Hello. I'd like to use LTC6948 as a common VCO controlling output freq with an external DAC. But to start VCO i have to keep PLL loop closed. Is there any way to start VCO without the loop at all?
Best regards, Eugene
The LTC6948's wide frequency range is made up of several smaller VCO's. There are plots on page 7 of the LTC6948 datasheet that show the approximate width of each VCO range. The LTC6948 will need to run a VCO calibration to select the correct VCO initially. After that, if you vary the this initial VCO frequency more than 0.5% then you will need to perform another VCO calibration so a new VCO can be used. The speed of the VCO calibration will be based on the PFD frequency = reference frequency/Rdivider (see page 18 - equation 1 of datasheet).
Is the DAC in your case providing a variable Reference frequency or the Vtune voltage?
Also how wide of a VCO range are you trying to cover?
Thanks for your response. Yep, I want to control Vtune voltage in a wide frequency range.
Does the above concept of supplying a reference and recalibrating the LTC6948 VCO work for you?
If not, if you can let me know your frequency range and sweep time requirements, we maybe able to come up with something else.
ADI does have some wide band VCO's that may be of interest
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