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LMX2492: Subsequent Ramps with Frequency Steps

Dominik Eyerly
Prodigy 20 points

Part Number: LMX2492
Other Parts Discussed in Thread: LMX2491

Hello,

I would like to use the LMX2492 in a special ramping application. Essentially, I need to be able to increase the start frequency by a fixed step size after every ramp. The bandwidth of each ramp is identical. The time between ramps is very short, only a few microseconds so I would like to avoid reprogramming via SPI between ramps. Is this possible by simply having an up profile and a down profile that doesnt return fully to the starting position? Are there limitations? What would be the recommended approach for this?

 

Also, I was wondering how the external triggers from GPIOs operate. Are they sampled using the PFD clock? Are there any specifications to their setup and hold times? Thanks!

 

best regards
Dominik

  • 0
    TI__Guru** 119750 points

    Hi There,

    Once the ramping is enabled, it cannot be interrupted. To change the start frequency after ramping, I am afraid programming the PLL to the new frequency is the only way to make this happen. 

    Right, GPIOs are sampled by the fpd clock. Since fpd is an internal clock, setup and hold time should be reference to the input clock (fosc). I don't have the data, my best guess would be in the range of 2ns. 

  • 0 in reply to Noel Fung
    Prodigy 20 points

    Thank you for your reply! I have a couple of clarifying questions...

    Could you explain a bit further what you mean by "it cannot be interrupted"? 

    Also, the datasheet mentions using the RST flag in the ramp config to avoid "ramp creep". Couldn't this be used to do exactly what I want? 

    RAMP0_RST: "Forces a clear of the ramp accumulator. This is used to erase any accumulator creep that can occur depending on how the ramps are defined. Should be done at the start of a ramp pattern."

    Can you explain how this ramp accumulator works? The datasheet does not mention it beyond this one line. I would imagine that if I have a up ramp, the ramp accumulator increments the fractional numerator by my step size. What happens if the numerator overflows? Does the integer part get +1 and the fractional numerator simply overflows back down to a very small fraction? If that is the case, then wouldn't this perfectly cover my use-case?

    cheers

    dominik

  • 0 in reply to Dominik Eyerly
    TI__Guru** 119750 points

    Hi There,

    After RAMP_EN is programmed to 1, the device will execute the commands as set in RAMP0 to RAMP7 profiles. The device does not expect any interruption until the ramping is finished. One exception is when we have setup RAMPx_NEXT_TRIG with a trigger signal to temporary pause the ramping. 

    The ramp engine changes VCO frequency by adding or subtracting value (RAMP_INC) to the N divider. For up ramp, it adds value. For down ramp, it subtracts value. The process is in a continuous fashion until the RAMP_LEN counter is time out. For example, RAMP_LEN = 10 and RAMP_INC = 1MHz, then the accumulated frequency change is 10MHz. That is, the frequency has changed from F1 to F1 + 10MHz. If the next ramp is a down ramp, then the frequency is subtracted from (F1+10MHz). RST is used when the start frequency of next ramp does not equal to the end frequency of the current ramp. For example, if we want to create a sawtooth ramp, i.e. F1 to F2, then F1 to F2, ...... In this case, the start frequency of the new ramp is not equal to the end frequency of the previous ramp. This is doable as long as we have RST = 1. At the beginning of each ramp, the added N divider value is clear so the VCO jumps to the initial frequency (F1) immediately before ramping up. 

    The ramp engine takes care both the integer and fraction portion of the N divider, there is no overflow in numerator.

    Suggest read the EVM user's guide (www.ti.com/.../snau160e.pdf) and LMX2491 datasheet (www.ti.com/.../lmx2491.pdf) for more ramp examples.