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LMX2492-Q1: Help with Ramp Function Start

David Langan
Intellectual 270 points
Part Number: LMX2492-Q1
Other Parts Discussed in Thread: LMX2492, CODELOADER

I'm attempting to use the hardware ramp function of the LMX2492-Q1.  I am able to lock the device to a 100 MHz reference signal and produce a clean output.  I can easily change the frequency by adjusting the PLL R or N dividers.  I can sweep over several GHz of bandwidth by modulating the reference signal and generate a clean LFM frequency spectrum.  My issue is that I am unable to produce a ramp signal using the hardware functionality in the LMX2492.  I have been able to in the past using a different VCO, and have repeated the register configuration to the extent possible, but the VCO frequency range is different which leads to changes in the ramp configuration and ramp limit registers as well as the R and N divider registers. I have tried all manner of toggling the ramp_en bit, as well as writing to the N divider in an effort to start the ramp.

What are the minimum steps required to load and start a continuously looping ramp or series of ramps?

What could prevent the ramp from starting?  comparators (not used), DLD (have tried POR value and recommended value), 

My register settings:

# Software Reset
0x0 0x2 0x4

# PLL_R = 1
0x0 0x1A 0x0 0x1

# PLL_N = 96
0x0 0x11 0x0 0x60

# CPPOL = 0 (Positive VCO per codeloader, Negative per datasheet), CPG = 21 (2100 uA)
0x0 0x1C 0x15

# Ramp limits config from TICS Pro
0x0 0x52 0x18 0x0 0x0
0x0 0x4F 0x0 0xF 0x0
0x0 0x4C 0x0 0x0
0x0 0x46 0x9

# Comparator limits from TICS Pro

# Ramp configuration from TICS Pro
0x0 0x5C 0x4 0xC3 0x50
0x0 0x59 0x0 0x0 0x3
0x0 0x56 0xEF

# RAMP_EN = 1, RAMP_CLK = 0, RAMP_PM_EN = 0, RAMP_TRIG_A = 0
0x0 0x3A 0x1

# Enable PLL -> output locked and visible on spectrum analyzer

Thanks!

  • 0
    TI__Expert 5370 points
    Hello David,

    Could you reach out to us on clock_support@list.ti.com and share your basic settings. We will send you a configuration file that should work.

    Regards, Simon.
  • 0 in reply to Simon Damphousse
    Intellectual 270 points
    Will do. Part of the challenge it that the device is integrated into proprietary hardware, so we are unable to use Codeloader or TICS Pro directly. Rather, they are used to check the inputs generated by my own software routines.
  • 0 in reply to David Langan
    TI__Expert 5370 points
    understood. Regards, Simon.
  • 0 in reply to Simon Damphousse
    Intellectual 270 points

    My issue was resolved with the help of the TI team (Noel Fung).

    1) The LMX2492 must be in fractional mode for the ramping functionality to work.  To do so the FRAC_ORDER bits in Register 18 must be configured to a value of 1, 2, 3 or 4.  A value of 0 will place the device in integer mode and it will not start.

    2) If the ramp profile does not return to the start frequency via a ramp then the RAMPX_RESET bit must be set to 1 for the first ramp in the sequence.  If a reset bit is not set, then subsequent ramps will continue incrementing the accumulator until the VCO hits a high or low limit, or the PLL hits the configured ramp limit.  If the reset bit is set the fractional numerator will be reset to jump between frequencies instantaneously.

    3) RAMP_LIMIT_HIGH and RAMP_LIMIT_LOW should be properly configured per the datasheet to ensure the ramp limits do not interfere with the ramp functionality.