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LMH0346: LOCKDETECT

Geoffroy CHANQUER
Prodigy 10 points
Part Number: LMH0346
Other Parts Discussed in Thread: LMH0344

Hi,

 

Context:

Our design is able to regenerated 4 distorted and Noisy HD-SDI video signal. In order to achieve this purpose, each channel use:

- 1 input connector (MCX 75Ω - RADIALL - R213424800);

- 1 Equalizer (TEXAS - LMH0344SQ/NOPB);

- 1 Recklocker (TEXAS - LMH0346SQ/NOPB);

- 1 driver (LMH0303SQ/NOPB);

- 1 output connector (MCX 75Ω - RADIALL - R213424800).

To deal with Build In Test requirement, we wanted to use the LMH0346's LOCKDETECT signal. For this reason, it is very important that this function is fully operationnal.

 

My problem:

When no input signal is present the LMH0346's Phase Loop Lock should be “unlock” and, consequently, LOCKDETECT signal should remain "Low". But in some channel, the LockDetect is not stable... On the unstable channel, the LOCKDETECT signal stay low during long period but get some oscillation (some High state occurs). Progressively, high state become longer and longer to finally remain High (about 1 minute after).

 

More information:

- Except LMH0346’s LOCKDETECT signal, the regenerated function works perfectly well.

- During my investigation, I use 3 different electronics boards. For each board, the problem occurs on different channels (Board#1 => Channel#1 Board#2 => Channel #1 Board#3 => Channel#3&#4).

- I think the loop signal is very sensitive because the problem disappear just by touching the 56nF Loop Filter capacitor.

- I already study http://e2e.ti.com/support/interface/f/138/t/556985?tisearch=e2e-sitesearch&keymatch=LMH0346

 

My question:

- Have you already seen this kind of problem ?

- Could you help me to solve my trouble on the LMH0346's LOCKDETECT signal ?

 

Best regards,

  • 0
    TI__Guru* 93670 points
    Hi Geoffroy,
    1). Actually LMH0346 is widely used and this is a good solid part. Touching loop filter cap, adds parasitic and thus behavior of the VCO changes. Normally, on pcb layout, we void ground or VCC under the loop filter cap just to reduce any parasitic or noise coupling. Also, we try to isolate traces away from loop filter cap just to make sure it is as quiet as possible.
    2). One question, I am assuming you are DC coupling between LMH0344 output and LMH0346 input. Correct?
    a). Is it possible to put high speed/high impedance scope probe on input of the LMH0346 to see if there is any activity?
    b). When this issue occurs, assuming DC coupling between LMH0344 output and LMH0346 input, can you force MUTE active on LMH0344 to see if this problem goes away?
    I am thinking perhaps there could be activity on LMH0344 output and this causes LMH0346 to show intermittent lock. LMH0346 should have some activity on it's input so it can attempt to lock. Without any input signal, signal detect on LMH0346 would not get activated and thus CDR would not lock.
    Regards,,nasser