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LMX2492

AZ1
Prodigy 100 points
Other Parts Discussed in Thread: CODELOADER

Hi,

We are considering using the device and performing PSK and FSK modulations.

We are trying to understand what would be the maximum modulation rate.

I guess this is, among the rest, a function of the loop bandwidth as well as the PFD frequency.

Any suggestion on how to estimate these numbers (for FSK and PSK)?

Thanks. 

  • 0
    TI__Mastermind 28011 points

    Hello,

    I would expect the loop dynamics to limit the programming speed and pin programming is very fast.

    As a rule of thumb, the loop bandwidth should be at least twice as wide as the maximum modulation rate.   Or anothe rway to say this is that your maximum modulation rate is 1/2 of your loop bandwidth.  

     

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    Prodigy 100 points

    Thanks Dean.

    will the PSK maximum modulation rate be the same as the FSK maximum modulation rate?

    Thanks.

  • 0 in reply to AZ1
    TI__Mastermind 28011 points

    For this, I'm not totally sure, but I think so.   The frequency is the rate of change of phase, so that makes me ponder a little.

    Here's another reasoning ...

    I did some quick estimates and it seems the lock time for a frequency change of 0.01% (to 1 kHz) is about the same as a phase change of 180 degrees (to 1 deg).

    The lock time is inversely proportional to the loop bandwidth, s otherefore it implies tha the same rule should apply for PSK.  But these are really rought estimates.

    Regards,
    Dean

  • 0 in reply to Dean Banerjee
    Prodigy 100 points
    Thanks Dean,

    The device can actually effect the loop bandwidth using the Fastlock mechanism. I don't know if this is being used "automatically" by the device at modulation mode (PSK or FSK) but maybe the modulation speed can be improved by applying this mechanism?
  • 0 in reply to AZ1
    TI__Mastermind 28011 points
    Fastlock works by switching in a wider loop bandwidth while the pLL is changing frequency, and then switching in a narrower right before it finishes settling. The benefit is that you get a narrower loop bandwidth when switching. WIth the FSK/PSK modulation, if you want a wider bandwidth, then just design for that in the first place. So I don't think it could be of much benefit unless you wanted to use a wide loop bandwidth while doing the modulation, and then switch to a narrower one when you are not modulating.
    Regards,Dean
  • 0 in reply to Dean Banerjee
    Prodigy 100 points

    Thanks Dean,

    I appreciate your support.

  • 0 in reply to AZ1
    Prodigy 100 points

    Hi,

    We are interested in producing quite large FSK deviation (2.5-5MHz). If I read the data sheet correctly, an FSK deviation of Fpd×FSK_DEV / 2^24 where FSK_DEV can go up to full 33 bits range can be produced.

    However when I run the CodeLoader program and right click the FSK_DEV field to get help on this filed I get the attached message: .

    It says that this is the integer part of the FSK dev and that the field length is only 8 bits.

    Is this a bug at the the help text?

    Thanks.