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Appropriate Oscillator

Discussion in 'General Electronics Discussion' started by TheMaster, Aug 7, 2016.

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  1. TheMaster

    TheMaster

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    Jul 20, 2016
    I am in need of assistance regarding the constructing of an oscillator which will best meet the following criteria for an application which I am designing.
    I attempted to find a solution with the search function, though found nothing definitive.
    The necessary criteria for the application are:

    - ~12V Signal, Powered by a 12V DC battery source
    -Ideally square wave (Sinusoidal gives worse results, but is also possible)
    -Accurate frequency of 55khz with 3% margin of error
    -Built with small components soldered to board (Typical sizes, nothing extraordinarily large) ICs are permissible.
    -Requires no manual tuning or adjusting (The circuit may be produced in large numbers)

    I attempted construction of the circuit with a 555 chip. The results were problematic due to the 555's 200ma output limitation. (I tried having the output control a transistor as a switch to the power, results are still problematic for various reasons).

    I do not need detailed instructions pertaining to construction, simply a point in the right direction as to which method may best meet the criteria.
    Any help is greatly appreciated
    -Thanks
     
  2. duke37

    duke37

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    Jan 9, 2011
    To get accurate frequency, you need a crystal oscillator followed by a divider.
    To get more consistency than a 555 you could use a ceramic resonator (dirt cheap). A 455kHz resonator divided by 8 may be near enough.

    There should be no problem getting a more powerful square wave output using a fet as a switch. What are your various reasons?
     
    davenn likes this.
  3. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    Use a 6-pin PIC chip and a FET.
     
  4. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
  5. Alec_t

    Alec_t

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    Jul 7, 2015
    If you go for the oscillator/divider, a 3.5MHz oscillator followed by a divide-by-64 chip will give you 54.69kHz.
     
  6. Sunnysky

    Sunnysky

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    Jul 15, 2016
    another counter, oscillator CD4060 and a cheap crystal @3.5MHz 2 small cap, 2 R's and a MOSFET switch.

    What is this for? drive current and load type?
     
  7. BobK

    BobK

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    Jan 5, 2010
    I agree with Colin. You cannot make it simpler than using a micro. You will get 1% accuracy without a crystal, and much higher with a crystal. With an SMT PIC and MOSFET you could fit this in 1 square cm.

    Bob
     
  8. TheMaster

    TheMaster

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    Jul 20, 2016
    Thank you all for your responses

    I believe that my "various reasons" causing problems are related to the transistor I've been using as a switch.
    Also my 12V source is comprised of batteries in a series, so I've been experimenting with overcoming high internal resistance.through various means.

    I have little experience with electronics, this circuit is a part of a larger application in my field of work, so please excuse my naivety with these questions.

    -What advantage does a MOSFET provide over a typical transistor in such a circuit?
    -What specific product could be best used as a 64 divider for a crystal oscillator?
    -What specific PIC chip would best suite this circuit?
    -Also, while I'm at it: Does the current rating for a component (for instance an IC) refer to P-P or RMS, when dealing with a changing current?

    I will test your advice in the simulator and check for results
    -Thanks
     
  9. duke37

    duke37

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    Jan 9, 2011
    1. A fet needs negligible drive current but does need a fair amount of voltage. The fet when turned on has a very low resistance so will dissipate very little heat. A npn/pnp transistor needs considerable current drive and will drop about a volt so may need a heat sink.
    2. A cmos 4060 oscillator/divider may suit you.
    3. I am ignorant of pics.
    4. Current ratings are normally maximum at any time. An exception is often given for diodes where they can be over-run for a very,very short time. The advice would be to keep well inside the specifications.
     
  10. Alec_t

    Alec_t

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    Jul 7, 2015
    One of these PICs perhaps?
     
  11. AnalogKid

    AnalogKid

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    Jun 10, 2015
    In your first post you mention the 555's 200 mA output current as a "limitation". Why? That is an exceptional output current for a small signal device. What output current do you need? Does your output need to only source current, like a single power transistor switch, or does it need to pull the output to GND rapidly during the "off" part of the cycle like a logic gate or 555 output stage?

    And regarding the use of a PIC for this task:
    1. Do you have a PIC development system and/or programmer?
    2. Do you have a PIC compiler for C, C++, BASIC, or whatever?
    3. What is your skill set for microcode development?
    4. What is your time budget?
    5. If the 555's 200 mA output stage was not beefy enough, a PIC output pin will be a major disappointment. Plus, you will need a voltage regulator for the PIC. Plus, you will need a voltage amplifier and current amplifier on the PIC output.

    After all of that, note this: the internal oscillator in most PIC devices is not very accurate; some are factory trimmed, but most are not. To get 3% accuracy you might have to add an external ceramic resonator or crystal. Once you do that, you're better off using a CD4060.

    And while I'm not a huge 555 fan, it does have some plusses. Besides an output stage that probably can drive whatever you need for power, its internal circuit contributes very little error to the oscillation frequency. With a 0.1% resistor and 1% capacitor, you should be able to hit 3% accuracy without calibration, depending on the operating temperature range, power quality, etc.

    ak
     
    Last edited: Aug 8, 2016
  12. Sunnysky

    Sunnysky

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    Jul 15, 2016
    What is this for? drive current and load type?

    It may affect the best choice of driver.
     
  13. BobK

    BobK

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    Jan 5, 2010
    PICs internal oscillators are specced at ±2% from 0C to 60C and are better for a narrower temp range.

    If the temperature variation is small and you calibrate it, it will be well within the 3% requirement without a crystal.

    The tools / experience question, however is relevant. I would use a PIC because I could have it running in less than an hour, but that is not true for someone with no experience.

    Bob
     
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