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NE555: ne555 oscillator frequency question

Intellectual 1365 points

Replies: 12

Views: 159

Part Number: NE555

Hi,

My customer is using NE555 for 50Hz oscillator output, but they found the frequency is swinging from 49.5Hz to 50.5Hz.

Pls check the schematic and waveform as below, thanks.

  • James,

    Is this frequency "drift", cycle to cycle (jitter), device to device, or some other change?

    1/2 Hz for a 50Hz signal is 1% accuracy. Getting accuracy that good or better with diodes, capacitor, timer, and resistor variances with their temperature coefficients and supply voltage will not be practical. It is easy enough to get rid of the diodes. Getting a very <<1% NPO/COG capacitor can be expensive. 

    A crystal , crystal driver and a divider will be more accurate and less expensive than 0.1% caps and resistors. 

    Regards,
    Ronald Michallick
    Linear Applications

    TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

     

  • In reply to Ron Michallick:

    Hi Ron,

    This is a cycle to cycle drift, not a device to device or other changes.

    is it correct that the resistor/cap accuracy would only effect device to device drift? 

    So if the resistor and cap is already on the board, the frequency should stay at some point at 50Hz or nearby 50Hz. And it should stay still, instead of drifting from 49.5 to 50.5Hz? 

    Thansk.

    James

  • In reply to Ron Michallick:

    Hi Ron,

    Do you have any updates for this question?

    Thanks.

    James

  • In reply to James Zhan:

    James,

    The main cause for cycle to cycle drift is AC voltage on VCC and control pin.

    In the monostable section the data sheet says "The timing interval is, therefore,independent of the supply voltage, so long as the supply voltage is constant during the time interval."

    In the a-stable section the data sheet says "As in the mono-stable circuit, charge and discharge times (and, therefore, the frequency and duty cycle) are independent of the supply voltage".

    Regards,
    Ronald Michallick
    Linear Applications

    TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

     

  • In reply to Ron Michallick:

    Hi Ron,

    But the VCC/control PIN voltage doesn't seems to have some AC voltage change during all the time.

    Could you please help to check if you have any ideas?

    Thanks.

  • In reply to James Zhan:

    James,

    Can you provide AC coupled VCC and control pin voltages so I can see the noise? 

    Early next week, I can measure jitter in a clean setup to get a baseline.  

    Regards,
    Ronald Michallick
    Linear Applications

    TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

     

  • In reply to Ron Michallick:

    James,

    I setup a NE555 timer with Ra = 14.3k, Rb = 732k, C = 20nF (no diodes) clean power supply. The jitter width was 10us over nearly 100k samples. That is a frequency difference of +/-0.012Hz.

    Regards,
    Ronald Michallick
    Linear Applications

    TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

     

  • In reply to Ron Michallick:

    Hi Ron,

    Thanks for the test.

    Customer has a questions that if they only have 1% resistor and 10% capacitor, which would have huge impact on the output frequency.

    Do we have other solution that we could keep the output frequency within 2% precision or even 1% precision under this limited 10% cap choice?

    Thanks!

  • In reply to James Zhan:

    James,

    I'll present a 1% or better solution tomorrow. I'm' not quite done yet.

    Regards,
    Ronald Michallick
    Linear Applications

    TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

     

  • In reply to Ron Michallick:

    James,

    The basic idea is using a low cost 32,768Hz crystal supported by SN74LVC1GX04 then divide that by 328 using CD4040B with an AND gate on Q8, Q6 and Q3 to reset. That gives 99.902Hz but not 50% duty cycle. SN74LVC1G80 can divide Q8 by 2 yielding a 50% duty cycle clock at 49.951Hz. 

    Regards,
    Ronald Michallick
    Linear Applications

    TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).