DP83849IF: Media converter - CML or PECL conversion

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Replies: 6

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Part Number: DP83849IF

Hi,

I am using DP83849if chip as RJ45(PORTA) to Fiber(PORTB) media converter and Maxim's MAX3799( laser driver and limiting amplifier) for the fiber optics conversion. MAX3799 has differential CML input and output.

From the other thread, I understood that DP83849if has differential CML inputs and outputs for the fiber operation. 

Now the question is : 1. Should I follow the circuit for converting CML to PECL as shown in the schematic of the dp83849if evaluation kit ? or Should I connect the TD+- and RD+- pins without this?

2. Does signal detect input have to be connected for fiber optic operation? Any recommendation for connecting the LOS signal from MAX3799 to dp83849if?

Thanks. 

6 Replies

  • Hi Yash,

    Yes, you will need a termination scheme to connect CML pins of DP83849 to PECL fiber optic transceivers. Please note that some transceivers may not have internal AC coupling capacitors so you might have to add them on the RD pair as well. If the fiber transceiver that you are planning to use is also a PECL then you can use the same schematic. You can compare the datasheet of the transceiver that you plan to use with the transceiver that is in the evaluation kit. 

    It is generally recommended to use the signal detect pin of the PHY for reliable connection. If the MAX3799 SD is same as the one used on the evaluation kit then use can use the same termination.

    -Regards

    Aniruddha

  • In reply to Aniruddha Khadye:

    Hi Aniruddha,

    Thank you for your quick response.

    In my case, the fiber optic transceiver has CML inputs and outputs including SD. So my question is - Do I have to connect any termination resistor ?  

    Thanks in advance.

    Regards,

    Yash

  • In reply to Yash Naik:

    Hi,

    Can someone please answer my question above?

    Thanks

  • In reply to Yash Naik:

    Hi Yash,

    The termination scheme for the fiber optic transceiver is generally mentioned in the transceiver datasheet. If you look at the DP83849IFVS-EVK, the transceiver used there is the HFBR-5803. In the datasheet for HFBR-5803, there are termination resistor listed to be used for 5V and 3.3V operation. The MAX3799 datasheet does list the outputs as CML and does not show any termination requirement. I think you can remove the terminations if you have used any, please confirm this with the transceiver datasheet.

    -Regards

    Aniruddha

  • In reply to Aniruddha Khadye:

    Hi Aniruddha,

    Thank you for your reply.

    You are right, MAX3799 does not need termination resistor as it is internally biased. Thus, no external components are required. Only ac coupling capacitors are required.

    I made the termination for the DP83849IF as follows --> (TX+- pins[CML Driver]: 50 Ohm to 3.3V, RX+- pins[LVPECL receiver] : 82 Ohm to 3.3V and 130 Ohm to GND, 0.1 µF coupling capacitors in series with all 4 lines).

    With this termination the communication did not work. I noticed that the PORTB(100BASE-FX) does not transmit  through TXD+- differential signals. This is the reason for the failure.

    Therefore, I tried below setup as described in section 3.4.1(https://www.ti.com/lit/an/snla086b/snla086b.pdf) without firmware (no read & no write) to monitor the data on RXD lines for both PORTA and PORTB.  DP83849if is strapped for the media converter mode.

    I have noticed that the PORTA - RXD0_A to RXD3_A lines are always 0. These lines show no  pulses when I connect the RJ45 to PC. This means that the PORTB transmit pins are not transmitting.

    Could you please confirm if I can read in extender/media converter mode from PORTA's RXD0_A to RXD3_A when I connect an ethernet cable to a computer? If yes, why can not I see pulses on RXD lines for the PORTA?

    Thank you.

    Regards,

    Yash

  • In reply to Aniruddha Khadye:

    Hi Aniruddha,

    I sent the schematics to your colleague(Mr. Walter Ahrend) in Germany. I hope you have reviewed the schematic.

    My observation in my board as well as in the  EVM kit:---

    I've noticed that the PORTB, which acts as a 100BASE-FX, always sends 800mVp-p signal though the PORTA is not connected to the computer with an Ethernet cable.

    Is this normal with 100BASE-FX outputs? or Is this due to a wrong configuration?

    How can I verify if 100BASE-FX's transmit pins send data or not?

    Thank you