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Coil Research

CenlasoftCenlasoft Posts: 265
edited 2009-04-16 17:18 in General Discussion
Hello,
I have been experimenting with LC circuits (coil and capacitor) for a while and I am thankful for everyone's help. I can now achieve resonance with different coil-capacitor combinations. I found my big problem that was stopping me. I took a 1.236 uH coil and a 560 pF capacitor. I got resonance at 191.8 khz. The calculated value was about 191 khz. different LC circuit got me similar results. The problem I was having before was with my ground. I took a long wire and attached it on an iron rod and stuck it in the ground. I used this as one of my grounds. My question is that when I take the circuit from the breadboard and make a pcb, how do I get a good ground. My portable scope is working great with this new ground. Could someone explain this problem? I am not an expert, but it seems that everything is right on the money now.
Thanks,
Cenlasoft

Comments

  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-04-14 04:19
    Cenlasoft,

    Can you post a complete schematic? There shouldn't need to be anything driven into the literal ground. The term ground often just refers to the negative terminal, and most of your sub-circuits will like to see a common ground, meaning that all of the negative power connections are all tied together.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • CenlasoftCenlasoft Posts: 265
    edited 2009-04-14 18:09
    Hello,
    Thanks Beau, Here is my schematic:

    SG (+) >
    |c1|
    |SP|
    |L1|
    <|SG (-), SN, GW|

    SG (+) = Signal Generator Positive
    SG (-) = Signal Generator Negative
    C1 = 560 pF
    SP = Scope Probe
    SN = Scope Negative
    L1 = 1.236 uH
    GW = Ground Wire from rod in yard

    I I don't add the GW then the voltage (PP), etc is about half and the Fr (resonant frequency) is higher. My scope is a portable Velleman I am using a small bread board. With the GW, everything is fine. I am using a BK precision SG.
    Thanks,
    Cenlasoft
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-04-14 19:12
    Still, you should not need a literal earth ground connection as you describe.

    Is your scope grounded?
    Is your signal generator grounded?

    If so, that could be the problem... One or both might need to be isolated.

    You also realize, that the scope can significantly load the LC's resonant frequency and cause it to shift from the actual resonance of the LC.
    Try placing a 10 Meg resistor in series with your scope probe when measuring, or couple the scope connection with a 1pF capacitor to minimize any loading effects that the scope might introduce.

    Is this circuit constructed on a solderless breadboard? That can also add additional, usually unwanted, capacitance to your circuit.

    Note:
    If your signal generator IS grounded, you could damage the SG(-) by also making that connection to ground. Chances are, even if your signal generator is portable, there is a screw terminal post on the case of the portable signal generator that should be the ground connection, not the SG(-) terminal.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • CenlasoftCenlasoft Posts: 265
    edited 2009-04-14 20:15
    Hello Beau,
    I have tried the suggestions you gave me. I called BK Precision (SG) and the tech stated that I should unplug the SG and test the ground of the BNC (outside connection) to the case with a continuity meter. If there is a beep then it is grounded. It worked and there was a beep. I plugged in the SG to a different receptacle where there was a ground for sure. No difference. It must be my scope. I want to try something. I want to make a signal generator with the prop demo board I just got in. Is it possible? Can I modify some of the code on your inductive proximity demo? I am new to prop, but I'll try. I ordered a nice scope (two channel). The one I have is a battery powered Velleman pocket scope.
    Thanks,
    Cenlasoft
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-04-15 02:30
    Cenlasoft,

    Just curious, does the Signal generator produce a differential output? ... can you take a picture and post it? ... how about the scope also?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • CenlasoftCenlasoft Posts: 265
    edited 2009-04-15 04:01
    Hello Beau,
    This is my scope: www.vellemanusa.com/us/enu/product/view/?id=351263
    This is my signal generator: www.grainger.com/Grainger/items/5BB58

    I will get some pictures if you still want them.
    Thanks,
    Cenlasoft
  • CenlasoftCenlasoft Posts: 265
    edited 2009-04-16 16:38
    Hello,
    This is an update. I read in a forum how someone was having a similar problem. The solution suggested was putting a 10 k resistor in series with the (+) signal generator lead. It worked and I no longer need the ground wire from earth. I tried various resistors and the 10 k worked best. My scope output is even better. I think this isolates the signal generator from the circuit, but I am not sure. The only problem left is my calculated resonance frequency is 191 khz and the actual is 230 khz. Will I need the 10 k resistor if I use the prop demo board to excite the LC circuit? Any help would be appreciated.
    Thanks,
    Cenlasoft (Curtis)
  • vaclav_salvaclav_sal Posts: 451
    edited 2009-04-16 17:18
    Your oscilloscope should have an option to read AC or DC. Since you are after the LC circuit frequency you scope input should be AC.
    The variation between calculated and real LC circuit frequency is normal.
    Make sure your signal generator output is set to minimum near or at the LC circuit resonant frequency.
    Have fun.
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