DP83848K: About 100 Base-TX UTP Differential Output Voltage test

Hi Tao,

To answer some of your questions:

1. DP82848K-MAU-EK is confirmed to pass the test.
2. Not having those decaps should not be a major issue, since you have the other decaps that are closer to the PHY.

I have a few questions of my own:

1. What is the RBIAS resistor value you are currently using?
2. Are you terminating the MDI lines with 100 ohms?
3. Can you provide a block diagram of your test setup?

Regards,

Adrian Kam

Hello Adrian,

Thank you for reply.

To answer some of your questions:

1. What is the RBIAS resistor value you are currently using?

(Ans.)

RBIAS resistor value is 4.87ohm. It isn't changed.

2. Are you terminating the MDI lines with 100 ohms?

(Ans.)

Yes. It is set to 100ohm on test fixture.

3. Can you provide a block diagram of your test setup?

(Ans.)

Please see attached file.

Regards,

Tao_2199

Hi Tao,

Is the RBIAS resistor within 1% tolerance? If not, I would suggest getting a resistor with that tolerance. Otherwise, I can also suggest adjusting the RBIAS resistor value to see if it improves.

Regards,

Adrian Kam

Hello　Adrian,

They use DP83848K-MAU-EK and the RBIAS(R24:4.87k) isn’t changed.

So, the RBIAS resistor is within 1% tolerance

I attached block diagram of test setup in my first question.

Could please give us some comment or advice about block diagram?

Regards,

Tao_2199

Hi Tao,

I do not see any issues with the block diagram.

Even though you are using the EVM, can you still try to adjust the RBIAS resistor value and see if there is a beneficial difference?

Regards,

Adrian Kam

Hello Adrian,

Thank you for reply.

About RBIAS resistor,

As you know, if RBIAS resistor(4.87k±1%) is exceed, IEEE standard can’t be met.

They need more high precision resistor to adjust within 1% precision.

Also, they referred below thread.

https://e2e.ti.com/support/interface/f/interface-forum/806498/question-about-dp83848/2984979?tisearch=e2e-sitesearch&keymatch=DP83848%252520rbias#2984979

So, they are hard to adjust RBIAS resistor.

They have some question.

1)Could you please tell possible reason why the Vout Differential Output Voltage test is fail on their test environment.?

For example;

・Difference in revision of DP83848 used for EVM

･Impedance mismatch of PCB pattern layout of EVM

･Difference in test environment between TI’s and my customer’s

2)About impedance mismatch of PCB pattern layout of EVM

They confirmed TD+/TD-(single end output) but it exceeds the ideal lower limit of amplitude (2.80V)

by about 60mV. (Please see attached file.)

Why does it exceed the ideal lower limit of amplitude? Could you please tell us possible reason?

They guess it is impedance mismatch of PCB pattern layout of EVM as possible reason.

Regards,

Tao_2199

【DP83848K-MAU-EK】Voltage swing of TD+ and TD-.pdf

Hi Tao,

1. About RBIAS resistor, the value that is suggested in the datasheet is used to meet IEEE standard. We usually inform customers not to change it to avoid issues. However, if you are failing a test even though the RBIAS resistor is the same as the suggested value, it would be ok to try and adjust it to see if it passes.
2. Let me consult with my team about exceeding the lower limit of amplitude issue and I can get back to you tomorrow.
3. As for why the test is failing, I may have overlooked something on the block diagram. It seems you are using a link partner for the test. How are you ensuring that the signal is coming from the DUT and not the link partner? If you are not sure, would you be able to try the test without the link partner?

Regards,

Adrian Kam

Hello　Adrian,

Thank you for reply.

(1)

>However, if you are failing a test even though the RBIAS resistor is the same as the suggested value,

>it would be ok to try and adjust it to see if it passes.

(Question)

If the test passed with changed RBIAS resistor value, is there no problem to use changed resistor value for their product？

(3)

(Answer)

They are terminating the MDI lines with 100ohm on test fixture as differential output voltage test.

Regards,

Tao_2199

Hello Adrian,

Additional question and answer.

(1)

(Question)

Or the changing value is use for only test and after test, RBIAS resistor will be restored value that is suggested in the datasheet(4.87k±1%).

Is there no problem about above?

(3)

(Answer)

They tested without Link Partner on test fixture section, but it was failed similarly.

Regards,

Tao_2199

Hi Tao,

I am looking into a couple of things regarding your issue. I will respond back by Tuesday at the latest.

Regards,

Adrian Kam

Hello Adrian,

Thank you for reply.

I will inform to my customer your comments.

As an additional question.

･They are using 16cm Short RJ45 cable(DUT- test fixture). (Please see below)

How many lengths did you use as Short RJ45 cable(DUT- test fixture) on your test?

Regards,

Tao_2199

Hello Adrian,

I have one more question.

>I would suggest either increasing the external termination resistors to lower the differential output voltage to pass the test,

About “increasing the external termination resistors”,

Do these mean the below resistors on TPDM/TPDP?

(For example, these are increased to 51ohm.)

Regards,

Tao_2199

Hi Tao,

1. On our test, we used a cable around 8 cm. Since your results are higher than the max limit, shortening the cable probably would not help.
2. Yes, those are the resistors I was referring to.

Regards,

Adrian Kam

Hello Adrian,

Thank you for good suggestion

My customer will try to test with increasing the external termination resistors and

has some question about them.

(1) DP83848K-MAU-EK(EVM) is designed 49.9Ω±1% as the external termination resistors.

Is there any concern or impacts to increase the external termination resistors as specification of PHY?

If no problem, why is there no problem to increase the external termination resistors?

When it increases to 51Ω, was there any problem as specification?

The increasing the external termination resistors or changing RBAIS resistor, Which one has less impact?

(2)When TI tested compliance test, was the external termination resistors changed?

Regards,

Tao_2199

Hi Tao,

1. There should be no issues increasing the external termination resistors. It should only affect the voltage levels on the MDI lines. The PHY's functionality should not be affected. From our experience, there were no issues when it comes to changing the external termination resistors. However, I have not tried specifically on the DP83848 increasing to 51 ohms. Overall, changing the external termination resistors should have less impact compared to changing the RBIAS resistor.
2. The resistors were not changed.

Regards,

Adrian Kam

Hello Adrian,

Thank you for comments.

They tested by changing the external termination resistors and confirmed to pass Differential Output Voltage test.

(All test item of compliance test was passed.)

From their test result, they will change resistor value to 45-47 ohm.

I have a qustion.

They will use AUTO-MDIX and need change RX side external termination resistor.

When RX side external termination resistor is change within the range IEEE specified (96ohm +-20%),

is there any impact on PHY specification?

(All test item of compliance test is passed.)

Regards,

Tao_2199

Hi Tao,

If you are using auto-mdix, then the external termination on the resistors on the RX side may need to be changed depending on the results of the differential output voltage test when the PHY is set to MDIX mode. I suspect you may have to make the same changes as the TX side, but I suggest you run the test in MDIX mode to see if those pins need the termination resistors changed.

Same with changing the external termination resistors on the TX side, it should not have any impact on PHY specification.

Regards,

Adrian Kam