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VCXO frequency isn't high enough

Discussion in 'Electronic Basics' started by Anthony Fremont, Mar 13, 2007.

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  1. Hello all,

    I was playing around and saw that my junk box had all the parts so I started
    tossing this together:
    The problem is (well I think it's a problem) is that I'm all the way down to
    a 10pF cap for the crystal trimmer and the highest frequency I can get out
    of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm
    thinking that the 20uH inductor is supposed to pull the colorburst crystal
    high in frequency then the adjustable cap should be able to tweak it back
    down. Higher capicitance does result in even lower frequency. I made the
    20uH inductor by wrapping 17 turns on an old amidon FT50-61 ferrite toroid.
    I don't have any way to measure the inductance, but by my calculations it
    should be approximately 20uH. Is the ferrite saturating and messing things
    up? I see that touching the 20uH inductor it will cause the frequency to
    shift down as much as 4-5kHz.

    I believe this is direct conversion and that the crystal should exactly
    match the desired station (3581kHz) I want to recieve, that's right isn't
    it? Maybe this is close enough? I have to wait til tonight to see if I can
    actually hear anything.

    I mostly tinker with PIC chips so this is pretty much out of my league. I
    just wanted to throw it together and listen to the nightly report. It seems
    to be working as I can touch the 7.8uH coil (40 turns on a T50-2 toroid) and
    I get increased noise out of pins 4 and 5 on the NE602 (hey it's what I had
    in the box ;-). I haven't added the 386 yet, I just wanted to test the
    front end first. I'm cramming it all on a RS 276-259 pc proto board, but
    the layout is coming along pretty well for not planning it out. ;-) Thanks
    for helping.
  2. It's interesting that I don't see that oscillator circuit arrangement
    I guess you're attempting to drag the xtal off freq? Why not just use
    a tank?
  3. Thanks for the reply. I should have been more clear, the desired frequency
    is 3581kHz so yes I need to pull the crystal up in frequency about 1.5kHz.
    Right now I'm only able to get to just under 3580. It seems that much
    inductance should have more effect than moving it up only about 500Hz, but
    this isn't my bag.

    By tank I guess you mean to rid myself of the crystal and use an ordinary LC
    circuit? I think I can come up with something simple, but I'd rather just
    get this working if at all possible. Do you have any idea what impedance
    the 602 wants to see on it's oscillator? I have a couple more chips and I
    could try and cobble something together if I knew what impedance to target.
  4. Try going smaller than 10 pF (you can put a 10 pF fixed capacitor in series
    with the trimmer).
  5. Uncle Peter

    Uncle Peter Guest

    Get rid of the series choke and just use the small value variable
    capacitor to go higher in frequency. I believe the inductance will
    lower the frequency--the combination works to give an increased
    VXCO range if I am not mistaken.

    Pete k1zjh
  6. Ian Jackson

    Ian Jackson Guest

    If the series capacitor is very small, the oscillation will stop. In
    these circumstances, I have successfully pulled VHF crystals (3rd, 5th
    and 7th overtone, between 70 and 200MHz) HF by adding an inductor IN
    PARALLEL with the crystal. The value will be somewhat less than that
    required to neutralise (ie parallel resonate with) the parallel
    capacitance of the crystal. The lower the inductance, the higher the

  7. Andrew Holme

    Andrew Holme Guest

    You have a capacitor and inductor in series.
    Capacitive reactance = -j/(wC)
    Inductive reactance = +jwL
    The net reactance [jwL-j/(wC)] may be capacitive (negative) or inductive

    Reducing the inductor makes it less inductive i.e. more capacitive.
    Reducing the capacitor also makes the net reactance more capacitive.

    You turned your trimmer all the way down to 10pF and it wasn't enough, so
    try taking a few turns off the inductor.
  8. Use less inductance or replace it with a wire link.

    73, Ed. EI9GQ.
  9. I did this and now I'm up to 3580.050; an increase of about 70Hz or so.. I
    don't understand how this was designed to work. What is making the crystal
    oscillate above its design frequency? I thought the inductor had something
    to do with it, obviously not. Is it being operated in series mode instead
    of parallel?

    I tried putting small cap in series with the adjustable one and that just
    greatly narrowed the tuning range and only slightly raised the frequency.
    It seemed to be getting unstable at that point as my old frequency counter
    started showing variations of quite a few hertz each update instead of
    remaining steady. It could be the amplitude is too low and the counter is
    missing ticks.
  10. Uncle Peter

    Uncle Peter Guest

    If you want a VXCO, you use the capacitor and inductor to achieve
    maximum range of the crystal frequency.

    An inductor in series will lower the frequency.
    A capacitor in series will raise the frequency.

    Eliminate the coil and just use the series trimmer to see how
    how high the crystal frequency can be raised before it becomes
    unstable or quits oscillating.

    As Ian suggested, you can add an inductor ACROSS the
    crystal to raise the frequency. Think of the crystal as being
    a parallel LC circuit (over simplification) and you'll
    be better able to visualize how this works.

    Try a few other color burst xtals in the circuit for the heck
    of it. You may not be able to pull the crystal that far.
  11. Jamie

    Jamie Guest

    You need to reduce the inductor.. that is causing a down swing in your freq.
  12. Thanks, I took it out and the frequency increased by only about 70Hz, but it
    did increase. :) Do you know of anything else I can do to increase the
    frequency by about another couple of kcs?
  13. colin

    colin Guest

    maybe the Q of you inductor is too low, try an air core,
    needs lots more turns, a trimmer core as a core is good too.

    Also you should test it with the crystal shorted,
    then when you get close to the frequency you want put the crystal back in.

    Colin =^.^=
  14. Jamie

    Jamie Guest

    try a different Xstyle, it sounds like your at the limits..
    it's possible the chip has too much capacitance on the
    osc legs, i suppose you could try a stand alone osc with a
    high freq transistor to lower the cap. use a buffer on the
    stage to drive the chip .
    Also, have you considered the board mounting? you could
    have to much cap via your board construction.
  15. Arv

    Arv Guest

    My W1AW receiver uses the crystal with just a 5-47 pf variable inductor, and it nets right on frequency. Try shorting
    the inductor and see if this gets you closer to the required
    frequency. Not all color burst crystals were created equal. If you
    have another crystal you might want to try it.

    Your ferrite will not be saturating at the small amount of signal you
    are sending through it as part of an SA-602 oscillator.

    Arv - K7HKL
  16. John Fields

    John Fields Guest

  17. Looks like you are quite likely correct. Without the inductor and with a
    10pF cap in place of the variable cap, I can get the frequency up to just
    over 3580kHz. I did some looking around and it appears that if I were to
    parallel another crystal, I could get a bigger spread on frequency. Or I
    could just try a different crystal too. I think I will tinker on a
    breadboard to see how high in frequency I can get the crystal to vibrate.
  18. I did this and it will go slightly higher in frequency, but it gets a tad
    unstable (could be the counter too, I wish my new scope would get here). I
    guess I'm at the limits of that particular crystal.
  19. It hasn't stopped, but it has gotten weak or unstable (I can't be sure right
    now) with capacitance below 10pF.
    Are you referring to the capacitance inside the crystal, or the 100pF
    external cap? Do you have a ballpark number on the inductance value I
    should try?
  20. I understand that, but I don't use values like j and w, just 2, f and PI.
    I took it out, and it's still too low in frequency. :-( Back to the
    drawing board I guess.
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