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3.3V VCO for 25MHz clock generation

bruce nepple
Prodigy 20 points
Other Parts Discussed in Thread: CD74HC297, CD74HC7046A, SN74LS628, LMX2582, LMX2571, LMX2541, LMX2592, LMX2594, LMX2581

I'm looking for a VCO to produce a 12Mhz clock with a tuning range of at least +- 10000PPM . Cost is not a major issue.  Simplicity is!

I've looked at

 tlc2934(really poor tempco which causes an initial frequency startup issue) and marked as NFND

CD74HC297, CD74HC7046A, CD4046   too low freq at 3.3V

SN74LS628 is 5V, but I like that it has a range input.

I looked at the VCO section of more stable/complex  devices with external loop filters and a Vin pin, such as LMX2582, but I'm not sure what would be the best choice once I get that complex.  Also, the dynamic  tuning range at that frequency isn't obvious. (and how to best convert to a logic level clock).

I guess I can hook up a spi for initial setup, but I can't be doing anything interactive once I start the system.

So, what is the 3.3V (or lower) VCO of choice to generate a logic level clock at 12MHz (or multiples thereof). (low jitter, good tempco, etc)

Thanks

Bruce

  • 0
    TI__Expert 5370 points
    Hello Bruce,

    thank you for your interest in our products. It sounds like you are looking generate 12 MHz. The clear answer to your question is that we do not have a 12 MHz VCO a such...However, depending on your use case you may find interesting to be able to generate 12 MHz from a fully integrated PLL+VCO with modest power: LMX2571.

    A few questions:

    1) what performence level are you looking for?
    2) what frequency is the reference?

    Regards, Simon.
  • 0 in reply to Simon Damphousse
    Prodigy 20 points
    The LMX2571 won't work because the VCO cannot be driven by an external phase detector.

    Please re-read my original post. If you look at the LMX2582 PLL you will see that it has a separate Vin pin (not connected internally)

    There is no reference, because I'm looking for a VCO.
    I could go into tempco, jitter, etc but there is no need since there seems to be nothing that comes close to what I require in the first sentence.
    An expert in the LMX2582 could probably tell me if it could be made to work, but I already found a Maxim part that does what I need (MAX2605).

    Thanks for trying

    Bruce
  • 0 in reply to bruce nepple
    TI__Mastermind 27581 points
    Bruce,
    Our group does not make standalone VCOs, however we do make them integrated with PLLs.
    You could use the LMX2541, LMX2581, LMX2582, LMX2592, or LMX2594. To do this, you would first need to program the PLL and lock the VCO, then power down the PLL (set charge pump current to zero, .. ect) and let your phase detector take over.

    The LMX2581 has a 3.3V charge pump, but the other charge pump s are 2.5V.

    Regards,
    Dean