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LMX2595EVM: Ramp output clipping problem

darren reis
Intellectual 385 points

Part Number: LMX2595EVM
Other Parts Discussed in Thread: LMX2595

I have been working on the LMX2595, trying to do an FMCW ramp.  I do not have a Spectrum Analyzer that can go up to the frequency that I will work at, so I have been using the frequency divider functionality of the 2595.  I output with the 4x divider so that my 53310A Domain Analyzer can watch the frequency waveform (upto 2.5 GHz). 

I'd like to build 3 different ramp waveforms, a 100MHz, 200MHz, and 400MHz wide sweep.  From the datasheet I understand I need to keep the slew rate at no faster than 1 MHz per 1 us, so for the 100Mhz case I prescribe the ramp to take 100us for each leg.  Here is the setup for the 100MHz waveform. 

Here is the output

Notice that the two-leg period is 200 us and 25MHz, which is what I expect after running through div4 channel divider.  Everything looks great up to this point.  For my actual system, I use the second channel output where there is no frequency divider.  I am not using any calibration, because when I do there are interrupts in the ramp that is disruptive to my goal.  

But then I go to run the 200MHz ramp in 200 us per leg, I run into a clipping problem .  

Then the output in div4 channel divider mode looks like this.

The period of the chirp is correct, 500us, but the ramp has been cut off.  I show the instrument markers at the expected levels, at 50MHz for the 200/4 setting.  But no matter what I do I get this ugly clipping. 

How can I get a wider chirp without this ramp clipping?  Thanks for your help!

  • 0
    TI__Mastermind 28031 points

    Darren,

    The VCO is divided into 7 different cores, which are further divided into 183 bands.  So there are  7 x 183 = 1281 different frequency bands.  There is a substantial amount of overlap, but the tuning range is limited.  If you want to ramp over a long range with the internal VCO, you must allow the VCO to re-calibrate;  that's the way it is.

    The first one has no issues because you can do 100 MHz in the frequency band without needing to re-calibrate.

    In the second one, I see you told the part to re-calibrate after 200 MHz, and you get a warning.  This is because the VCO band is not that wide.  So what you are seeing is the device trying to tell the VCO to ramp, but the VCO gets rammed against the edge of the band.  If you reduce the setting to allow the VCO to recalibrate, then you can ramp the 200 MHz, but you will have to allow the VCO to re-calibrte.

    Regards,

    Dean