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Original question:

LMX2594

LMX2594: maximum slew rate for ramping without calibration

user6343606
Prodigy 40 points
Part Number: LMX2594

A TI employer answered that the minimum ramp duration is 1/fpd.

Does this mean we get in this small time a nice linear ramp with

a very high frequency slew rate?

E.g., if fpd= 100 MHz and the ramp is from 14 to 14.1 GHz (inside

VCO7, so no calibration necessary) the slew rate is 10^16 Hz/s,

i.e., 10 GHz/us?

It is mentioned that for ramps with calibration the slew rate 

is maximum 250 kHz/us.

  • 0
    TI__Guru* 94280 points

    Hi There,

    Frequency will be incremented once every fpd cycle, each ramp duration is therefore 1/fpd.

    However, there are many ramp between 14GHz and 14.1GHz. For example, if there are 100 ramps (defined by RAMPx_LEN), each ramp will change the VCO frequency by 1MHz only.

  • 0 in reply to Noel Fung
    Prodigy 40 points

    Noel, thanks for your answer.

    So if RAMPx_LEN= 1 (which is not forbidden), with my example the slew rate

    (the slope of the frequency ramp) is 10 GHz/us= 10000 MHz/us. Can the

    PLL follow this?

    For example, in "An Ultra-Wideband Fast Frequency Ramp Synthesizer […]" 

    () they mention a maximum ramp slope of 22 GHz/0.35 ms= 63 MHz/us. This is much below

    the 10000 MHz/us.

  • 0 in reply to user6343606
    TI__Guru* 94280 points

    Hi There,

    If RAMPx_LEN = 1, you are not doing a frequency ramp (or sweep), you are switching the VCO from one freq to another. 

    For example, given fpd = 100MHz, we can increment PLL_N by 1 to change the VCO frequency from 14000MHz to 14100MHz. We may be able to do this without calibration.

    Without calibration, the analog PLL loop response will be similar to this.

    You can use PLL Sim to do a similar simulation for other settings.

  • 0 in reply to user6343606
    Prodigy 40 points
    user6343606 said:

    Noel, thanks for your answer.

    So if RAMPx_LEN= 1 (which is not forbidden), with my example the slew rate

    (the slope of the frequency ramp) is 10 GHz/us= 10000 MHz/us. Can the

    PLL follow this?

    For example, in "An Ultra-Wideband Fast Frequency Ramp Synthesizer […]" 

    ( ) they mention a maximum ramp slope of 22 GHz/0.35 ms= 63 MHz/us. This is much below

    the 10000 MHz/us.

  • 0 in reply to Noel Fung
    Prodigy 40 points

    Noel, thanks again. I did not know about PLL Sim and will check it out.

    Effectively, the slew rate is about 6 MHz/us, because the 100 MHz change

    is finished after about 16 us.