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low frequency micropower vco needed

Discussion in 'Electronic Design' started by Albre, May 2, 2005.

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  1. Albre

    Albre Guest

    I need a low frequency vco (to operate between 55 and 65 Khz) that
    draws very little power (microamp supply current if possible). Goggle
    turns up uhf and microwave vco's that draw 10's of ma. It needs to be
    a square wave, but can live with a triangle or sine wave output.

    A single chip solution would be preferred, but can build it from
    discrete components if necessary.

    I'd like to have it sweep between 55 and 65 Khz with the varying
    control voltage.

    A cmos r/c oscillator might work just fine, but I can't figure out how
    to vary the frequency with a dc input voltage:>:

    Any suggestions?

    Thanks,

    A
     
  2. Jim Thompson

    Jim Thompson Guest

    Vary VDD

    ...Jim Thompson
     
  3. Albre

    Albre Guest

    Thanks Jim,

    It sounds too good to be true though, is it really that easy?

    A
     
  4. Peter H

    Peter H Guest

    Use a cmos 555
    Typically draws 60uA


    Peter
     
  5. Jim Thompson

    Jim Thompson Guest

    I know it works for ring oscillators used as VCO's, but as soon as you
    add R/C's I'm not sure if it still works.

    (The ring oscillator is really varying gm.)

    ...Jim Thompson
     
  6. Albre

    Albre Guest

    Again, thanks Jim.

    After I read your first message, I googled with different keywords. As
    a result, I did find a schematic which showed a 4584 cmos gate
    oscillator with the input (Vcc) varied to control the output
    frequency. With an R of 2.2 meg and a C of 220 pf, the author claims
    that 2.5v in produces 1 Khz out and 9v in produces 20.8Khz out.

    I'd post it, but I think binary attachments are forbidden here.

    Thank you so much for taking to the time to reply, I appreciate it.

    A
     
  7. Frequency shouldn't change more than a few percent with Vdd over the
    normal operating range of a CMOS gate.

    You could make a VCO with an op-amp and a comparator, but how about
    the VCO portion of a 4046?



    Best regards,
    Spehro Pefhany
     
  8. Jim Thompson

    Jim Thompson Guest

    [snip]

    Post it to...


    ...Jim Thompson
     
  9. Jim Thompson

    Jim Thompson Guest

    That works because the 4584 has hysteresis. I think an ordinary
    inverter will vary only a small amount.

    I have made custom chips where the inverters had current mirrors in
    the rails.

    ...Jim Thompson
     
  10. That's a ST hex inverter.
    just post a link, like this one:

    http://www.discovercircuits.com/PDF-FILES/4584vco.pdf


    Best regards,
    Spehro Pefhany
     
  11. Albre

    Albre Guest

    I looked at the 74HC and 74HCT variants on the Phillips www site. They
    make some similar chips with slightly varying part numbers, but I
    checked them all.

    These chips are intended to run as high as 19 Mhz, and they all draw
    way to much quiescent power too. There might be a chance of one
    working if there was a slower speed part, but I don't see anything
    like that. I'm not sure why they draw so much current when they aren't
    switching, but they do.

    Thanks,

    A
     
  12. The On MC74HC4046A has maximum Iq of 4uA -55 to 25°C, 40 uA < 85°C.
    That's not so bad.



    Best regards,
    Spehro Pefhany
     
  13. Albre

    Albre Guest

    OK, th e file is at:

    http://www.discovercircuits.com/PDF-FILES/4584vco.pdf

    It appears to met my needs very nicely, although I don't need nearly
    as much frequency variability as this circuit provides.

    If a standard 4000 series gate can provide a 10 percent variance, I
    could probably use it.

    I think the 4584 is easily available and cheap enough, I'll probably
    use it.

    Thanks to all who helped out.

    A
     
  14. Genome

    Genome Guest

    Barry hat on.

    If you cared to take some time and consider the data provided by the OP then
    I am sure you will notice that he requested that the supply current was in
    the order of a microamp. Obviously your solution is unsuitable by a factor
    of four and, perhaps, an order of magnitude.

    Pick one of the following

    1) I have personally designed such circuits which worked perfectly.
    2) I have personally been involved with people who design such circuits and
    had many interesting discussions in order to steer them in the right
    direction.
    3) I have personally been involved with people who design such circuits and
    had many interesting discussions with them so I know what I'm on about.
    4) I was personally involved in a project where such a circuit was designed.
    No-one took my advice and it didn't work.
    5) I was personally involved in a project where such a circuit was designed.
    My advice was taken, it didn't work and they were subsequently proved to be
    wrong. If they would have taken my advice properly then it would have
    worked.

    Now, I think I have asserted my credentials and thus proved that you are
    wrong and I am right.

    Would you care to respond and give me an opportunity to............

    Oh Perlease!, I'm gagging for it

    BNA
     
  15. Perhaps, but his "microamp level" statement is sufficiently hazy that
    10uA or 50uA might also be okay... he mentions 10mA as being
    unsuitable.
    Well, 1) except for all the differences, but hey, you've convinced me.
    Okay, go for it. 1uA (maximum? typical?) at 70kHz operating current
    VCO, with realistic parasitic capacitances.


    Best regards,
    Spehro Pefhany
     
  16. John Fields

    John Fields Guest

     
  17. Genome

    Genome Guest

    [Derf Transform Applied]
    I see no reason to *go for it*. If you know the answer then I look forward
    to your explanation. I will not waste my time with needless arguments.
    I see that you understand. Perhaps we should wait for the OP to respond so
    we can further discern his actual requirements.
    Yes, but if you read OPs question again you will see he was asking for
    65kHz-70kHz operation. This is a very narrow range to achieve given your
    original solution of a 74HC4046 which operates to 19MHz.
    You are quite right. I should have posted to ask the OP to further define
    his actual requirements. This is probably a battery application and we
    should be more concerned about the gradual draw down due to internal
    impedances.
    Welcome

    BNA
     
  18. Albre

    Albre Guest

    If it was possible to do it with picoamps, I'd say go for it! It's not
    likely to happen for awhile though.

    I said 'microamps'. The statement was meant to discourage those
    replies from people that had milliamp solutions-after all, some people
    consider milliamps to be low power.

    Thanks again to all who had suggestions.

    A

    k
     
  19. Best regards,
    Spehro Pefhany
     
  20. Ah, okay, perhaps I misunderstooood what you meant when you said you
    were "gagging for it". Perhaps that's just as well.
    3.0/11 MHz min at 3/4.5V Vdd for the one I mentioned, but that's more
    like 19MHz when you convert to GBP. Anyway you don't want CMOS sitting
    inbetween the rails too much of the total period or it will blow away
    the uA limit. Or get rid of the CMOS bit. IIRC there were some really
    low power oscillators that put high-value resistors in the drains of
    the MOSFETs in a 4007.

    Best regards,
    Spehro Pefhany
     
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