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Original question:

SN74LV4046A: Frequency fluctuation

SN74LV4046A: What is the Minimum Frequency of VCO?

Liu Yang
Expert 6005 points
Part Number: SN74LV4046A
Other Parts Discussed in Thread: CD74HC4046A,

Hi Team,

According to the datasheet, the Maximum frequency of VCO is 24MHz or 38MHz.
I can’t find the Minimum Frequency.

Best Regards,
Tom Liu

  • 0
    TI__Guru* 96842 points

    Hi Tom,

    The VCO uses three external components to determine the operating frequency of the device, R1, R2, and C1.  These are the limitations of those components:

    The test for Center Frequency uses minimum values for R1 and C1 with R2 removed for zero offset (and Vcc/2 input voltage):

    I wouldn't expect the device to drive any slower than about 1/2 of the shown minimum center frequency. For example, at 3.3V supply and 7 MHz center frequency operation, the VCO will definitely operate down to 3.5 MHz, but I probably wouldn't try to push it below that.

  • 0 in reply to Emrys Maier
    TI__Expert 6005 points

    Hi Maier,

    Thank you for your answer.
    Could you tell me What is the Minimum Frequency when Vcc=5V, C1=0pF, R1=9.1kΩ, R2=∞ (Maximum frequency is 38MHz)?

    Best Regards,
    Tom Liu

  • 0 in reply to Liu Yang
    TI__Guru* 96842 points

    Hi Tom,

    I would recommend against operating the device with C1 = 0pF (which is where the typical maximum of 38 MHz is measured on the datasheet). The datasheet minimum for C1 is 40pF for normal operation, and the frequency values given above are given at this condition.

    The CD74HC4046A has more detailed plots for operation. Since the SN74LV4046A was designed as a direct replacement to the CD74HC4046A, Those plots can be used to setup the SN74LV4046A.

    Figure 12 shows the expected center frequency across a variety of C1 values:

    The above plot is on a logarithmic scale. You can see that at minimum resistance (3kohm), the center frequency starts to roll off towards the lower capacitance values (less than 100pF). 

    Figure 21 in the same datasheet shows the VCO frequency as a function of VCO input voltage:

    I chose this figure since it shows operation under very similar conditions. You can see here that the VCO output frequency varies (at 4.5V) from ~4MHz to ~17MHz, with a center frequncy (at VCOin = 2.25V) of ~9MHz.

    You can also see that the supply frequency doesn't significantly impact the VCO frequency, except that it extends the top end of the frequency scale -- ie the 3V supply range overlaps with the 4.5V and is very close to the 6V scale.  Ie when you use 5V, you will find that the numbers match closely to 4.5V... and if you have a non-imaginary power supply, I would recommend using the 4.5V values since supplies vary and designing for the worst-case is a good practice.