LTspice simulator issue?

Discussion in 'General Electronics Chat' started by KCHARROIS, Nov 13, 2012.

  1. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    Hi so I below is a tuned RF amp for 7.2 Mhz. I have one LC circuit coupled to another LC circuit, both are set to be at a center frequency of 7.2 Mhz but when I simulate my circuit to an AC analysis it shows that my center frequency is around 5.2 MHz. Am I doing somethinf wrong?

    Thanks
     
  2. tshuck

    Well-Known Member

    Oct 18, 2012
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    When you inductively couple an inductor as in a transformer, the overall inductance seen on the primary changes... this may be what you are experiencing....
     
  3. Papabravo

    Expert

    Feb 24, 2006
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  4. vk6zgo

    Active Member

    Jul 21, 2012
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    Certainly,there is some effect on the primary inductance,but not enough to change the resonant frequency by nearly 30%.

    Consider the retro-type instrument beloved by Hams,--the Grid Dip Oscillator (GDO).

    This consists of a tube oscillator,with the inductor of its resonant circuit mounted on the front of the instrument.

    In use,this coil is brought in the proximity of a resonant circuit,& the GDO oscillator is tuned through its range.

    At the resonant frequency of the circuit being tested,the GDO's grid current drops (dips) ,due to the other circuit "stealing" some energy from the GDO tuned circuit

    The GDO dip is broader when it is closely coupled,& narrows as coupling is reduced,but the centre frequency remains the same.

    Coupling mainly affects the "Q" of the tuned circuit,not its resonant frequency.
    Either you are doing something wrong,or there is a "bug" in LTSpice.

    PS:It might be wise to put some decoupling on the V2 line & try again.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I don't know how to calculate the equivalent inductance, but if you simulate those unity-coupled resonators, the resonant frequency is equal to what you would get with a single inductor of the same value, and the two capacitors in parallel with it. See attachment. If you assume the capacitance is 270pF, the inductance calculates to be 3.84uH.

    EDIT: LTspice does not make errors on simple passive circuits like this.
     
    Last edited: Nov 15, 2012
  6. vk6zgo

    Active Member

    Jul 21, 2012
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    In my comments re the GDO above,I did not take into account the classic case of "overcoupling",as in the GDO case,& many others,this level of coupling is seldom reached.

    As coupling is increased beyond "critical coupling",the response begins to take on a "double-humped" shape,with resonant peaks above & below the "centre frequency".

    The centre frequency is at,or very close to, the resonant frequency of each circuit considered separately.

    These links cover this subject:

    http://www.ee.bgu.ac.il/~intrlab/lab_number_7/Two inductively coupled RLC circuits.pdf

    http://frank.yueksel.org/other/RCA/Radiotron_Designers-Handbook_Fourth-Edition/09-Tuned-Circuits.pdf

    Perhaps LTSpice was not set to sweep higher in frequency & did not see the higher frequency peak.
     
  7. Tesla23

    Active Member

    May 10, 2009
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  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    That's what I think too. Can you explain the theory?
    I ran another simulation, with the two tanks rsonating separately at 9.9MHz and 10.1MHz. With k=1, LTspice put one peak at Fr=1/(2*pi*√(L*(C1+C2)), where L=1uH, C1=258.4pF, and C2=248.3pF.
    There was actually another peak at 315GHz, but I suspect it was an artifact of the limits of mathematical precision.
     
  9. vk6zgo

    Active Member

    Jul 21, 2012
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    I apologise for not picking up on this,but I guess,as a Tech,I'm not used to working with the impossible!:D

    My version of LTSpice is not functional,(not sure why,& haven't yet tried to fix it) so I couldn't download the OP's simulation.
    Does it use k=1 for its default, or did he make a mistake?
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    He has K1 set to 1 on his schematic (K1 L1 L3 1).
     
  11. vk6zgo

    Active Member

    Jul 21, 2012
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    Yep,Ron,I see it now!

    I guess I don't speak LTSpice!:D:D

    The OP is a beginner with RF,so it was probably just an oversight on his part.
    A "K" of around 0.2 might have been more appropriate.
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You're probably right. And, as you know better than I do, the response is highly dependent on the value of "K". As you pointed out, a grid (or gate) dip oscillator can be used to tune the circuit, including the coupling.
    Or, if you happen to have a spectrum analyzer lying around...:p
     
  13. Tesla23

    Active Member

    May 10, 2009
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    The paper in the link shows the analysis. For a symmetric circuit like this, odd and even mode analysis shows it quite clearly. Even simpler, when k=1, for equal coils, you can simply move the load on the secondary to the primary (as long as it was floating), so it is equivalent to a resonator with the same L and with 2C, hence frequency reduced by 1/√2.
     
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