thing. Your mid rage frequency for your local oscillator will be offset by the frequency of the crystal filter depending on if you use high side or low side injection. The formula will apply directly to a signal generator for testing the hf band.
     
  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    You think wrong - no need (or use) to calculate the reactance, you just need to calculate the inductance of the coil from the resonant frequency required with the specific capacitance you're using.

    The formula has been posted here multiple times, but I'm sure you already knew it.
     
  11. flat5

    flat5 Member

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    ok, thanks. Everybody is saying that I should not think about a desired reactance to shoot for but just match the tuning cap that I can find somewhere. That will be the practical thing to do.
    Since I will have to order any tuning caps I need I'll think about tuning range required and go from there.

    At resonance reactance is ideally infinite or zero so I'm barking up the wrong tree anyway :) arf arf
     
  12. Mikebits

    Mikebits Well-Known Member

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    Instead of using all those calculations, you could just follow the steps I suggested. The result is the same, but less work.
     
  13. flat5

    flat5 Member

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    Hi, Mike
    I just tried the Resonant Frequency Calculator and inputing any cap value between 2 and 250 pf and frequency 8 mhz I only get NaN for an inductance. (Firefox v3 java enabled).
     
  14. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It only seems to calculate frequency, nothing else.

    20pF and 20uH gives about 8MHz.
     
  15. Space Varmint

    Space Varmint New Member

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    L = 1 / (C*[2pif]squared)

    what's in the brackets is squared...kapeesh?

    Anyway, I was just showing the relationships. It's absurd to say that reactance doesn't matter. In a Hartley circuit we know it should be a high Z. So it really goes back to what I've been trying to say from the beggining. If one of them doesn't matter in practical application, then none of them do. The best advise I can give is the physical construction of the coil. After you get it running, you can go back into your mad scientist closet or what have you, and calculate away if you want. Me, I'd much rather just get the thing running and play with it until it looks and acts right, then start licking my chops at building the next stage, and cross my fingers and hope that it doesn't load the piss out of my oscillator to where I'm practically back to the drawing board (figuratively speaking). This is what my original instructions should help him avoid.
     
    Last edited: Apr 19, 2009
  16. Mikebits

    Mikebits Well-Known Member

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    It is spelled capisci if your Italian... :)

    If you pay attention you will note.
    L= 1/(4pi²CF²)

    2 squared is 4 and C x f squared is same thing as you wrote above. Capisci?

    If you prefer, use attached formula, same thing. :rolleyes:
     

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    Last edited: Apr 20, 2009
  17. Space Varmint

    Space Varmint New Member

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    Hey Mike? How do make that little 2 ? The squared sign.
     
  18. Mikebits

    Mikebits Well-Known Member

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    I am sorry, I don't quite understand your question...
     
  19. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    You cut and paste from Mikes post :D

    4pi²CF²
     
  20. Mikebits

    Mikebits Well-Known Member

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    Okay, I think I get your question now. Use the advanced editor option. It allows use of symbols and such like Ω²√'

    Look for advanced option, was that your question?
     
  21. Space Varmint

    Space Varmint New Member

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    What the heck are you guys doing up? Well I know why Nigel is.

    The "2". The square sign. Did you do that or cut and paste it from somewhere?
     

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