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LMH1982: GENLOCK

Zeming Kong
Expert 3515 points
Part Number: LMH1982
Other Parts Discussed in Thread: LMH1983,

Hi team,

The output clock frequency will change if we turn on/off GENLOCK, and it is not acceptable. Do you have any method to decrease the frequency change?

Best,
Zeming

  • +1
    TI__Guru** 108270 points

    Hi Zeming,

    This is a normal behavior.

    In Genlock mode, PLL is locked to the input video reference, so the output frequency is correct.

    In free run mode, output frequency will be equal to the VCXO frequency, which is dependent on the Vc voltage. We can enable the LOR holdover function, when the input video reference is removed, PLL will switch to holdover mode, Vc will be the voltage that was last detected when the PLL is locked. This could reduce the frequency error in short term. 

  • 0 in reply to Noel Fung
    TI__Expert 3515 points

    Hi Noel,

    Thanks for your suggestion.

    The customer told me that holdover mode cannot reduce the frequency change, it can only reduce the settle time, but the frequency change will not improve.

    Could you teach me if the frequency can be improved if we change the VCXO filter? or LMH1983 is better than LMH1982?

    Best,
    Zeming

  • +1 in reply to Zeming Kong
    TI__Guru** 108270 points

    HI Zeming,

    If you use the same loop filter as shown in EVM user's guide, you don't need to change this filter. The op-amp between the loop filter and the VCXO is useful to reduce current leakage in VCXO and as a result help reduce frequency drift problem. In addition, different VCXO from different vendor will have different leakage performance. Try different VCXO to find out which VCXO return less leakage. Use tighten frequency accuracy and smaller tuning range VCXO also help improve frequency drift.

    LMH1983 should has the same holdover performance as LMH1982.