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LMX2572: Manual ramping issues

abhishek singh
Prodigy 60 points
Part Number: LMX2572

Hi,

I have LMX2572EVM with reference pro. My goal is to generate a frequency sweep between 2400 MHz and 2500 MHz in 2 MHz steps on the RFoutAP output within 400microseconds.

I am able to generate ramp according to my requirement but it takes time in milliseconds and also having less output power   .I have programmed LMX2572 by STM32 . My SPI clock frequency is 3Mhz and ramp clock frequency is 200khz.

  • 0
    TI__Guru* 85330 points

    Hi Abhishek,

    Could you provide your ramp configuration? I am particularly interested to know the setting for below registers.

    Please note, there will be several VCO calibration in the middle of the ramp, so the total ramp time is not equal to number of ramp x ramp clock period. 

  • 0 in reply to Noel Fung
    Prodigy 60 points

    keeping ramp0 as 4Mhz gives me step size of 2Mhz in output frequencies

  • 0 in reply to abhishek singh
    TI__Guru* 85330 points

    Hi Abhishek,

    4MHz is correct because you have a output divider of 2. All the frequencies in the RAMP page are refer to VCO frequency.

    You have setup VCO calibration Threshold of 50MHz. That is, whenever the accumulated ramp frequency reaches 50MHz, the device will do a VCO calibration. The calibration may take more than 100µs. 

    Picture 1: no calibration (Threshold is 30MHz)

    Picture 2: calibration happen (Threshold is 30MHz)

    The calibration will happen every 30MHz (VCO) or 15MHz (RFout, div/2) increment. Your configuration has Threshold = 50MHz, you will see the calibration happen every 25MHz (RFout). 

    Since your RampClk is 200kHz, you are ramping the VCO every 5µs, which is much shorter than the calibration time. As a result, you may not get correct ramp frequency. Actual ramp time would also be longer than expectation. 

    I may have an idea to workaround this calibration issue, I will check this out in next week.

  • 0 in reply to Noel Fung
    Prodigy 60 points

    Shall I use the feature of turning off VCO in calibration duration ..

    Can i use Full Assist in Manual Ramping  for 100 MHz Ramp.. will LMX 2572 will support it.

    basically i have to generate two ramps @100 MHz & 300 MHz.. is it achievable using LMX2572

  • 0 in reply to abhishek singh
    TI__Guru* 85330 points

    HI Abhishek,

    The workaround I told you, unfortunately, it dose not work. We are not able to reduce the calibration time during ramping.

    If your goal is to continuously change the output frequency but not a linear frequency ramp, you may consider to continuously program the device to change frequency. Use the full assist feature of the device we can completely eliminate VCO calibration time. Below is the information how to use full assist. 

    https://www.ti.com/lit/pdf/snaa336

    If you want to use the ramp feature, there is restriction on the ramp range without VCO calibration. For LMX2572, this range is approx. 50MHz.