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LMX2581: FIN Pin together with internal VCO

Josef Lex
Prodigy 20 points
Part Number: LMX2581

Hello

My question is, can I use the FIN Pin together with the Internal VCO? Chapter 8.6.1.6.8 in the data sheet has only two modes listed, Full Chip and PLL only mode (by the way, isn't there a typo in the last column, because I would have expected that the FIN PIN is powered up in the PLL only mode?). What I want to do is use the built in VCO with RFoutA or RFoutB Pins, do some things with it externaly and feed it back to the FIN Pin to use it as input for the N-Divider / Phase detector. I know that this is a rather untypical application, but it could help me here solving a tricky problem (I know what I am doing, having almost 20 years of experience with PLLs). Could it be that one of the reserved modes can be used for that?

Thanks and Best Regards

Josef

  • 0
    TI__Mastermind 28001 points

    Josef,

    1.  Can you use internal and external mode on the LMX2581?  

    Yes, you can hook up an external VCO to the Fin pin and also use the internal VCO.   For instance, perhaps you want the broadband coverage of the internal VCO, but there is some specific frequency band you need a narrowband external VCO.

    2.  Can I feed the internal VCO through the Fin pin and follow the datasheet?

    No.   When you program the R0 register, it calibrates the VCO based on the internal N divider.  There is no muxing or bits to power up the Fin pin

    3. Are there test modes to allow me to do step 2?

    Not really.  You could theoretically calibrate the VCO using the internal path and then try switching to the path through the Fin pin.  So you are looking for a power up bit for the Fin pin and something bit for the N divider to take the Fin path instead from the VCO. I don't know of the bits to do this.  

    If you want to use just the internal VCO and use your own PLL, you could lock the internal VCO with our device and then tri-state the charge pump and switch in your own.

    Regards,

    Dean

    As for your comments in the datasheet, you are correct, there is a typo in the table.

  • 0 in reply to Dean Banerjee
    Prodigy 20 points

    Dean,

    Thank you very much for your prompt and well-founded answer. It saved me a lot of time which I might have used in playing around to find out myself.

    Best Regards

    Josef