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DP83620 PHY: Fiber Optic mode signal levels?

Kaitav Sapre
Prodigy 70 points
Other Parts Discussed in Thread: DP83620

Hi,

We have developed a board using DP83620 Ethernet PHY and using it for Copper as well as Fiber Ethernet, one at a time. The question I am about to ask is with respect to Fiber Optic transceiver by Apac Opto which is interfaced with this PHY using the application schematic reference given in the TI's PHY datasheet.

The interface is working OK, but I want to know the acceptable/optimal levels of TX and RX signals.

We are using 3.3V for both DP83620 PHY and Optical transceiver power supply.

The following results were taken be referencing RX+/TX+ wrt GND, RX-/TX- wrt GND and then taking difference (CH1-CH2) of the two by using MATH function of the oscilloscope. I had enabled AC coupling mode on the oscilloscope for the measurements. I used two probes in single-ended mode.

TX+: 1.1Vpp

TX-: -1.1Vpp

TX+ - TX-: ~2.2Vpp

RX+: 1.9Vpp

RX-: -1.9Vpp

RX+ - RX-: ~3.8Vpp

Then, I took following measurements by putting probe directly across TX and RX pairs for "kind of" typical differential signal measurement.

TX pair: ~0.98Vpp

RX pair:  ~1.74Vpp

I referred the datasheet, but it only gives the following information:

VFXTD_100: 0.5V typ., 0.93V max.

Even the FX_SD voltage levels are not mentioned.

Now, I have two questions:

1) Are my measurements above proper?

2) What are the acceptable levels or ranges for TX and RX signals, Vpp both in single-ended and differential mode, when working with a Fiber Optic transceiver?

Again, the fiber optic Ethernet is working fine on our board, but I am a little skeptical about the levels.

It will be highly appreciable if someone can please help me with these questions.

Please let me know if anything is unclear in my explanation.

Thanks.

  • 0 in reply to Kaitav Sapre
    TI__Mastermind 19780 points
    Hi Kaitav,

    I am looking into this.

    Best Regards,
  • 0 in reply to Rob Rodrigues
    Prodigy 70 points

    Hi Rob,

    Okay, thanks for the consideration. Please let me know if you have any questions for me. I will be more than happy in assisting you with solving this question.

    Regards,
    Kaitav

  • 0 in reply to Kaitav Sapre
    TI__Mastermind 19780 points
    Hi Kaitav,

    Your single-ended measurements are OK and the differentially probed signals are what we see in the lab with our devices that are properly setup via the terminations suggested.

    The signals in and out of a fiber transceiver are typically LVPECL and as such you need the terminations you see in the diagram. The TX/RX pins on the PHY are specced to 2V single ended and as such, your signals are in spec.

    FX_SD pins can take an input up to ~1.5V single-ended but this signal hardly ever gives issue as it toggles much more infrequently than the TX/RX pins.

    Signal level compatibility is going to have to come from the datasheet of your specific FX transceiver. Your high RX swing is probably due to the FX outputting a maximum signal swing. Either way these numbers are acceptable.

    Best Regards,
  • 0 in reply to Rob Rodrigues
    Prodigy 70 points

    Hi Rob,

    Thanks a lot for your detailed reply. I would want to further clarify the questions.

    I am attaching the datasheet of Apac Opto FO transceiver (at the end of this post) we are using in our design. However, we have used the reference of application schematic given in DP83620 datasheet and the signal measurements I have posted above are with this termination design. 

    As per FO transceiver's datasheet, 

    1. Differential input voltage for TX is 0.4V to 2.0V. I am in this spec if I consider the differential measurement done directly across the TX pair, but not if I consider the single-ended measurements which you said are OK. 

    2. Differential output voltage for RX is 0.5V to 2.0V.  I am in this spec if I consider the differential measurement done directly across the RX pair, but not if I consider the single-ended measurements which you said are OK.

    3. Signal Detect Output voltage is somewhat tricky, in that since it is an LVPECL output it is given as:

    Voh - Vcc = -1.02V to -0.88V, which equates to Voh = 2.28V to 2.42V. I am getting 2.36V as high with respect to GND, which is in this limit.

    Vol - Vcc = -1.81V to -1.62V, which equates to  Vol = 1.41V to 1.68V. I am getting 1.60V as low with respect to GND, which is in this limit.

    However, Apac Opto has measured FX_SD with 10Kohm to ground, whereas is our design we have this signal connected to Vcc via 130R resistor and to ground via 82R5 resistor, as mentioned in TI PHY datasheet. 

    Are above FX_SD levels OK from TI PHY point of view?

    I would appreciate if you could please look into all the levels again and confirm that my measurements are OK and it won't cause issues with the DP83260 PHY in long run.

    Thanks & Regards,

    Kaitav Sapre

    Apac_LM34-A3S-PI-B-EC-V1_1_2014-06-24.pdf

  • 0 in reply to Kaitav Sapre
    Prodigy 70 points

    Hi Rob,

    I am awaiting your reply...

    Thanks,

    Kaitav Sapre

  • 0 in reply to Kaitav Sapre
    TI__Mastermind 19780 points

    Hi Kaitav,

    1. You are within the single-ended and differential voltage specs of the Apac FO as Vin is listed as acceptable up to Vcc in the abs max section.  That is the a single-ended measurement you have to consider.

    2. Your RX diff measurement is inside the spec and the single-ended measurement is also OK as the DP83620 accepts up to 2-V input single-ended.

    3. As you pointed out the SD line is trickier as we are interfacing two different interface standards.  The Apac schematic assumes interfacing with another LVPECL device which isn't valid with the DP83620's.  The SD input line follows the same parameters as the RD+/RD- pair with the exception that there is no differential signaling to be accounted for.  Your signals are within specification on this pin for the PHY.

    Best Regards,

    Rob

  • 0 in reply to Rob Rodrigues
    Prodigy 70 points
    Hi Rob,

    Okay, thanks for the detailed explanation. So I understand that all my voltage levels are acceptable for both DP83260 and FO transceiver.

    However, unlike what you mentioned, I am still not close to Absolute Maximum Ratings if I see the single ended of TX & RX, which are just under 2V at maximum of 1.9V.

    Thanks again for your time and answer.

    Regards,
    Kaitav Sapre