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Questions about ONET1151L

Other Parts Discussed in Thread: ONET1151L

Hi,

I'm trying to develop a laser driver circuit for fiber sensing application. I'm using pretty much the same circuit given in datasheet, figure 20.
 Here are some other details,

-Circuit is connected to an FPGA board through an FMC connector.

-Input DIN signal is LVDS, coming from FPGA (AC coupled- as in datasheet)

- Laser diode is RIO Planex (with custom TEC driver)

- Detector is SIR5-FC

- Instead of two series coils, I used one with a value of 1500nH

-In bias path, I didn't use an coil, since laser itself already has one in package

I made one assumption that this device can work with lower speeds (thus longer pulses), since SDD11 is given from 0.01 to 11 Gbps.

What I see is that pulse amplitudes varies too much Please see the waveform. Bias register is set to 1, and modulation current is set to 500. I'm ruling out temperature change in laser, cause when I monitor thermistor voltage at package pin, I don't see any correlation. Event though, temperature shouldn't change this fast.

Do you have any idea what might be the problem(s)?

Thanks,

Murat

  • Hello Murat.

    It is difficult to say what the issue may be. One problem may be the signal type itself. The part was not designed or tested for puse mode operation. The part was designed for DC balanced continuos data inputs such as used for Ethernet and SONET. The issue may be related to the low repetition rate of the pulses.

    Regards,

    Alex.

     

  • Hi Alex,

    Thanks for your quick reply. I'm inclined to rule out the unbalanced codes and low rep rates. Please see the screenshot below. I'm using much faster rate, however, same problem is still there.

    I only chose ONET1151L, because it was in free samples program.  If you can suggest any other TI part better suited for my application, I'd be more than happy to hear it.


    Thanks,

    Murat

  • Murat,

    If that is the case then I believe the issue is related to the bias-Ts. For low data rate operation the inductors need to be fairly high value in the uH region (for example 47uH to 100uH). You said you only had a 1500nH inductor and no AC block on the bias connection because it was integrated with the laser. I'm sure this is not a large value inductor.

    This is why we recommend at least 2 inductors. Typically one is a ferrite bead (such as BLM15HG601SN1 from Murrata) to provide a high AC impedance across a wide frequecny range. The other is a high value inductor (such as GLFR1608T470M-LR from TDK)  to provide an AC block for low frequencies. I would experiment with this to see if it solves the issue.

    I do not know of other suitable parts from TI for pulsed laser applications.

    Regards,

    Alex. 

  • Alex,

    I ordered some wideband bias chokes from coilcraft. Let's see how much this will help.


    Thanks for your help.

    Murat

  • Hi Murat,

    Do you have any feedback on the proposed solution from Alex. Did it solve the problem in your case?

    Regards,
    Christophe
  • Christophe,

    It didn't help much. So I implemented my driver using discrete RF transistors and some ADCs/DACs.

    Thanks,
    Murat