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LMK04616EVM: How to change VCXO to an external oscillator?

Guenther Rehm
Prodigy 10 points
Part Number: LMK04616EVM
Other Parts Discussed in Thread: LMK04616

Hi,

I've successfully used the LMK04616EVM to generate various output clocks. Now I need to change from the supplied on-board VCXO at 122.880 MHz to an external voltage controlled oscillator.

I see that there are facilities to install SMA connectors to that end, but I looking at the circuit diagrams in the datasheet, there remain some DNC in the existing board that need to be filled. Maybe other parts need to be removed, for instance the currently installed VCXO? Are there any instructions?

Best regards

  • +1
    TI__Mastermind 33430 points

    Guenther,

    When you say you wish to replace with an external voltage-controlled oscillator, is this intended to fulfill the same role as the VCXO? Just making sure: PLL1 supports external VCO (usually a VCXO), and you're not trying to use an external VCO as the PLL2 VCO.

    The EVM connects the tuning voltage output port from the LMK04616 to the control pin of multiple common EVM footprints in parallel. The original VCXO should ideally be removed if a new device is going to be used, but this is sort of annoying to do in practice because of the size of the board and the specific interface between the on-board VCXO and the EVM... I've pulled more than a few pads off in-house boards by accident. A slightly less risky approach removes external connections to the VCXO:

    • Remove R57, and solder the connection of R57 directly to the other end of R58 with a small wire. There are no signal integrity concerns, the loop bandwidth at PLL1 is low enough that any wire will be sufficient to carry the signal from one pad of R57 to the other pad of R58.
    • Short pin 2 to pin 3 on VCXO U4; this pulls the enable pin to ground and disables the oscillator to prevent crosstalk.
    • Remove C25 and C28 to break the connection from the on-board VCXO outputs to the LMK04616.
    • Populate the OSCIN_P and/or OSCIN_N SMA connectors (both for differential, either one for single-ended).
    • Populate R34 and R43. Default 0Ω will DC-couple OSCIN connectors directly to 50Ω single-ended load, and C2/C29 will AC-couple the signal to the device pins. If external signal must be AC-coupled to the 50Ω single-ended load, R34 and R43 could be populated with the capacitors from C25 and C28 instead. R37/R42 could be replaced with R39 for DC-coupled or AC-coupled 100Ω differential connection as well.
    • If the external VCO is single-ended, and you do not use a balun to split into a differential input, the unused leg of the differential pair should swap either C26 to C27 or C29 to C30; the unused single-ended OSCIN pin at LMK04616 must be AC-coupled to GND.

    Regards,

    Derek Payne

  • 0 in reply to Derek Payne
    Prodigy 10 points

    Thanks Derek,

    Your answer helps me a lot. It addressed precisely what I want to do: disable the populated VCXO in an elegant way and connect to an external oscillator (in my case a Si569 on a Si5xxUC-EVB. I need this, as I require the VCXO to run at a special frequency (249.567 MHz), and this 'programmable oscillator' offers a solution.