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DS90UB960-Q1: SSC Clarification

David Shumate
Expert 7875 points
Part Number: DS90UB960-Q1

Team,

Can you help clarify the spread spectrum clocking requirements for the 960. In the datasheet we specify a +/-.5% or -1% as an amplitude and we then have a 33khz requirement as well.

I don't understand what is meant by amplitude in the spec, is this supposed to be how much we can deviate from from the center frequency?

For example if we start from a 25Mhz clock and we use the -1% does this mean that we can go down to 0.99*25Mhz=24.75Mhz and then back up to 25Mhz?

If above is correct, what is the 33khz spec suppose to indicate?

Thanks for helping clarify!

Best,

David

  • +1
    TI__Guru** 107810 points

    Hello David,


    The most important parameters of spread spectrum clocking are:


    • Amount of SSC ==>  In the table above this is the +/-0.5% or -1%
    • Center or down spread
    • SSC profile ==> This can be Sawtooth Shape, Hershey Shape, or Sinusoidal Shape
    • Modulation frequency ==> In the table above this is the 33KHz

    The SSC can be either centered around the output frequency or down spread. Normally, when it isspecified using "±", it means that the SSC is centered. When only "–" is used, then it means that it is down spread. For example, if ±0.5% SSC is selected for a 25MHz output frequency, it means that the output frequency oscillates between 25.125MHz (25MHz + 0.125 MHz) and 24.875MHz (25MHz – 0.125MHz). This means that some clock cycles are above 25MHz. This can be a problem if the system is already running at its maximum frequency (for example 26MHz), in this case, use only a down-spread SSC.

    As an example, we used in our validation the following SSC enabled oscillator:

    Spread type: Center spread
    Speed percentage: +/- 0.5%
    Modulation frequency: 25.4 KHz
    Modulation profile: Hershey-kiss

  • 0 in reply to Hamzeh Jaradat
    TI__Expert 7875 points

    Thanks Hamzeh, please confirm if my understanding is correct:

    The +/-0.5% and -1% specifies the min and max frequency that we change to(eg: 25.125MHz to 24.875MHz as you've shown above)

    The 33khz modulation frequency is the max speed that we can change from the min frequency to the max and back (so it would take 1/33khz=30.3u seconds to change from 24.875Mhz to 25.125Mhz and back to 24.875Mhz)

  • 0 in reply to David Shumate
    TI__Guru** 107810 points

    David,

    Yes correct! The 33KHz is the frequency of the modulating wave. which is 30.3uS takes to change from min to max and then min again.