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CD74HC4046A: CD74HC4046A VCO Frequency characteristic

yoshie
Prodigy 60 points
Part Number: CD74HC4046A
Hello.
I'd like to use the product CD 74 HC 4046 A under development as an oscillator.

Specifically, using only the function of the VCO, R1 = R2 = 2 MΩ to 10 MΩ, C1 = 0 to 10 pF, VCOIN is connected to GND, the value of C1 is adjusted and a rectangular wave of a certain frequency is output from VCOOUT Take out it, and input it to another device.

So, I designed it with reference to the graph below, but I would like to know what effect this has on VCOIN = 0.5 *Vcc, and how does it affect VCOIN = 0 V? Does the linearity of the frequency to the capacity be lost?
Also, in p22 of the Application Report (SCHA003B), there is the following description.

Here, Stray capacitance at pins 6 and 7 are not limited by the pin-to-pin capacity, but what other factors do you have?
And, Could you please tell me the specific value of Stray capacitance at pins 6 and 7?
It is described as "6pF" in "Design Examples With Measured And Calculated Results".Is this value correct?
Best regards.
Yoshie.K
  • 0
    TI__Guru* 97046 points

    Hello and welcome to the forums!

    I do not see the images you tried to add. I have found the best way to add images is to first save them to your hard disk, then use the "Insert/Edit Media" button to add them to your message.

    I don't know which graph you mean for the first part of your question.

    "Stray capacitance" refers to total parasitics from your system design. I cannot tell you what you designed.  If you can provide a schematic, layout, and stack-up, I can try to give you an estimate of the capacitance.

    With this type of device, it is typically best to just build a prototype - this will give you the ability to test values directly rather than guessing.

  • 0 in reply to Emrys Maier
    Prodigy 60 points
    I'm sorry. Since I added the image, please confirm it.

    Thank you for answering "stray capacity". understood.

    I have an additional question.
    The value of R2 is limited by 300 kΩ in "DC Electrical Specifications" in the datasheet, but what is the meaning of this graph up to 11 MΩ?

    I understand that this limiting value is a limitation to guarantee the linearity of the frequency with respect to the voltage change of VCOIN, and it is understood that oscillation of the VCO itself is possible, is it?


    Thanks for the suggestion, But I can not provide "schema, layout, and stack-up" information directly from confidentiality restrictions here.

    And I already tried to measure the direct value by using prototype.
    However, I am anxious about the variation at mass production, not the value at the time of the test.
    When checking the function block of the VCO shown in the figure below, the N, P - FET and diode are connected inside the 6th and 7th pins, and this parasitic capacitance may affect the dispersion of the transmission frequency as the system thinking about.
    What is the variation and the value of this parasitic capacitance?


    Thank you.
  • 0 in reply to yoshie
    TI__Guru* 97046 points

    Please understand that this device was developed at least 20 years ago by Harris Semiconductor and was acquired by TI. The data available to me is very limited. If you are looking for a low cost oscillator, this is the right part.  If you are looking for a high reliability and high accuracy device, then I would recommend moving to a more modern VCO. There's a list of offerings from TI here: http://www.ti.com/clock-and-timing/oscillators/products.html#p2192=Standard 

    The value of R2 is limited by 300 kΩ in "DC Electrical Specifications" in the datasheet, but what is the meaning of this graph up to 11 MΩ?

    The graph shows typical operation across a wide range of values. You are correct that these typical values do not guarantee all datasheet specs -- you must remain within the limitations provided (3k to 300k) to keep within the characterization of the device.

    I understand that this limiting value is a limitation to guarantee the linearity of the frequency with respect to the voltage change of VCOIN, and it is understood that oscillation of the VCO itself is possible, is it?

    "Typical" specs are only guaranteed at 25C. Yes, the device will oscillate when using these values at 25C. No, it is not guaranteed to meet all specifications when operating outside the characterization limits.

    And I already tried to measure the direct value by using prototype.
    However, I am anxious about the variation at mass production, not the value at the time of the test.

    Variations are within the min/max limitations of the datasheet. If this limitations are beyond your system's requirements, you will have to move to a newer oscillator design.

    When checking the function block of the VCO shown in the figure below, the N, P - FET and diode are connected inside the 6th and 7th pins, and this parasitic capacitance may affect the dispersion of the transmission frequency as the system thinking about.
    What is the variation and the value of this parasitic capacitance?

    The internal capacitance variation is already included in the frequency variation data provided on the datasheet. Internal design structures are not characterized or included in the datasheet. Use the min/max values in the datasheet.

  • 0 in reply to Emrys Maier
    Prodigy 60 points
    Thank you for your answer.

    I can not use the VCO IC recommended for you. Because the cost and frequency are too high for the system requirements.

    And, I understand about other answer contents.