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Interfacing the CDC3S04 to the LMK03000

Alan Langman
Prodigy 20 points
Other Parts Discussed in Thread: CDC3S04, LMK03000, LMK00105, LMK00304, CDCLVC1104, SN65LVDS105, LMH7220

Hi,

I am using a CDC3S04 to distribute a 26MHz TCXO clipped sinewave clock to multiple devices, including the LMK03000. What is the best way for the CDC3S04 to drive the LMK03000. The CDC3S04 specs a maximum ouput load of 30pF (approx 200ohm at 26MHz). The evaluation board for the device is matched for 50 ohm; what effect does this have on the signal? I can't find too much information on the input characteristics of the OSCin for the LMK03000, other than one can drive it single ended; looks like the clock should be able to drive a 50 ohm load and  the power levels for best performance are prescribed by Figure 16; although the graph for singled ended only starts at 50MHz.

I did notice that the eval board for the LMK03000 uses a 1:2 impedance RF transformer.

Any input would be greatly appreciated.

Best

Alan

 

  • 0
    TI__Mastermind 28550 points

    Hello,

    The input can be considered high impedance and capacitive.

    At < 30 MHz, the input impedance of OSCin is greater than 3.3 kohm = | 2500 -j 2200 |

    At < 30 MHz, the input impedance of OSCin* is just less that 4 kohm = | 3400 -j 1800 |

    Alan Langman said:
    The evaluation board for the device is matched for 50 ohm; what effect does this have on the signal?

    This helps match the 50 ohm RF trace to the input.

    Alan Langman said:
    I am using a CDC3S04

    Is there a reason you are using a sine wave buffer instead of a buffer like CDCLVC1104/LMK00105 (single ended) or SN65LVDS105/LMK00304 (differential)?

    Alan Langman said:
    Figure 16; although the graph for singled ended only starts at 50MHz.

    That's because input slew rate is important, with a single ended sine wave it is best to have slew rate > 0.15 V/ns.  0.5 V/ns is better.  Higher frequency sine waves improve slew rate, especially when single ended.  Less than 50 MHz sine wave is not recommended.  Solve this by using a buffer which outputs a square wave, but it is possible your clipped sine wave clock will have enough slew rate.

    73,

    Timothy

  • 0 in reply to Timothy T
    Prodigy 20 points

    Hi Timothy

    Thanks so much for your rapid response!

    The reason I am using the CDC3S04 is that I need to provide the 26MHz reference to a RFD2081 and RFFC5071 from RFMD (sorry not TI parts). They recommend clipped sine wave for the reference input.

    Do you have a TI part you can suggest to convert sine wave to square wave for best performance for the LMK03000?

    Thanks!

    Best

    Alan

  • 0 in reply to Alan Langman
    TI__Mastermind 32575 points

    Hi Alan,

    A simple TI device to convert sine wave to square wave is LMH7220 high-speed LVDS comparator.  The LVDS output of LMH7220 should provide a sufficiently fast edges to drive the clock input of LMK03000.

    Regards,
    Alan