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  • TI Thinks Resolved

CDCM7005-SP: Sine Wave input to device

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Replies: 13

Views: 214

Part Number: CDCM7005-SP

Hello, 

I would like to use the CDCM7005-SP but I have a sine wave input.  Can I please get some design suggestions as to how I could make this work optimally?

Regards,

  • Hello Kimberlee,

    Please refer to the following thread, I believe it answers your question.

    https://e2e.ti.com/support/clock-and-timing/f/48/p/349887/1227083

    Thanks,

    Vibhu

  • In reply to Vibhu Vanjari:

    Vibhu,

    That thread is talking about the VCXO input.  I am inquiring about the PRI_Ref.  

    I have a 10MHz input with nominal of 6.5dBm RF output power. Given that you are suggesting the same parameters apply are you saying that this would be an acceptable PRI_Ref input with no external circuitry?

    Thank you. 

  • In reply to Kimberlee Shimizu:

    Hello Kimberlee,

    Please ignore the previous suggestion, you are correct, that was for the VCXO input not PRIREF.

    Assuming the impedance of the line is 50 ohms, the 6.5 dBm signal power can be converted to a little over 1.3 Vpp.

    I'm worried that this may be a bit too low in power.

    "7.3 Recommended Operating Conditions" requires a VIL under 0.3 Vcc, which is ~ 1 V and VIH above 0.7 Vcc, which is ~2.3 V.

    This requires a minimum voltage swing of 2.3 V - 1 V = 1.3 V

    I was going to suggest, AC coupling the reference and then using a 100-ohm to Vcc, and another 100-ohm to GND to DC bias it at Vcc/2.

    If you are confident you can get above 1.3 Vpp and can meet the VIL and VIH requirement with your input, this is an option.

    Thanks,

    Vibhu

  • In reply to Vibhu Vanjari:

    Vibhu,

    Understood which is why I am asking for suggestions for external circuitry to generate a compliant waveform.

    As far as increasing the amplitude of the signal, I can directly couple the signal using a bias circuit.  Since there is a 150k pullup (to VCC), I can then add a pull down of 150K and add an AC coupling cap between the RF input and the PRI_REF pin.  This would bring the signal up with a Vpp of a little less than 3V.  

    So are you suggesting that if I do what I mentioned about, that it would be an acceptable input?  What about the 4ns maximum rise time?  

    Thanks.

  • In reply to Kimberlee Shimizu:

    Hello Kimberlee,

    You will still need to meet the rise time specification. For example for the 1.3 Vpp.

    So the fastest slew rate achievable will be 2*pi*f*V = 2*pi*10000000*1.3 = 0.082 V/ns => rise/fall time is about 9.5 ns

    Can you explain your suggestion further, how would would you get the Vpp higher with the 150k pull down?

    Thanks,

    Vibhu

  • In reply to Vibhu Vanjari:

    Vibhu,

    You would bias the signal such that it is centered around 1.65V then use some type of high speed comparator to create the square wave.  Of course this would involved some type of voltage detecting circuitry amongst other things.  There would be a lot that goes into this solution.    

    I guess that is my question and why I am on this thread.  That is one of the way that I thought of to make the PRI_REF input compliant but there are a lot of extra features and circuitry that would need to be fully developed and thought out.

      What is the best external circuit that I could use to get that 6.5dBm 10MHz clock compliant for the PRI_REF?  That example above is just something I've been thinking about but I'm sure there are other possibly simpler ways. 

    I'm sure there may have been others who wanted to use an RF sine input to this part and was wondering what external circuitry TI could recommend (perhaps out of the TI space catalog) or even some design options or suggestions.  

    Thanks,

  • In reply to Kimberlee Shimizu:

    Hello Kimberlee,

    Using a comparator or op-amp is one way to do it but these may add some phase noise/jitter. Another option to consider using a buffer or level shifter to boost the signal or convert to sin wave. I assume you require an -SP grade part, is this correct? Can you confirm?

    What is your RF input characteristics, can you confirm this is single ended? Is there any adjustments that can be made to this device, with respect to its frequency or voltage to help with the slew rate?

    Thanks,

    Vibhu

  • In reply to Vibhu Vanjari:

    Vibhu,

    Yes I would need components that are either -SP or -SEP rated.  We could consider commercial components as well if there is a white paper or radiation report written.

    This is what I have for the RF characteristics.

    Frequency	10 MHz
    RF power	6.5 ± 1.5 dBm
    Waveform	sine wave
    VSWR		< 1.5
    # of outputs	1
    Connector	SMA

    This is fixed.

    Thanks!
  • In reply to Kimberlee Shimizu:

    Hello Kimberlee,

    Clarification here, is the input S-E? This is what I assumed.

    What is the CDCM7005-SP being used for, to buffer the 10 MHz or generate a different frequency /  set of different frequencies? Maybe I can suggest a different part or combination of parts if I understand this better.

    Thanks,

    Vibhu

  • In reply to Vibhu Vanjari:

    Vibhu,

    Yes S-E input.

    It is being used to generate multiple instance of an 80MHz clock.

    Thanks,

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