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CDCS501: Pulsed oscillations on output.

Slower
Prodigy 40 points
Part Number: CDCS501

When the clock is turned off through an enable line, the clock output that feeds into the CDCS501PWR is low. During this, the output of the part has a pulsed oscillation.

I have tried making sure that the signal fed into the CDCS is pulled low and pulled high. The same problem occurs on the output either way. Checking with the oscilloscope this problem does not exist on the input of the CDCS.

I have removed the 18pF and 15pF capacitors on the output. I have removed a ferrite bead on the output. I have removed the resistor on the output. I have even tried pulling the output up with a 1k resistor up or down. I always have the same output when the clock is disabled with every individual change.

This problem does not exist when the clock is enabled.

Any help is greatly appreciated.

  • 0
    TI__Guru* 81280 points

    Hi Slower,

    We don't have an immediate explanation on this, we will need to check and get back to you later.

  • 0 in reply to Noel Fung
    TI__Expert 6140 points

    Hi Slower, 

    Just so I understand better, the clock here is the XLH335050 component? 

    So when it's providing and output which goes into the CDCS501, the CDCS501 shows you the same output - as expected. 

    When you change the enable line on the XLH335050 (presumably disabling it's output), CDCS501 shows a pulsed output? You have probed this and you know for certain that there's no input clock going in and you have forced a low or a high, and the CDCS501 still shows the pulse. What is the voh of the pulse? 

    This is an older device and the information on it is very limited, but from what I can tell the whole spread feature is essentially disabled once you pull SSC high - so what benefit is it providing in your system? 

    Thanks and regards,

    Amin 

  • 0 in reply to Amin Eshraghi
    Prodigy 40 points

    Yes, XLH335050 is the clock being used.

    Yes, when there is a signal applied to the CDCS501, I get the expected signal on the other side.

    Yes this is correct, I have probed both input and output at the same time while attempting to pull the line high and low. ~3.2V.

    Sorry I had not caught that when I posted up the schematic. The part for pulling SSC high is actually not populated on the board and that should show up in the schematic with an NF over it but the software I use sometimes decides not to do that.

  • 0 in reply to Slower
    TI__Expert 6140 points

    Hi Slower, 

    Does it matter at all whether the clock was ever provided or not? As in do the pulses only occur if the system had ever gotten an actual clock vs if you powered up in disabled mode? 

    Also is there an inductor on the output pin? 

    Thanks and regards,

    Amin 

  • 0 in reply to Amin Eshraghi
    Prodigy 40 points

    The behavior is the same before the clock signal is initially provided and after the clock signal has been provided but then disabled.
    There was a Ferrite bead on the output of the device. This was removed while trying to debug this issue.

    Thanks

  • 0 in reply to Slower
    TI__Expert 6140 points

    Hi Slower, 

    Is this the first time you've come across this issue? Have you used the device previously in any other system? 

    Is it at all dependent on the input frequency or does it occur no matter what? Can you send us scope captures of the output pulse? 

    Apologies I haven't been more helpful but this is an older device with very little amount of technical information on, so I'm just limited to the datasheet as well and I can't see anything in there that potentially even hints at you're seeing. 

    Thanks and regards,

    Amin 

  • 0 in reply to Amin Eshraghi
    Prodigy 40 points

    Only on every board manufactured so far haha. I have only used this part in 1 design to date.

    It occurs no matter what, when there is no signal present. When the actual 50MHz signal is applied the problem no longer exists. I will try to grab a scope output and post back.

    I initially checked over the datasheet. I guess I was hoping that I could get confirmation from TI that this is an issue with the chip or that I have something else going on at my end.

    Either way I do appreciate the help Amin.

  • 0 in reply to Slower
    TI__Expert 6140 points

    Hi, 

    Just to let you know I am checking with other folks if anyone has seen this issue or has any guesses as to what's happening. So far no luck. 

    Regards,

    Amin