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CD4046B: CD4046B Maximum Frequency

yoshie
Prodigy 60 points
Part Number: CD4046B
Hi, Support team!!
I am using CD4046B with Vdd = 10V, R1 = 300k, C1 = 50p, VCOn = 0V.
So we check the oscillation frequency of the VCO. And when I checked the data sheet, I found the following statement.

There is no description for the upper limit of frequency, but what is the value? Is it about twice the value of TYP? Or does it change up to about 10 times?

Which part of the VCO circuit is this variation?
Thanks and best regards.
  • 0
    TI__Guru 57490 points

    Hi,

    Fig 4 and Fig 5 gives the center frequency at 25C and across temp. The frequency depends on the R and C values. 

    The listed datasheet spec and which you have in the query is at 25C ambient temperature data. The max frequency is guaranteed as a minimum value on the datasheets. The device can definately go higher than that depending on the passives chosen but the datasheet gives the min expected frequency of the device under that condition.

    Let me know if you have any concerns.

  • 0 in reply to ShreyasRao
    Prodigy 60 points

    Thank you very much for your quick reply.

    I have additional questions.

     

    •I understand that the frequency changes depending on R and C.

    But, This electrical specification table states that VDD, C, R, VCOIN, and temperature are fixed conditions.

    It was understood that even if the above parameters were fixed, the oscillation frequency of the VCO would deviate. Is my understanding wrong?

     

    • Under the above constant conditions, 20 sets of circuits were prepared to measure the frequency dispersion of the VCO. The tolerance of R and C is ± 1%. At this time, the oscillation frequency of the VCO fluctuated from 120kHz to 160kHz. Is this error within the specification range of the IC?

    Also, although the theoretical formula for calculating the VCO frequency is unknown, please tell us specifically.

     

    ※ We use stabilized power supply

    ※ Frequency is the average value of 50 data

    ※ Frequency measurement oscilloscope (DSO-X 3014A)

  • 0 in reply to yoshie
    TI__Guru* 96681 points

    Hello,

    It might help you to see how the VCO works.  There's a great app note on this device located here: http://www.ti.com/lit/an/scha002a/scha002a.pdf

    And that includes this schematic:

    Plus a lengthy explanation on how it works.

    In short - yes, you will expect to see a difference in frequency even if all external components are the same to 0.0001% accuracy. There is always some minor difference in internal components from one IC to another.

    This isn't the most accurate VCO around -- I wouldn't recommend using this device solely as a VCO as there are many newer and better options available on the market.  it only really becomes useful when inside a PLL loop, which is what the CD4046B is designed for. The minor frequency variations don't matter when you've locked your frequency through a feedback loop -- just like the exact open-loop gain of an opamp doesn't really matter when used in a feedback configuration since only the total loop gain matters.

  • 0 in reply to Emrys Maier
    Prodigy 60 points

    Hello,

    Thank you for the useful advice.

    I checked the app note. It was speculated that this was due to the ON resistance of the transistors that make up the circuit, and the variation in Vgs of P1 and P2.
    I think that the maximum frequency is limited by these components , but do not know the approximate value?
    I don't think it will be 10 times the value of TYP, but what about?

    Also, could you introduce a better VCO-IC?