100Mbps Ethernet LVDS Transceiver Chip/Eval Board for Media Converter Prototype

[Saturn929]

Hi everyone,

I have an in-house built optical transmitter (LED) and ROSA (PIN & TIA) that we are developing as our optical link. I would like to demo this link by building a prototype 100Mbps Ethernet - Optical and Optical - 100Mbps Ethernet media converter.

In other words, I have a converter on one end converting from 100Base-Tx to optical and another box on the other end converting Optical - 100Base-Tx.

My question is, what kind of 100Mbps Ethernet Transceiver chip is suitable for this purpose based on my transmitter and receiver specification below.

Transmitter - Input Signal : Works best with LVDS. But it will be able to take low voltage differential in the region of 200mVpp to 500 mVpp differential. The common move voltage is not important as I will AC-couple the signal before entering the device.
UPDATE: since this device is basically an LED, even a current input signal is compatible where 2.5mApp to 3.5mApp single-ended would work best.

Receiver - Output signal is SLVS, which is Vcm = 200mV and Vswing = 200mV (single ended). I will use the output in differential mode
My Tx-Rx link can achieve up to 700Mbps using LVDS input signal to my transmitter (I used TI DS15BR400). However, the target of this demo is only at 100Mbps using 100Base-T configuration. Based on my transmitter requirement and receiver output, I think it works best if there is an Ethernet - LVDS transceiver chip, but I'm not sure if such a thing exists.

I realize that there are many 100Base-T to SFP media converters out there. However, our current transceiver product is still in prototype form and are not in the SFP module form. Thus, I'm looking to purchase the transceiver chip (or its Eval board) and try to tap the signal direct to my transmitter, and also take my receiver's signal to the chip for media conversion.

Thanks in advance!

 

[FvM]

A number of 10/100 MBit Ethernet PHY has an additional 100BASE-FX interface, provided as NRZ differential PECL port. It can be easily adapted to LVDS.Just search for "100BASE-FX PHY".

 

[Saturn929]

I have thought of something simpler.

In a typical media converter where there are RJ45 and LC ports and SFP transceiver is used, forgetting the complicated signal conversion going on inside it, the SFP transmitter must be driven by a differential voltage which comes from 'something'. And also, conversely, the SFP receiver output voltage will be processed by that 'something' back to Ethernet signal.

Do you know what type of chip that 'something' is and what is the typical VIH, VIL, VOH, and VOL of it? I understand that different media converters might use different chip and have different input/output specifications. I just would like to know what is the generic name of such a chip.

Thanks.

 

[FvM]

-FX and -TX use different PMD layers. A simple way to make a converter between both media is to use two back-to-back connected PHYs, see e.g. **broken link removed** (Media Converter Operation pg.22f).

- - - Updated - - -

The -TX specific operation are specified in ANSI X3.263-1995, essentially a pseudo-random scrambler and MLT-3 (three-level) coding. I didn't look into the details, but the scrambler needs synchronization, so I guess you'll have difficulties to implement a standalone scrambler without decoding 4B/5B symbols to determine the protocol state.

 

[Saturn929]

Hi FvM,

Thanks for your feedback.

Actually, I'm not converting -TX to -FX or any of the 100BASe-X standards. I'm simply looking to find a type of Ethernet PHY that can convert the 100Mbps Ethernet signal into something that can drives my LED, which would work best with signals with voltage swing of 200mVpp to 500mVpp, such as LVDS.

At the same time, the Ethernet PHY must be compatible with the output of my receiver, which is an SLVS signal.

 

[FvM]

Suitable level conversion is the lesser problem I presume, but protocol-wise you need exactly what the said converter solution does. The "FX" output on the converter is still an electrical signal, ready to drive an optical transmitter.

If you don't need to be compatible with the 100BASE-FX standard, you might transmit a scrambled -TX bit stream, but still need to encode/decode NRZ to/from MLT-3. In addition, a standard -FX PHY will also process the signal detect output of an optical receiver, bit it may not be required in your application.

I still believe that the converter solution is a simple and straightforward, if you want -FX compatibility or not.

 

[Saturn929]

Hi FvM,

I'm quite new to this Ethernet thing so I hope you could bear with me.

So referring to the Micrel PHY you mentioned earlier, are you saying that for my intents and purposes, can simply use pins 40 and 41 (Tx- and Tx+) to drive my transmitter? And also, use pins 32 and 33 (Rx- and Rx+) for my receiver output? Am I correct to say that the specified TTL inputs is for my pin 32 and 33, and the specified TTL outputs are the outputs of pins 40 and 41?

I noticed that this PHY (and also other PHY from TI, Broadway that I found) are mostly to interface with LVTTL. For example, the VOH and VOL are (TTL outputs) are 2.4V and 0.4V respectively. However, LVTTL works by having 4mA and -4mA current source for differential signals (correct me if I'm wrong) which I can tap straight away into my transmitter without having to use any LVTTL receiver.

However, on the receiver side, the VIH and VIL are 2V and 0.8V which in no way can be compatible with my receiver output of SLVS. In there a different PHY that actually interfaces to LVDS/SLVS instead of LVTTL? If there isn't, do you have an idea on how I can use my receiver output with an Ethernet PHY?

Thank you so much.

- - - Updated - - -

I think I got it wrong.

The TTL inputs are outputs are simply the IN/OUT ethernet signals. So which PIN actually provides the signal to power an optical module after conversion from TTL?

 

[FvM]

The Micrel PHY is just one example of chips with an -FX transceiver connect option. Some have dedicated pins for it, the Micrel chip is apparently using the twisted pair port. In any case, the ports are differential (usually implementing the PECL standard) and are more or less LVDS compatible. You have to refer to the PHY documentation for details about signal levels and required external termination.

 

[Saturn929]

Hi FvM,

After some searching, I found this particular product from Vitesse, which even has an eval board.

**broken link removed**

It looks to be able to do what I want, by connecting the TDN/TDP and RDN/RDP pins to a fiber optic transceiver, which in my case, I would design them with an SFP interface.

I'd like to purchase the eval board to start tinkering with it and potentially use it for a demo. But I'm really not certain that I know for sure what I'm doing here is correct. Can you give your opinions? I still do not understand certains things like SGMII vs Serdes. It seems to offer both, but am I able to tap into the SGMII signal?