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LMH0318: is not reclocking

Simon Brown
Intellectual 340 points
Part Number: LMH0318

I have a cable equalizer driving the LMH0318.  The signal coming into the cable equalizer is SD-SDI and it drives out at 100R differential, just what the LMH0318 needs.

If I look at the output eye pattern from the LMH0318 it is WORSE than the signal being fed to it.  It is clearly not managing to do the job it says it can.  Register programming is done as per the datasheet and SNLU183, and I am reading back lock status, LOS status and datarate.  The datarate is reported as 0x20 which is correct for 270Mbps, the lock status is 0x58, so it is locked, the LOS is read as 0x02 which implies input 1 (the one I'm using) has a signal.

This is my initialisation:

raw318 (0xff, 0x04, 0x07); // Select channel registers
raw318 (0x16, 0x25, 0xff); // Enable full temperature range
raw318 (0x3E, 0x00, 0x80); // Init CDR State machine control
raw318 (0x55, 0x02, 0x02); // Init CDR state machine control
raw318 (0x6A, 0x00, 0xff); // Init CDR state machine control
raw318 (0x03, 0x00, 0xff); // Setup CTLE - this is for 10 inch FR-4
raw318 (0x0A, 0x0C, 0x0C); // Reset the CDR
raw318 (0x0A, 0x00, 0x0c); // Release CDR reset
raw318 (0x31, 0x01, 0x03); // Enable IN0->OUT0, power down OUT1
raw318 (0x2F, 0x00, 0xC0); // Enable SMPTE modes.
raw318 (0x0A, 0x0C, 0x0C); // Reset the CDR
raw318 (0x0A, 0x00, 0x0c); // Release CDR reset

The communication is being done by i2c and the status registers mentioned above are reading back appropriately.  I've also tried reading the HEO and VEO, but appreciate that that isn't valid in SD-SDI.

Regards,

Simon Brown

  • 0
    TI__Guru* 93670 points

    Hi Simon,

    The fact you are reading 0x02 = 0x58 this means device is locked and we must get a good eye opening at the output. However, with a bad 75-ohm eye opening and device locked, this means there could be perhaps a poor return loss.

    1). Can you please read reg 0x02 multiple times - 10-20 times - to make sure device is locked all the time(i.e. it is not intermittent).

    2). What happens if you take 75-ohm output to a video analyzer. Is video ok?

    3). Instead of SD, can you try HD or 3G to see if you can get a decent HEO/VEO?

    4). Have you done return loss measurement? Result looks ok?

    Also, if you like you can send me your schematic for review as well(Nasser.mohammadi@ti.com).

    Regards,,nasser

  • 0 in reply to Nasser Mohammadi
    Intellectual 340 points

    Hi Nasser,

    Many thanks for your reply.  I have repeatedly read register 0x02 and get 0x58 consistently.  The 75R output to a video analyser shows the video is ok, but the output to a scope shows a very poor eye.  If I look at the signal on the scope before the reclocker I get a much cleaner eye pattern.

    I have tried HD and my HEO and VEO measurements are roughly 55 and 64 respectively.  For SD they're <30 and 64.

    I've not done a return loss measurement, but this isn't the first SDI circuit I've laid out.  I'll send you pictures of the eye patterns before and after the reclocker, and the schematic by email.

    Regards,

    Simon

  • 0 in reply to Simon Brown
    TI__Guru* 93670 points

    Hi Simon,

    Judging your HEO/VEO measurements, it means we are getting 0.8UI(55/64) of horizontal eye opening while we have 200mV(64X3.125). I was expecting to see VEO at least 400mV or higher. Have you changed reg 0x03 content to see if you can get a higher VEO value? I just received your email as well and will respond shortly.

    Regards,,nasser