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Using EVAL-ADF4355-2 as a VCO with a sweepable frequency via an analogue input

I'd like to use the EVAL-ADF4355-2 board as a VCO with an analogue input that can sweep the frequency. My system produces an analogue signal, and I'd rather not go for the complicated option of adding an arduino or something to convert this to a digital signal to interface with the registers.

As far as I can tell there are 2 options. Could you please advise me on what you think the best one is (or provide a better idea), and answer a couple of questions related to them?

My ideas are:

  1.  Providing an external reference frequency from a function generator or similar and using an analogue input to sweep that. It seems that using an external reference requires the internal reference to be disconnected by unsoldering a resistor, is that correct? Also, how well would the board be able to follow changes in the reference frequency?
  2.  Simply attaching an analogue signal to the VTUNE input on the board. I understand that the second option would probably remove the PLL functionality, but that's not necessary for my application. I'm unsure if simply soldering a wire would work, is there anything else that would need to be done to disconnect the PLL? Also, while I found a graph for the sensitivity of the VCO system as a function of frequency in the datasheet for the ADF4355-2, I was unable to find the absolute voltage values that should be used there, is that information available?

Thank you

  • Hi,

    ADF4355-2 must be programmed to have a valid output frequency. The most of the registers does not have a POR value and they are 0 on power-up. So, you won't be able to control the output without any programming. 

    This condition is valid for all VCO integrated PLLs, you are very limited without any SPI programming.


  • Hi,

    Sorry if I was unclear before, I'm happy to set up the chip with the software and then leave it on continuously, it's just that while it's already running I need to be able to change the frequency with an analogue channel.

    Since posting the question I have briefly tried changing the voltage VTUNE while the chip was outputting and I didn't see any change in frequency, so it seems that that won't work. I also tried using the REFIN SMA ports, and again they seem not to work as-is. I think the next thing to try is unsoldering the resistors R12 and R27, but that seems like an awkward thing to have to do just to supply my own reference frequency.

    Do you know if there's anything I'm missing here?

  • Sorry, I misunderstand that. 

    There are 4 VCO core and 256 sub-band for each core. Autocalibration process selects a core and a sub-band for a given PFD frequency and N divider value. Frequency can be shifted by changing Vtune voltage or shifting the reference frequency. This shift will be limited to the coverage of the sub-band. 

    If you want to use your external reference on EVB, you need to remove R12 resistor which provides supply to the on-board crystal. If your external reference is single ended connect it to REFIN A, if it is differential connect it to both REFIN A and REFIN B.