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micro power square wave oscillator

Discussion in 'Electronic Design' started by nukeymusic, Jul 1, 2008.

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  1. nukeymusic

    nukeymusic Guest

    I'm looking for a schematic for a square wave oscillator which draws
    around 10uA, powered by 2 to 5V and oscillates at around 100kHz with a
    50% duty cycle. I tried out some schematics around a 4007 but none can
    meet all the requirements.

    any suggestions here?

    regards,
    nukeymusic
     
  2. An MSP430 micro might be able to do that, worth checking. Not 5V though.

    Dave.
     
  3. Guest

    A simple two transistor astable multivibrator should do the job. You'd
    need low capacitance transistors - the SD214 is one part that might
    just do the job

    http://www.datasheetcatalog.org/datasheet/linearsystems/SD214.pdf

    The BFR92 5GHz broad-band transistor would walk it, since it is an
    appreciably lower capacitance part

    http://www.nxp.com/acrobat_download/datasheets/BFR92_CNV_2.pdf

    but you'd need to put some 22R of low inductance (surface mount)
    resistor in series with each base to stop them oscillating at a GHz or
    so on the stray inductances and capacitances around your layout. A
    ferrite chip (non-wound) inductor might do the same job - they weren't
    widely available back when I was playing with the BFR92 and it's PNP
    complement.

    Note the base of the BFR92 can't take more than 2V of reverse voltage
    - on a 5V supply you have to be careful that your circuit doesn't
    destroy the transistor when it turns it off. You should really build
    and test the circuit in LTSpice before you risk blowing up real parts
    (not that they are all that expensive - a dollar or so each).
     
  4. I am thinking about a PUJT as a relaxation oscillator, perhaps coupled
    to a CD4060, maybe? Have you considered that approach?

    Jon
     
  5. I suppose you are right. Dispels the PUJT relaxation thingy, I think,
    as it dumps the charge each cycle and I think the 2n6028 has 100's of
    pF, roughly. Not good.

    Trapezoidal rise/fall, then? ;) Or perhaps draw some charge at first
    and then just move it back and forth in an LC + xtl tank with high Q,
    replacing losses?

    How do the watch folks do it? There must be some serious remodeling
    of crystals for simulation work there (vastly more complex
    understanding of them) in order to design something as efficient as
    I've seen from them. Anyone know the details here?

    Jon
     
  6. MooseFET

    MooseFET Guest

    Its hard to beat the LTC1540 for low current. You will need really
    high resistor values so don't breath on the PCB.
     
  7. Guest

  8. Phil Allison

    Phil Allison Guest

    "D from BC"

    ** Nonsense.

    The load charging current only flows during the rise-time of the square
    ave - which even in your hand-picked example is only some 10% of the time.

    The average supply current is therefore ( for your example) only 1 uA.

    Similar math applies for faster rise times making the AVERAGE current draw
    essentially independent of the rise-time for fixed frequency and load C.


    ...... Phil
     
  9. You just reminded me of the HA7210, which just needs a crystal and a
    cap to make a (mostly) square wave oscillator. The data sheet
    suggests supply currents around 21uA @ 5V to 38uA @ 8V. Not 10uA,
    though.

    I see that the LTC1540 has low requirements when it just sits there
    doing nothing, but would it do better than the HA7210 once you
    surround it with four resistors and a cap, or so.

    Jon
     
  10. .... for 100kHz.

    Jon
     
  11. Guest

    Since I'm posting via a telephone modem - my brother's house in Sydney
    has every luxury except ADSL - posting a schematic would be time-
    consuming. An LTSpice text file would work, but who needs it?

    The patent I pointed to does include a number of schematics, and if
    you need a schematic for conventional two-transistor astable
    multivibrator, you can find one with google.

    You don't seem to appreciate precisely how simple a two-transistor
    astable actually is. I did once see someone build a two-transistor
    multivibrator that blew up its base-emitter junctions, whence the
    warning, but it isn't a complicated circuit.
     
  12. Frank Buss

    Frank Buss Guest

    Posting binaries in this newsgroup would be not a good idea. But why is it
    time consuming? Lets assume a 56k modem. A JPEG image of about 200kB needs
    less than a minute to upload, for which you can use one of the free image
    hosting services and then posting a link to it.
     
  13. Guest

    Posting binaries to ths news-group is not an option - when I'm at home
    I could post to abse, but I can't here.

    I'd have to stick it on my web-site as a new page - which takes quite
    a lot of fiddling around - if I thought that it was worth the effort.
    Since astable multivibrators are almost trivially simple, I'd prefer
    not to further clutter up my web-site for benefit of the terminally
    dim.
     
  14. Frank Buss

    Frank Buss Guest

    Not that I'm interested in an astable multivibrator schematic, but
    uploading images is as easy as clicking on the upload button, e.g. with
    this image service:

    http://www.imageshack.us/

    A breakout board for a display connector I designed and soldered some weeks
    ago:

    http://img329.imageshack.us/my.php?image=displaysteckerxb0.jpg

    I routed this by hand. It was interesting to see how the Eagle autorouter
    result looked like, with two sides and multiple vias :)
     
  15. MooseFET

    MooseFET Guest

    The LTC1540 would be spending most of its time "doing nothing". Most
    of its time it will spend near a rail.

    The input current doesn't rise much when the difference voltage goes
    up so the resistors can be many meg.
     
  16. Guest

    Post it yourself, if you are so keen to see it. The OP hasn't asked
    for it, and I'm not motivated to do you any favours.

    If you wanted to prove how clever you are. you could try to post a
    circuit for a 50% duty cycle emitter-coupled astable multvibrator -
    the smaller voltage swing at the active devices would reduce the
    current consumption over the more familiar collector-coupled design,
    and would make it easier to preserve a thin base-emtter junction, if
    you could get it to work.
     
  17. legg

    legg Guest

    Pity about the square wave requirement - that's where a lot of the
    power goes, and a sinusoidal output can have fair dv/dt through the
    logic transition levels.

    The usual complications and expense show up in anything with inductive
    energy storage..........

    RL
     
  18. James Arthur

    James Arthur Guest

    That drives U1A's input diodes...naughty naughty!

    Any idea what the average crossover current on that
    gadget is? The spikes are mA; the average could be
    a bunch. A 74hcu04 might help, or one of those CMOS
    transistor arrays things, CD4007 (?).

    Here's a low-tech method:

    Vcc +2v (regulated) / 8uA
    -+-
    |
    +--------------+--------------------+
    | | R4 |
    R1 | | 100K
    100K Q1 | R3 |
    | 2n3904 |/ .--470K--+-------+------> 102KHz
    +------+-----| C1 | | | 250mV p-p
    R2 | | |>. 100nF | |/ | (semi-sinewave)
    680K | | .--||--+------| |
    | | | | Q2 |>. |
    === | | === 2n3904 | |
    GND | | GND | |
    | '--------------------+ |
    | R5 | |
    | 220K |
    | | |
    | === |
    | Ct GND |
    | 20pF |
    '----------------||------------------'

    I didn't use all the current, leaving some for a buffer.

    With regular transistors you have to keep everyone from
    saturating otherwise they're way too slow.
    RF transistors would be easier--you could probably just
    spin a bog-standard multivibrator and get better symmetry
    to boot.

    Yours is simpler and cuter, if the current's okay.

    Best regards,
    James Arthur
     
  19. Joerg

    Joerg Guest

    I can't see the OP's post because he uses Google. He could check whether
    he can make an oscillator out of one of these since they've got a nice
    hysteresis:

    http://ww1.microchip.com/downloads/en/DeviceDoc/21434h.pdf

    Get the lowest voltage part available to minimize capacitive voltage swing.
     
  20. Joerg

    Joerg Guest

    First, low voltage. A fresh silver cell is just 1.55V. Then the clock
    never really feeds anything except the input of a divider chain within
    the chip. They'll probably do their darndest to keep its capacitance to
    a minimum.

    A square wave oscillator won't be a nice resonant architecture. So
    you'll have to muscle capacitive charges around and it will consume more
    power. It's like wanting to rapidly move the pendulum of a grandfather's
    clock between its end points. This is why the OP might want to think
    about whether it really has to be a square wave.
     
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