[SI-LIST] Re: CML versus ECL/PECL

CML is often said to be for point-to-point signaling only but RAMBUS uses
this for multipoint signaling to memory devices. RAMBUS uses a 42 ohm t-line
that is loaded to 28 ohms by the capacitance of the devices hung on it.
The speed limitation should be set by the figure of merit of the TX-RX. That
is, low device capacitance and high drive current will give faster
transitions. RAMBUS uses N-ch MOS devices and so does not have the figure of
merit of a bipolar device.

    Dave Cuthbert
    Consultant

On Dec 27, 2007 11:46 AM, Srivats Partha <vats123@xxxxxxxxx> wrote:

> Hello Wolfgang
> My two cents on the advantages of CML. The first being for high speed
> designs the fundamental limitation is the load capacitance, It is the the
> load capacitance that determines the switching speed. In the case of CML the
> load capacitances are charged or discharged depending on the signal
> swing,thus enabling faster switching. I assume your signal swings are
> extremely small at high speeds. The other advantage I found with CML is that
>  since we have a  NMOS current sink providing constant current  ,
> switching noise is greatly reduced, thus helping with the speed of
> operation.
> Regards
> Srivats
>
> --
> P.Srivatsan
>
> --
> I have no special talents. I am only passionately curious. AE
>
> ----- Original Message ----
> From: "wolfgang.maichen@xxxxxxxxxxxx" <wolfgang.maichen@xxxxxxxxxxxx>
> To: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
> Sent: Thursday, December 20, 2007 2:13:29 PM
> Subject: [SI-LIST] CML versus ECL/PECL
>
>
> Here is a question to all design experts: I have been wondering for a
> while what the design advantages (or disadvantages) of CML signaling
>  vs.
> ECL/PECL signaling are. I can think of a few, but I may be wrong, and
>  I'm
> sure there are more considerations:
> - both schemes are typically differential, so no advantage here
>
> - I noticed most high-speed drivers use CML; I guess this is because it
>
> can easily provide double termination (50 Ohm internal pullup at driver
>
> and 50 Ohm pullup at receiver) which is important for signal integrity
>  at
> 10+ Gb/s
>
> - but I guess that combined 25 load means CML is typically more power
> hungry
>
> - ECL/PECL needs a third voltage (Vcc - 2V) for end termination while
>  CML
> can usually be terminated to Vcc. Of course I can use a approx. 150 Ohm
>
> pulldown to GND for the driver (assuming 3.3V PECL) to get around this,
>
> and add a series resistor at the driver and bridge termination at the
> receiver, but that makes for a smaller swing - not good.
>
> - CML can driver larger swings, while ECL/PECL is normally restricted
>  to
> standard values (0.4V or 0.8V swing, respectively).
>
> I'm looking forward to more insight on this.
>
> Regards,
>
> Wolfgang
>
>
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