ONET1101L: The input and output waveforms of onet1101L do not match

Hi,

Due to the US Christmas and New Year holiday, the response to your question will be delayed. We have assigned an engineer to this E2E thread and will look into your question when we return. Sorry for the wait and any inconvenience it may cause.

Thanks,

David

Hi Adah,

Apologies for the delay, I just got back from vacation.

Has there been any further debug on your side on this since your original post?

A couple questions related to debugging this:

• Is it possible to share your schematic? You can share it with me over E2E private message if you'd like to keep it off the public forum.
• Have you tried testing with a DC balanced pattern like 1010 or 5 GHz sine/square wave?

Thanks,

Drew

Hi Adah,

I didn't receive an email from you.  Are you able to follow up on E2E?

Thanks,
Drew

Hi Drew,
Happy New Year!

I made two versions of PCBs, one with a single ended signal output impedance of 25 ohms and the other with a 50 ohm impedance; The previous consultation image was based on a 50 ohm output impedance, and the schematic provided below is also based on this version.Please refer to the attachment for the drawings.
My application is to output high-frequency and extremely narrow pulses.
My purpose is to trigger the output of narrow light pulses from laser diodes, and the following phenomena have been observed through testing:
1. 25 ohm version: The higher the input pulse frequency, the worse the waveform shape, but the peak output is roughly the same as the actual input waveform pulse width (which may also be caused by output waveform deformation), which can make the LD output narrow light pulses;
2. 50 ohm version: The higher the input frequency, the better the waveform shape, but there may be a situation where the output waveform has twice the pulse width of the input waveform; The actual excitation of LD will also widen the pulse width of the light pulse;
3. The R86 (49.9R) in the drawing is used to match the impedance of the LD internal resistance line, but in reality, after adding R86, the laser diode cannot respond to pulses normally. After removing it, there is no problem;
I hope you can help analyze the cause of the problem based on the above phenomenon.
In addition to the previously raised questions, there is an additional question I would like to consult:
1. Do you have any better suggestions for chip usage or recommend a more suitable chip for my application.

We look forward to your reply

Thank,

Adah

Hi Adah,

Happy New Year!

Thanks for sharing details on project.  I'm working on coming up with some suggestions to help improve the performance in your application.

I'd like to better understand your application.  I have some questions about your schematic:

1. What is the purpose and value of R61/R73?

2. Are you able to share details on how you determined values for R80/R81/R82/R83?

3. Are you able to share which laser diode you are using and any documentation on it?

Adah-Zhao said:

My purpose is to trigger the output of narrow light pulses from laser diodes

Regarding this, will you be triggering output pulses periodically?  The signal chain of this device is AC coupled, which typically relies on a DC balanced signal to work correctly.  I'd like to better understand more details on the signals will you be sending.  Are you able to share more details on the pulses you'd like to generate from the laser?  I'd like to understand if they will occur periodically, or are non-periodic.  Is there a minimum/maximum pulse width?

Also, have you configured the ONET1101L to operate in open loop control mode?

Thanks,

Drew

Drew,

I replied to your private message. Have you received it? I want to make sure that I can communicate with you normally in this way.

Thanks,

Adah

Hi Adah,

Yes, I received your private message.  Apologies for the delayed response.  I'm working on reviewing the information you sent and will get back to you shortly.

Thanks,

Drew

Hi Drew,

I haven't received your reply for about a week.Have you made any progress here?How much longer do you think it will take?

Thanks,

Adah

Hi Drew,

May I ask if you can still receive my messages or have you replied to my messages recently?

Thanks,

Adah

Hi Adah,

Apologies for the delay.  Do you observe abnormal behavior using a DC balanced pattern?  Trying to understand if this is strictly correlated with pulse input.

Thanks,

Drew

Hi Drew,

Thank you for your prompt. I am trying to change the input signal from AC coupling to DC signal. The AC coupling capacitor on the output side has not been changed. The test results show that when the DC signal is directly input, the output signal pulse width is consistent with the input signal and does not change with frequency.
I would like to ask you, 1. Why did this happen? Is direct input of DC signal feasible?
2. In the message I sent you, it was mentioned that the usage frequency ranges from mid frequency to high frequency. Can this modification meet the usage requirements?
Looking forward to your prompt reply.

Thanks,

Adah

Hi Adah,

1. Without further investigation, I'm not certain exactly why this occurred, although it seems related to non-DC balanced signal.
   • If you send DC balanced signal with narrow pulses, do you observe good output?  Just curious if pulse width is a factor.
   • Directly inputting a DC signal is possible, although you will want to match the DC bias of the device.
   • Your observations may also be related to DC offset compensation in the device.  You can try disabling this.
2. Given that this device supports up to 11.3 Gbps, I believe it will support both mid and high frequency usage.

Thanks,

Drew

Hi Drew,

Based on your suggestion, I have conducted the following verification:
1. Sending a narrow pulse DC signal, the phenomenon is that the amplitude of the output current signal (LD direct terminal) decreases, which may be due to measurement reasons, but this phenomenon has been observed so far;
2. Under the condition of AC coupling input signal, the part of the register related to DC offset compensation was modified, and this function was disabled, but the output signal did not improve;
3. Under DC signal, there is no significant deterioration in the quality of medium and low frequency pulse signals; Under the condition of AC coupled input signals, the quality of mid low frequency pulse signals significantly deteriorates;
Do we need more experiments to confirm if this method works properly?
Additionally, additional issues were identified during testing;
1. In the schematic diagram, resistor R86 next to the AC coupling output capacitor is an impedance matching resistor (49.9R), designed to match the impedance difference between the LD internal resistance and the cable. But the presence of this resistor will cause a decrease in the amplitude of the output current signal and prevent the normal excitation of LD pulses to light up. But after removing this resistor, the input pulse waveform will generate a certain degree of overshoot. Is the inability to excite the pulse due to the current limitation of this resistor? How should the placement of this resistor be modified? Or is there a better solution for impedance matching of this requirement.

Looking forward to your prompt reply.

Thanks,

Adah

Additional clarification:

1.Regarding the above (1,),after repeated testing and making the same changes, it was found that the consistency between several circuit boards seems to be poor. The attenuated output current amplitude presents different phenomena for lighting the same LD. Some circuits result in low light pulse amplitude output by the LD, while others have no significant effect.

2.The PCB version that has been tested has an output impedance of 25 ohms and did not use the impedance network recommended by TI. The output current pulse shape is severely deformed, and it is observed that the pulse width is basically consistent with the input pulse (due to the deformation of the pulse shape, it cannot be completely determined).

Hi Adah,

How are you controlling the bias current to your laser?  Are you using PD?

Thanks,

Drew

Hi Drew,

The bias current of the laser is operated according to the register setting method in the data manual, and the output bias current is less than the threshold current required by the LD, so that the driving current can excite narrower LD light pulses.

I did not use PD.

I guess the PCB board with current amplitude attenuation may have poor performance due to the driver chip (onet1101L) being lesioned. In previous tests, the onet1101L had poor output performance due to improper transportation, but the performance was restored after replacing the driver chip. Do you think it's possible?

Where should the resistor R86 be placed in the above consultation question? I hope to receive your advice?

Looking forward to your prompt reply.

Thanks,

Adah

Hi Adah,

Today (Feb 17) is a holiday in the US, so responses will be delayed. Drew will get back to you shortly regarding your question.

Best,

David

Hi Drew,

As of Feb 17 another week has passed and I still haven't heard back. Please reply as soon as possible.

Thanks,

Adah

Hi David,

As of Feb 17 another week has passed and I still haven't heard back. Please reply as soon as possible.

Thanks,

Adah

Hi Drew,

It has been a long time and this issue should be nearing completion. Could you please reply to my question as soon as possible.
One important issue is the use of R86 (49.9R) resistor for impedance matching of 50 Ω cable in the design of the drive output terminal. However, the placement of this position will result in a decrease in drive current, which will prevent the LD from outputting pulsed light. May I ask where it is more reasonable to place this impedance matching resistor?

Looking forward to your prompt reply.

Thanks,

Adah

Hi Adah,

Are you using a single ended cable between R86 and LD_CATHODE?  Are you able to share details on the cable?

Thanks,
Drew

Hi Drew,

Thank you very much for your reply！

There is a differential connection between R86 and LD_CTHODE, and the PCB layout is shown in the figure, where R85 is actually short circuited.

The current phenomenon is that R86 welds a 49.9R resistor with good signal quality and no obvious overshoot, but the pulse amplitude is low and the pulse driving current cannot trigger the LD; R86 short circuit processing may cause deformation of the pulse signal, but the pulse driving current can trigger the LD normally.
I hope you can assist in resolving this issue.

Looking forward to your prompt reply.

Thanks,

Adah

Hi Adah,

In a typical communications application, the laser bias current is set such that the laser is always emitting light.  Then the modulation current just changes the amount of light the laser outputs.

Is an implementation like this possible in your application?

Thanks,

Drew

Hi Drew,

Yes, it is also the phenomenon you mentioned in my application.
If R86 will cause a decrease in the amplitude of the driving current pulse, where should a cable impedance matching resistor be added?

Looking forward to your prompt reply！

Thanks,

Adah.