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Varactor tuning

Discussion in 'Electronic Design' started by Paul Burridge, Dec 10, 2003.

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  1. Hi,

    I'm currently working on this VCXO that achieves frequency shift by
    applying DC bias to two varactor diodes connected cathode to cathode
    (bias applied to the junction between them). If I can't get enough
    shift with the available bias voltage, is there any problem with just
    putting another pair of the same diodes in parallel with the existing
    ones?
    This is a ceramic resonator oscillator, BTW, so will stand a lot more
    'pulling' than a xtal would, so don't worry about that aspect of it.

    p.
     
  2. W3JDR

    W3JDR Guest

    Paul,
    The amount of tuning range is a function of the ratio of Cmax/Cmin. If you
    parallel varactors, Cmax will double, but so will Cmin. The ratio hasn't
    changed.

    If you're not already using a "hyper-abrupt" type of varactor, you should
    look into one. They offer a wider capacitance range.

    What type of varactor are you using, and what's the frequency of the
    resonator? What's the application...linear frequency modulation like FM or
    data keying like FSK???

    Joe
    W3JDR


    Winston Churchill
     
  3. Jim Thompson

    Jim Thompson Guest

    Paul, That will work, but will double *both* min and max capacitance.
    But I'm puzzled: "I can't get enough shift with the available bias
    voltage" implies you're on the *low* end of capacitance (highest
    voltage).

    ...Jim Thompson
     
  4. John Larkin

    John Larkin Guest

    Sure. Is this a coaxial ceramic resonator, or one of the low-freq
    piezo things? What's the frequency, Kenneth? [1]

    John

    [1] old Dan Rather joke, sorry.
     
  5. Marc H.Popek

    Marc H.Popek Guest

    that would the more total capacitance but not any larger dC/dV.....

    you need even better tricks :)

    Marco

    Winston Churchill
     
  6. Phil Hobbs

    Phil Hobbs Guest

    An inductor in series with the varactors, then another one in parallel
    with the series combo can get you a very wide range of impedance from a
    decent varactor.


    Cheers,

    Phil Hobbs
     
  7. Phil Hobbs

    Phil Hobbs Guest

    Right. I might make the series inductors symmetrical, if I needed the
    tap point to be near signal ground (e.g. with a centertapped coil or a
    differential pair driving it). Last time I used this trick was in a
    160-MHz phase shifter. The two inductors will generally be about the
    same size for best results with a hyperabrupt varactor--5 minutes with a
    math program will give you the right values. Generally you need to keep
    the reactance capacitive if you're resonating a crystal against this
    combination--there are multiple operating frequencies otherwise, since
    the resonator will look capacitive almost everywhere.


    Cheers,

    Phil Hobbs
     
  8. Tom Bruhns

    Tom Bruhns Guest

    So the right way to do this is to lower the _effective_ minimum
    capacitance. You can do that by adding an inductor, to cancel out
    capacitance. You can end up making the tuning range as wide as you
    want, but at the expense of the crystal (ceramic resonator in your
    case) being less of the overall frequency determination. In other
    words, there comes a point where you'd be as well off to just do an LC
    oscillator. But to double, say, the range, it's a good way to go.

    I guess I re-discovered what was already well known, but a few years
    ago I designed such a VCXO, and was amazed how linear the
    freq-vs-controlvoltage curve was (a good thing for use in a PLL).
    Don't know what range you're trying to achieve, but I had no trouble
    getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
    crystal.

    Cheers,
    Tom
     
  9. Oh bugger. Well how about using varactors with a higher C/V ratio?
    Not sure what you mean by that term, but imagine it amounts to simply
    a type with a higher capacitive reaction to applied voltage - as I
    mentioned above.
    I'm currently using BB149A diodes, but I've got some BBY40s as well,
    which might offer more shift per volt; I haven't checked the spec yet.
    The fundamental frequency is 8.00Mhz and I need to pull it by +/-32khz
    for tuning rather than modulating purposes.
     
  10. I'm using a 555 timer to generate a sawtooth waveform to feed the
    diodes, so I get a constant frequency sweep at the vcxo's output. Main
    problem is the limited voltage output range; starts above zero volts
    and peaks well before supply rail. So not much of a ramp; just around
    4 or 5 volts, I guess. I could try changing the diodes for more
    responsive ones but they're SMDs and I really hate messin' with 'em.
    :-(
     
  11. Phil Hobbs wrote...
    Sounds good. How about a specific example?

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  12. Thanks, is this the kind of thing you mean?



    +-------+
    | |
    | |
    | |
    C| |
    L1 C| |
    C| |
    | |
    | |
    V |
    D1 - |
    | C|
    Applied DC control voltage | C| L2
    Line --------------------+ C|
    | |
    | |
    D2 - |
    ^ |
    | |
    | |
    | |
    +-------+

    View in FP font.

    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
     
  13. Guest

    The 555 thresholds are set to charge and discharge the timing cap at
    1/3 and 2/3rds the supply voltage. If you using that directly the
    easiest way to get more "swing" is :

    Use an opamp to translate the voltage lower and add some gain
    (use at least 12v on the opamp). That can get you a Tuning voltage
    that is near 0 to near 12V (that should help).

    OR use a higher Vcc on the 555, say 12v. That will get your total
    swing to about 4V and the low will be 4v and the peak will be 8v.

    There are tricks that can be used to "offset" that 1/3 and 2/3 point
    but the total swing is usually the same. That can help as operating
    the Varicap closer to 0V will allow you to use more of it's
    capacitance range though it's usualy less linear at the bottom.

    Myself I'd use an opamp to create a saw generator and then I can
    control the swings.

    Allison
     
  14. Phil Hobbs

    Phil Hobbs Guest

    Last time I used this was with an MV104 common-cathode dual hyperabrupt,
    to make a 110-MHz phase shifter. It used a Mini-Circuits quadrature
    hybrid in the usual way, coming in the 0 degree port, coming out the 180
    degree port, and hanging matched reactances on the 90 degree ports.

    Each section had its own inductors, and the cathodes were bypassed
    heavily (1000 pF) to ground so that the two sides didn't interact too
    much. The component values were 45 nH in series and 43 nH in parallel.
    It was linear to within +-4 degrees, and the one section gave phase
    shifts from 12 to 164 degrees, both dramatically better than I could get
    with a bare varactor.

    The idea is to have the varactor resonate with the series inductor just
    off the low-voltage end of the range, and have the series combination
    resonate with the parallel L just off the high-voltage end of the range.
    Since the series-resonance doesn't even notice the parallel L, the
    design equations decouple nicely, too. You adjust the placement of the
    resonances to get the range and linearity desired.

    Cheers,

    Phil Hobbs
     
  15. Phil Hobbs

    Phil Hobbs Guest

    parallel with the series combo can get you a very wide range of
    impedance from
    Last time I used this was with an MV104 common-cathode dual hyperabrupt,
    to make a 110-MHz phase shifter. It used a Mini-Circuits quadrature
    hybrid in the usual way, coming in the 0 degree port, coming out the 180
    degree port, and hanging matched reactances on the 90 degree ports.

    Each section had its own inductors, and the cathodes were bypassed
    heavily (1000 pF) to ground so that the two sides didn't interact too
    much. The component values were 45 nH in series and 43 nH in parallel.
    It was linear to within +-4 degrees, and the one section gave phase
    shifts from 12 to 164 degrees, both dramatically better than I could get
    with a bare varactor.

    The idea is to have the varactor resonate with the series inductor just
    off the low-voltage end of the range, and have the series combination
    resonate with the parallel L just off the high-voltage end of the range.
    Since the series-resonance doesn't even notice the parallel L, the
    design equations decouple nicely, too. You adjust the placement of the
    resonances to get the range and linearity desired.

    Cheers,

    Phil Hobbs
     
  16. Thanks, nospam. I can't imagine why I didn't think of this before. Got
    just the circuit lying about for a 741 sawtooth too!
     
  17. ddwyer

    ddwyer Guest

    Thats the way its done to pull crystals a long way.
    The thought/real experiment to assist is to assume that the acoustic
    resonator is resistive (zero phase) at series resonance.
    The maintaining circuit can then be replaced by an equivalent resistor.
    The circuit with resistor should oscillate at approx the resonator
    frequency.The inductor across the varicap is selected to almost tune
    out/parallel resonate with the varicap.
    The series inductor is phase retard to ensure the maintaining circuit
    tis zero phase . Adjustment of the varicap then moves the circuit above
    and below the series resonance of the resonator.
     
  18. If he puts two diodes in parallel he will double the capacitance and
    will have to reduce the amount of inductance to have the same
    min. frequency. Im not sure that the high end won't be greater than
    before, even though the capacitance ratio is the same, since the
    fixed inductance is lower.

    Another idea would be to put the two varicaps in parallel, but switch
    one of them out as you approach the upper frequency.
     
  19. W3JDR

    W3JDR Guest

    If he puts two diodes in parallel he will double the capacitance and
    Kenneth,

    F=1/((2*PI)*SQR(L*C))

    If you double C, you have to halve L to maintain the same frequency. If you
    do this, you only changed the LC ratio, not the delta tuning range.
    The only way to get more delta F is to get more delta C. If you use the
    switching technique, you'll have a discontinuous tuning curve (Vtune vs
    Freq) which makes it hard to implement a closed loop tuning system. It can
    be done, but the control loop gets complicated.

    A previous poster suggested what I'd called a "synthetic reactance", which
    is a series-parallel LC combination. This technique can produce very large
    effective-capacitance changes with a modest varactor range, however it also
    comes with the susceptibility of mode-jumping in the output frequency.

    Joe
    W3JDR
     
  20. J M Noeding

    J M Noeding Guest

    Suppose you mean greater Cmax/Cmin, larger delta C was already
    achieved above, but as you say "it doesn't work"
    The tuning range can be calculated as follows:
    (Cmax/Cmin)^2 = Fmax/Fmin
    You could always divide the tuning into two ranges and it shouldn't be
    too difficult to adjust the trimmer capacitors

    73
    LA8AK
     
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