help please

Discussion in 'The Projects Forum' started by gelamarie02, Sep 4, 2010.

  1. gelamarie02

    Thread Starter New Member

    Sep 4, 2010
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    can someone tell me what pspice sweeps i can do with a hartley oscillator? thank you. :(
     
  2. gelamarie02

    Thread Starter New Member

    Sep 4, 2010
    6
    0
    oh and when i simulate my circuit, it says that my node for out is floating x.x
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Perform a transient analysis on it. It may take awhile to start oscillating. Try 200mS with a max timestep of 20uS. Start the supply at 0v, and skip the starting operating point.

    L2 seems to be VERY large.

    You might help start the oscillation by using an initial condition statement:
    .ic i(L2)=1uA

    Your OUT node is indeed floating. Connect a high-value resistor (say, 1MEG) from OUT to Ground.

    [eta]
    I ran a simulation of your circuit in LTSpice; it oscillated at ~17.4kHz.
    It starts just fine without the startup or skip initial operating point options, and the .ic wasn't necessary either.
     
    Last edited: Sep 5, 2010
  4. gelamarie02

    Thread Starter New Member

    Sep 4, 2010
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    what about dc analysis then? they say that the only analysis i can do is dc and transient but i need ac too.. x.x im not familiar with oscillators.. x.x help? im at lostx.x
     
  5. gootee

    Senior Member

    Apr 24, 2007
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    For AC analysis, you typically place an AC source at the input, often set to 1V amplitude, so that the AC Analysis can display the ratio of the AC output to the AC input over some range of frequency. It displays the "transfer function". But your circuit does not have an input.
     
  6. gelamarie02

    Thread Starter New Member

    Sep 4, 2010
    6
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    ive chosen another ckt for my project in pspice, cos we are required to have 3 analysis x.x ive chosen an instrumentation amplifier.. x.x im at loss now, x.x
     
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  7. gelamarie02

    Thread Starter New Member

    Sep 4, 2010
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    can anyone help me? i need at least 5 objectives, the analysis works but i dont know what to sweep/ where to put the markers.. x.x
     
  8. gootee

    Senior Member

    Apr 24, 2007
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    According to "A Designer's Guide to Instrumentation Amplifiers", by Charles Kitchin and Lew Counts:

    "Common-mode rejection, the property of canceling out
    any signals that are common (the same potential on
    both inputs), while amplifying any signals that are
    differential (a potential difference between the inputs),
    is the most important function an instrumentation
    amplifier provides. Both dc and ac common-mode
    rejection are important in-amp specifications. Any
    errors due to a dc common-mode voltage (i.e., a dc
    voltage present at both inputs) will be reduced 80 dB to
    120 dB by any decent quality modern in-amp.".

    Maybe you could compare your instrumentation amplifier's performance/specs to those of a single-opamp amplifier, by simulating both.

    For more about in-amps, you can find that paper on line, probably at analog.com. They also have lots of other material about instrumentation amplifiers.

    That paper also says this:

    "In order to be effective, an in-amp needs to be able to
    amplify microvolt-level signals while simultaneously
    rejecting volts of common-mode at its inputs. It is
    particularly important that the in-amp is able to reject
    common-mode signals over the bandwidth of interest."

    So maybe your first objective should be to characterize the gain bandwidth of the instrumentation amplifier, i.e. the difference between the frequencies of the upper and lower -3dB gain points, which you can get from AC Analysis.

    You would want to at least find the two -3dB frequencies, the gain, and the difference between the two frequencies (the bandwidth).

    Then maybe you could characterize how well the AC and DC Common-Mode Rejection work, over that bandwidth.

    More about that, from the same paper:

    "Common-mode gain (ACM) is related to common mode
    rejection and is the ratio of change in output
    voltage to a change in common-mode input voltage.
    This is the net gain (or attenuation) from input to
    output for voltages common to both inputs. For example,
    an in-amp with a common-mode gain of 1/1,000
    and a 10-volt common-mode voltage at its inputs will
    exhibit a 10 mV output change. The differential or
    “normal mode” gain (AD) is the gain between input and
    output for voltages applied differentially (or across) the
    two inputs. The common-mode rejection ratio (CMRR)
    is simply the ratio of the differential gain, AD, to the
    common-mode gain (ACM). Note that in an ideal inamp,
    CMRR will increase in proportion to gain."

    I would think that you should be able to apply both common mode and differential mode signals and plot the output for each (with AC Analysis, I assume), to show the common mode gain and the differential mode gain, versus frequency, over the entire bandwidth.

    And if there is a way to divide one by the other and plot the actual CMRR (i.e. diff-mode gain plot divided by common-mode gain plot, vs freq), so much the better! (But you might have to make two identical circuits in the same schematic, to be able to have both at the same time, so you could divide one by the other and plot the result (for both AC and DC cases separately, of course). [Or at least that's the only easy way I can think of, to do that.])

    P.S. If you are not absolutely required to use a 741 opamp, use a more modern one! The 741 is terrible.

    Cheers,

    Tom
     
    Last edited: Sep 7, 2010
  9. gelamarie02

    Thread Starter New Member

    Sep 4, 2010
    6
    0
    So maybe your first objective should be to characterize the gain bandwidth of the instrumentation amplifier, i.e. the difference between the frequencies of the upper and lower -3dB gain points, which you can get from AC Analysis.
    -- how can i do this in ac sweep then? x.x sorry, im just not familiar with pspice.. x.x
     
  10. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    You will have to LEARN all about pspice.

    You can ask for help. But no one can do most of the work FOR you, even if they wanted to try.

    Have you tried reading the Help files, or the manual, or a book about it, or some of the many on-line tutorials?

    It is not very difficult, once you get started.

    But it WILL require your TIME.

    You MUST be willing to invest almost ALL of your time in your studies.

    That is the only good way to do well and succeed.

    Good luck!

    Tom
     
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