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Sinusoidal oscillator configuration

Discussion in 'Electronic Design' started by Nick Zalutskiy, Aug 25, 2008.

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  1. I am looking for a minimal hardware circuit to generate a 125 kHz
    sinusoidal wave to be fed into the power stage that is in turn feeding
    an antenna. The wave is used to generate a carrier frequency for an
    RFID application.

    I was thinking of using this [ http://www.ecsxtal.com/store/pdf/ecs-31.pdf
    ] crystal in a Pierce Oscillator configuration. However I am not sure
    if this crystal will work in that configuration. Also, I can't seem to
    figure out what the output would look like. Would it be a square wave
    or a sinusoidal? I came across a diagram that pictured a sinusoidal at
    the inverter input and a square wave at the output for the Pierce
    configuration. Is that correct?

    I would greatly appreciate any pointers on this.

    Thank you!
     
  2. A minimal hardware circuit could be a tiny PIC micro with an RC filter
    - 3 SMD parts.
    Depends on the specs you actually need of course.

    Dave.
     
  3. Try a Phase-Shift oscillator. I use them easily for
    audio frequencies, not sure I've gone up to 125khz
    though.
    Ken
     
  4. Guest



    The 125khz crystal is the most difficuot item.

    Use bipolar autobiassed amplifier for crystal 100KHZ or so . collector
    load of 100kohm. 1M from base to collector.
    capacitors. of 470pf from collector to ground and base to ground
    crystal from collector to base.Oscillation will build till it clips
    ( during clipped part of the cycle gain is zero.)
    .
    Detect signal at the collector with high impedance and reduce the
    supply to the collector resistor when a suitable non clippng amplitude
    is reached. Should be able to limit harmonics to -30db when a simple
    lpf will get you to -40db.
     
  5. neon

    neon

    1,325
    0
    Oct 21, 2006
    maxim makes an oscillator chip $5 that generates up to 50mhz sine, square, triangle. you can vary the frequency by a pot from 10 hz to 50 mhz. it actualy makes a very good lab oscillator. runs on +5v and with an out amp into 50ohm load. it has being many years since i biuld it and still runs ok.
     
  6. Using a "cheesy" DDS sounds like the solution I am trying to replace
    actually. There is a 4 MHz crystal going through a 4060 counter, which
    brings down the frequency to 125 kHz (square wave at that point), and
    is then fed into an RLC network to filter out the harmonics, giving a
    pretty clean sinusoidal. My new design doesn't require a 4 MHz clock
    and the counter is pretty bulky and generate a square wave incurring
    the need for the RLC network. This solution isnt bad by any means, I
    am just investigating different ways of accomplishing the task with
    the goal of minimizing the hardware.

    This is a learning process for me, so I decided to inverstigate the
    Pierce configuration further. I have a 125 kHz crystal, so thats not a
    problem. However, I cant get it to oscillate in this configuration.

    Schematic: http://en.wikipedia.org/wiki/Image:Pierce_oscillator.svg
    I am using the following crystal: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=XC999-ND
    with 4.7 pF caps at C1 and C2 (also tried 12 pF) and 10M for R1 (also
    tried 1M) and its just not working. The circuit is very simple, so I
    am completely lost at what I am doin wrong here. Are my C1 and C2
    values wrong? Do I need a series resistor between the crystal and
    inverter output?

    Thanks everyone for the very informative replies!

    -Nick

     
  7. Howdy,

    I think your capacitor values need adjustment. You may find this
    article helpful http://www.fairchildsemi.com/an/AN/AN-340.pdf

    Note that the series combination of the two capacitors should equal
    the required load capacitance of the crystal and that increasing
    the value of the capacitor on the gate's output increases feedback.
    If the crystal is a bit sluggish you'll want more feedback.

    If you have a jfet handy, like the mpf102, you'll find it works
    much better than a gate. Though when a spare gate is handy I'm
    likely to use it.

    Also see,
    http://www.fairchildsemi.com/an/AN/AN-118.pdf

    This is excellent,
    http://ww1.microchip.com/downloads/en/AppNotes/00943A.pdf

    Armed with these I'm sure you can get it working if the
    crystal is functional.
     
  8. I've looked at the Fairchild paper before and overlooked the series
    combination part, in turn making an error in my load capacitance
    calculations.

    The load capacitance for the crystal is 12.5 pF. So C1 and C2 should
    be 25 pF. I tried 24 pF (two 12 pF in parallel) for C1 and C2, no
    luck. I tried giving it more feedback, as you have suggested, by
    putting ~27 pF at the output (22 pF in parallel with 4.7 pF), no luck.

    I also tried to take into account the gate input and output
    capacitance (assumed to be around 5 pF) by placing 17 pF (a 12 and a
    4.7 in parallel) as C1 and C2, still no luck.

    I also have a JFET, a 5457, so I attemped to put the crystal in the
    following configuration: http://www.aldinc.com/pdf/fet_11120.0.pdf
    with 10k for Rd, 10M for Rf, no Rl, no buffer at the output, and 24 pF
    as C1 and C2, no signs of life. I also tried, J201 and a PN4392 jfets,
    still no luck.

    At this point it seems like I am playing trial and error, and thats
    pretty disturbing in my mind. =(

    Thanks,
    -Nick
     
  9. If that is good enough performance-wise for the existing circuit then
    simply replacing the counter and crystal with a 125KHz square wave
    output from a tiny 50 cent 5 or 8 pin PIC micro is a good solution to
    minimise your hardware. Just one tiny chip and the existing RLC
    filter.
    If you use the smallest SMD parts you can get then the circuit
    footprint will be very small indeed. Double sided component load would
    make it even smaller.
    I doubt you'll get a smaller solution using more traditional means.

    Dave.
     
  10. Howdy,

    Do you have a URL for the crystal specification?
    I'll look for my gate oscillator spreadsheet and
    drop the values into it.

    Have you tried more than one crystal?

    Remember that the gate should be biased into the linear
    operating range to get things started. Sometimes a small
    resistance in series with the gate output is necessary in
    addition to the 10M-20M feedback resistor.

    Then the pi section (the two caps and the crystal) provides
    180 degrees of phase shift, which when added 180 degrees from
    the inverting gate satisfies the 360 degree round the loop
    requirement. The other requirement is loop gain greater
    than unity, but the gate has more than enough gain to
    compensate for the loss across the pi network.

    With low frequency crystals (~100KHz and below) the gate
    oscillator usually works, so this is a puzzle. If you have
    a 4049 inverter I'd try that. They make vigorous LF oscillators.
    With the 4049 use 10M feedback 49pf and 56pf on either side of
    the crystal (highest value on the gate's output side) Also use
    39pF between the gate output and the pi section (where some
    application notes show a few K-Ohms of resistance.) Of course the
    10M bias resistor must be directly connected from input to output
    of the gate to force it into linear operation.

    I have a pierce circuit with an inductor insted of a resistor
    in the drain(2.5mH because it's what was handy.) Every LF
    crystal or ceramic resonator that I've tried, mostly salvaged
    from computer junk, has oscillated if it was ever going to
    oscillate. I don't like the app note oscillator for LF crystals.
    The pi section capacitors and feedback resistor are too low in
    value and the drain resistor would be better replaced by an
    inductor. But I can't say it wouldn't work.

    Don't be too hard on cut and try (trial and error) somtimes a
    person stumbles onto something wonderful via that path.
     
  11. Guest

    I still think Tim's solution is the way to go. Isn't such a low
    frequency crystal expensive? In any event, he is not suggesting
    creating a square wave, but rather a pseudo-sine-wave that will have
    much reduced harmonics. I did a sine wave generator for telecom apps
    by simply creating a square wave with a flat spot in the middle (i.e.
    reduce the size of the jump), then filtering the signal with a
    switched capacitor filter. There are Walsh based circuits to make
    pseudo sine waves, but you would need to watch out for patent issues.

    If you want to use a dac and can make the sample rate a multiple of
    the desired frequency, then you just need a simple look-up table. If
    that timing cannot be achieved, a coordic can be used to compute the
    sine values. That scheme is stone age enough that there should be any
    patent issues. The coordic would have the advantage of being able to
    change the frequency independent of the clock, i.e. you could make
    125KHz and 13KHz with the same circuit.

    In any event, if you want a clean signal, it is better not to generate
    the harmonics in the first place rather than to filter off the
    harmonics.
     
  12. I have used 100kHz xtals in circuits like this:
    ftp://panteltje.com/pub/xtal_osc_125_khz.gif

    The fet I used was likely a BF245 (high gain).
    I cannot run simulation on this diagram without more info on the xtal.
    Your solutions use the xtal in series resonant mode.
    This one uses parallel resonant mode.
    Give it a try perhaps.
    The waveform at the source should be almost a sine already, with some
    flattening at the bottom.
    In my applicaton startup was really slow...

    You can also add a drain resistor of a few hundred ohms, and take the signal from there.
    Adapt the capacitors perhaps, ratio 1:2 is normally good.
     
  13. Guest

    might help on the filter requirements to use two pins and two
    resistors go get a three
    step wave form:

    _____ _____ _____
    pin1 |_| |_| |_
    _ _ _
    pin2 _| |_____| |_____| |___

    _ _ _
    combo _| |_ _| |_ _| |_
    |_| |_| |_


    adjust overlaps to get minimum harmonics

    -Lasse
     
  14. Nice idea!

    Dave.
     
  15. MooseFET

    MooseFET Guest

    The magic number is 60 degrees to get no 3rd.

    If your micro has tri-stated pins, you can do much the same by using
    the tristate condition to make the flat spot in the edges.
     
  16. What is your field requirement in A/m at __ distance?
    The antenna usually means a coil around a big piece of ferrite.
    Therefore, it is narrow band if you can't afford too many losses. What
    is your input power requirement? If efficiency is important, then you
    may have a problem: The antenna is an R-L-C where R is small and Q is
    high. Look at the variability of the inductance and resonant
    frequency. Also saturation of the inductor will change the frequency.
    You will find it is the antenna setting the frequency requirement and
    not the crystal.

    I would recommend either a free-running power oscillator at the R-L-C
    frequency or phase locking it to a crystal. Be careful when selecting
    the capacitor that resonates with the coil - the AC voltage can be
    hundreds of volts, depending upon your power output.

    Kaschke makes some antennas of varying power handling for these 125
    kHz applications. Some have the capacitor built-in and they are pre-
    tuned.

    Frank
     
  17. Guest

    yes,
    but using two pins might make it possible to setup two timers to
    generate the two waveforms autonomously. Don't think I have ever
    seen a timer that could tristate a pin on a compare

    -Lasse
     
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