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LC Oscillator Questions

Discussion in 'Electronic Design' started by Anthony Fremont, Mar 19, 2007.

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  1. I had to play around with this quite a bit to get it running near 10MHz. It
    seemed to be more picky about the cap ratios than their actual values.
    I figured that it would cause horrid clipping the way it was installed. I
    thought maybe the designer intended it to be the other way around to protect
    the transistor from reverse voltages on the gate, hmm.... always something
    to keep you guessing. ;-)
    I don't really "need" anything in particular, it's just an exercise to try
    and learn something. I just used the ceramic caps because they were handy.
    I didn't have a pile of NPO caps laying around. ;-) I d

    Time to add a buffer now since the scope probes load it down so badly now.
    I'm getting 1.1Vpp into the apparent 5M load of two scope probes. It's
    probably pulled a ton off frequency as well. Let's see.....(removes one
    probe).....yep, 90KHz rise in frequency after taking one probe off.
     
  2. Tim Shoppa

    Tim Shoppa Guest

    Oscillators have to have gain greater than one at the frequency of
    oscillation.

    When turned on, the amplitude builds up until something in the circuit
    cuts back the gain. In simple oscillators, that "something that cuts
    back the gain" is almost always the active device saturating and
    distorting its output.

    The higher your gain, the more reliable the oscillator starting up,
    but also the higher the distortion.

    If you take the output not from the output of the active device, but
    from a lightly-coupled tank, then you'll see something much more like
    the sine wave you were expecting. This is what you see at the L1/C3
    junction. But still you'll get lower distortion there if the active
    device isn't driven so far into saturation/distortion. And by
    definition you cannot suck much power out of the L1/C3 junction
    without decreasing the Q of the tank and making distortion there too.

    You can add a few more active devices and not only buffer things but
    also put a fairly linear AGC in the loop. This still has distortion,
    but this is done intentionally in a rectifier to derive the AGC
    control voltage, which is then filtered. The intentional distortion
    does not have to appear in the output!

    Clever use of devices can make the AGC loop quite beautiful. Look at
    the Wien Bridge or Meacham Bridge oscillators that use a light bulb in
    the bridge to not only be the loop-control device but also do
    filtering (thermal time constant of the filament).

    Tim.
     
  3. Thanks for the reply. Sounds pretty slick. Way out of my league though.
    ;-)
     
  4. The "conversation" that L1 and C3 sure looks nice on the scope. :)
    Well there sure isn't much talk about it out there. Material I find is
    like, "here's a schematic, pick a coil and cap and your done. No one seems
    to care what the result looks like. Seems like you can make a reasonably
    decent wave _and_ still have the oscillator start reliably.
    Seems to be the way people like to do it. ;-)
     
  5. It seams reasonable that if I can look at the junction with a scope and the
    wave looks good, I should be able to tap it with a secondary JFET without
    destroying it. Yet I see no examples of that being done. I guess it's just
    easier to accomplish the waveform repair by using a tank on the output of
    the oscillator and not loading down the primary tank circuit.
    This sounds like what Chris Jones was talking about. Do you have a link so
    I could check it out?
    Clever stuff. :)
     
  6. In message <>, Anthony Fremont
    Symmetry is a nice word to use where LC resonance is concerned.
    Symmetry for the drive and clipping mechanisms.

    Have a look at a long-tail transistor pair with cross-coupled
    collector-base feedback resistors, and collectors driving a
    centre-tapped LC resonant circuit.
     
  7. Tim Shoppa

    Tim Shoppa Guest

    A "classic" oscillator with AGC is the Sulzer Oscillator. Very nice
    pics and schematics at

    http://leapsecond.com/museum/sul25-1/
    For really nitty-gritty stuff about low-distortion oscillators, see
    Jim Williams' examples in Linear Technologies appnote AN-43. (Go to
    http://www.linear.com/ and do a search for "AN43" without the hyphen).
    Most of the examples there are bridges, and distortions in the
    sub-0.1% category are achieved AND THEN IMPROVED UPON BY FACTORS OF
    HUNDREDS! Also google "Meacham Bridge" and "Wien Bridge".

    Tim.
     
  8. Tim Shoppa

    Tim Shoppa Guest

    In addition to the low-noise-low-distortion-high-stability URL's I
    pointed you towards in my other followup, recent ARRL Handbooks have
    some really clever low-noise VFO circuits using a multitude of
    approaches, including explicit AGC circuitry. I have been slowly
    working my way through the cookbook examples and every example has its
    merits.

    In typical ham use, for better or worse, stability and reliability to
    start-up are often the most important criteria. What you are
    complaining about when you see a distorted output, is something that
    is actually a design goal of oscillators that are followed by
    multipliers.

    One very common method over the years of decoupling the frequency-
    determining tuned circuits from other frequencies generated in a radio
    is to run the oscillator grid tank at half the output frequency and
    depend on distorition to make the desired output frequency. In the
    simplest case a balanced or push-pull oscillator is a "No-No" because
    you WANT the second harmonic. The electron-coupled oscillator that was
    in the 50's/60's/early 70's handbooks is a classic design.

    Tim.
     
  9. K7ITM

    K7ITM Guest

    Some people worry a whole lot about it. What they worry about,
    typically, is phase noise first and stability second. Lack of
    harmonics in the waveform are generally lower on the list. That's
    because filtering out harmonics is relatively easy, compared with
    cleaning up phase noise and stability problems. You can find lots of
    articles on minimizing phase noise, but there's also quite a bit of
    trade-secret sorts of knowledge whose owners aren't particularly
    interested in sharing, understandably. It's far from trivial to get
    the phase noise and spurious performance you'll find in the good
    commercial signal generators. Good as they are, though, I know of
    none that's good enough to be used without harmonic filtering to do
    low-level harmonic distortion measurements.

    Cheers,
    Tom
     
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