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LMX2572: LMX2572

David Shelef
Prodigy 20 points
Part Number: LMX2572
Other Parts Discussed in Thread: LMX2491

Hi,

I want to ask if I can use ramp with the LMX2572 in wide range of 1 to 4 GHz or 4 FSK frequencies equally spaced from 1 to 4 GHz.

How can I synchronize it to external clock.

 

Thanks,

David 

  • 0
    TI__Guru* 81400 points
    Hi David,

    Ramp is possible with calibration. In FSK mode, the VCO will not be calibrated, so the FSK deviation cannot be too big, 1 to 4GHz deviation is not possible.
  • 0 in reply to Noel Fung
    Prodigy 20 points

    Hi

    Thank you for your answer.

    How can I define the ramp? Can I clock each step externally?

    David

  • 0 in reply to David Shelef
    TI__Guru* 81400 points
    Hi David,

    in Manual mode, ramp is clocked externally. In Automatic mode, ramp is clocked every fpd cycle. Please see datasheet 8.1.7 for details.
  • 0 in reply to Noel Fung
    Prodigy 50 points

    Hi Noel,

    David and I work on the same project, I tried the setting from the page 7, 8 and 9 but I didn't get any frequency output. I can only set a frequency by using the PLL tab. Below are the requirements and setting that we want to have. Please correct if need. Also to ramp I used the input RampClk and applied a 3.3V with a switch or with a function generator, nothing work.

    Here are the requirement.

    Below are the setting in the user control tab.

    Beow is the setting in the Ramp limits tab.

  • 0 in reply to hoang cam
    Prodigy 50 points

    Sorry, I think the screen snap shots pasted into Word are not accepted. Another question is the denominator setting, in which tab I can find it?

    Thank you.

  • 0 in reply to hoang cam
    TI__Guru* 81400 points
    Hi Hoang,

    Denominator setting is in the PLL tab.
  • 0 in reply to Noel Fung
    Prodigy 50 points
    Hi Noel,

    I can do now an automatic ramping and also a manual ramping. However, even when I increase the N divider one step at the time in the PLL tab (no automatic or manual ramping), I need to do a VCO calibration, if not, the frequency output will be wrong or sometime does not increase at all. It doesn't mater if it's a 25MHz or 50MHz step. Can we do the frequency stepping change like I did using the N divider without VCO re calibration? How about in the automatic and manual ramping mode, do I need to do the VCO calibration? i think I got the same problem. In ramping mode, can we do the VCO calibration without sending a command thru the SPI? Is the denominator setting the fraction number window? I have the LMX2572 EVM board with the TICS pro installed.

    Thank you.
  • 0 in reply to hoang cam
    Prodigy 50 points
    Hi Noel,

    I tried to set the VCO calibration based on the section 8.1.4 to 8.14.3. Is the section 8.1.4.1 important andwill that avoid the VCO calibration every time we increase or decrease the frequency 25MHz or 50MHz? At the section 8.1.4.3, i set all of them to 1 or zero, neither cases don't help. Can you send me a procedure to change the frequency without VCO calibration command, at least not via the SPI control?

    Thank you.
  • 0 in reply to hoang cam
    TI__Guru* 81400 points
    Hi Hoang,
    We have six VCO cores in the device, each VCO is divided into 183 bands. So if the VCO frequency change is big enough, a VCO calibration must be performed in order to find out the correct VCO core as well as band.
    In automatic ramp, once the RAMP_EN is checked, the ramp will run without further SPI programming. However, VCO calibration during the ramp is usually required unless the frequency change is small. Please see datasheet section 8.1.7.2 for details.
    If linear ramp with big frequency change is needed, I think you can try Full Assist. For example, if you need 100MHz ramp, you can program the PLL to change frequency 100 times, each time you change the output frequency by 1MHz.
  • 0 in reply to Noel Fung
    Prodigy 50 points
    Hi Noel,

    Thank you for your answer. What we need is to sweep from 1.8GHz to 3.6GHz with a step of 50MHz. In a regular PLL, every time we want to change a frequency, we need to send a command via SPI and it will take us a lot of time to do the sweeping as I showed above. Based on the frequency band, we need to send 37 times the SPI commands.
    However, if we can do the ramping mode without using the SPI every time, and based on the VCO frequency, I think we need to do it in two times to cover the full range (1 time with a VCO divider by 2 and the 2nd time with a VCO/1). From what you said, it may be very hard or even not possible for me since we need to have a linear ramp in manual mode so I can synchronize with another chip. Can you suggested us on how to do it or may be propose us another chip?

    Thank you.
  • 0 in reply to hoang cam
    TI__Guru* 81400 points
    Hi Hoang,

    Right, you are facing two problems with LMX2572. (1) you need to change the CHDIV to support the entire sweep. That means you have to do a SPI programming. (2) VCO calibration must be taken for every sweep step.
    Since your step size is big, looks to me that this is not a ramp but more like a frequency hopping. It will be better to use Manual Ramping Mode. However, since vco calibration is required, every time you jump to a new frequency, you have to wait for, say 50µs, before the new frequency is stable.
    Another option is to use LMX2491 with external VCO. This device supports ramping and since the VCO is external, there is no VCO calibration at all. However, you still have to design the loop filter carefully so as to minimize the VCO lock time as 50MHz jump is not small.