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SN74LV4046A: VCO Characteristics

Part Number: SN74LV4046A
Other Parts Discussed in Thread: CD74HC4046A, CD74HCT4046A

There seems to be a complete void for information on how to set the VCO characteristics of the SN74LV4046 with C1, R1 & R2. There are no usable charts or formulas that I can see in the SN74LV4046 part spec.

What am I missing?????

  • Hi Rick, and welcome to the forums.

    I can understand your frustration - the SN74LV4046A's datasheet really doesn't say much about the operating frequency of the VCO.  All I find in it is this section:

    The only info here is the expected center frequency with C1 = 40pF, R1 = 3kohm, R2 left open, and VCO_IN = Vcc/2.

    I pulled these two plots from the CD74HC4046A datasheet:

    It's a little hard to tell what the exact values are, but it looks like this is pretty consistent with the LV4046 device - which makes sense since the LV4046 was designed to be a "pin compatible" drop-in device for the older CD74HC4046.  You should be safe using the HC4046 datasheet's plots to setup the VCO. 

    I always recommend prototyping with a device like this -- the values should be tweaked to get exactly what you want.

  • Emrys,

    Thanks for welcoming me to the forum and for your response.

    I read that the LV4046 was designed to be a pin compatible device to the older part as well, but I noticed a significant difference in the specifications for the value range for R1 & R2. The LV4046 part says that R1 & R2 should be in the 3K to 50K range where the CD74HC4046 part has an R1 & R2 range of 3K to 300K. Whats even more confusing is that the "CENTER FREQUENCY vs R1" graphs in the later spec indicates values upwards of 7.5M.

    Up to this point, I've been prototyping with the SN74LV4046 part, but today I should receive some CD74HC4046A & CD74HCT4046A parts to experiment with as well. Hopefully, I'll be able to get the this sorted out soon and get my VCO parameters under control.

  • Hey Rick,
    It's unfortunate that I don't have more data for you.

    These parts aren't used very commonly, so they kind of get relegated to the bottom of the priority list for updates.

    I have added a ticket for us to update the datasheet with additional information, but I wouldn't expect that to happen for quite a while based on our current backlog of updates (probably 1-2 years).