[SI-LIST] Re: LVPECL termination and offset crossing
- From: bala <balaseven@xxxxxxxxx>
- To: Lee <leeritchey@xxxxxxxxxxxxx>
- Date: Fri, 18 Oct 2013 00:23:43 +0530
Tom,
Yes you are right.i would also prefer Vcc-2 termination.But one of my
vendor told me to use either vcc-2 or internal 100 ohm.Anyway thanks for
your time and response.
Regards
bala
On Thu, Oct 17, 2013 at 11:50 PM, Lee <leeritchey@xxxxxxxxxxxxx> wrote:
> Most of the LVPECL I/O I see are really CMOS rather than open emitter
> bipolar transistors as was the original ECL. These new I/O may well have a
> pull down built into the outputs. Somehow this needs to be checked in
> order to get the circuits right.
>
>
>
> -----Original Message----- From: Tom Dagostino
> Sent: Thursday, October 17, 2013 10:55 AM
> To: 'bala'
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: LVPECL termination and offset crossing
>
> Content-Type: text/plain;
> charset="us-ascii"
> Content-Transfer-Encoding: 7bit
>
> Bala
>
>
>
> An ECL type output will not be biased correctly with only a 100 Ohm
> differential termination. Where will the currents flow? How are you
> biasing the output devices? In a steady state condition how much current
> is
> flowing in the emitters? I suspect you are getting any output at all out
> of
> this termination arrangement because of CdV/dT currents where C is the
> parasitics in the circuit. You need to bias the outputs with a standing
> current which means you need a resistance to Vcc - 2V or the equivalent.
>
>
>
> Tom Dagostino
>
>
>
> Teraspeed Labs
>
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>
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>
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>
> tom@xxxxxxxxxxxxx
>
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>
> From: bala [mailto:balaseven@xxxxxxxxx]
> Sent: Wednesday, October 16, 2013 11:54 PM
> To: tom@xxxxxxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] LVPECL termination and offset crossing
>
>
>
> Tom,
>
>
>
> 100ohm resistor connected between true and complementary signals.
>
>
>
> If i use thevenin,i am getting the below waveform(figure1)
>
> Inline image 1
>
>
>
> When i use 100 ohm parallel termination i am getting(figure2)
>
> Inline image 2
>
>
>
>
> Reference:download.intel.com/.**../Class2_10_11_12_**
> Differential_Signaling.ppt<http://download.intel.com/.../Class2_10_11_12_Differential_Signaling.ppt>
>
>
>
>
>
> Regards
>
> bala
>
>
>
>
>
>
>
> On Thu, Oct 17, 2013 at 2:53 AM, Tom Dagostino <tom@xxxxxxxxxxxxx> wrote:
>
> Bala
>
>
>
> Can you define 100 Ohm parallel termination and what you mean by crossing
> offset? And what part are you using? Pictures would be worth a 1000
> words.
>
>
>
> Tom Dagostino
>
>
>
> Teraspeed Labs
>
> 9999 SW Wilshire St.
>
> Suite 102
>
> Portland, OR 97225
>
> USA
>
>
>
> 971-279-5325 Office
>
> 971-279-5326 FAX
>
> 503-430-1065 Cell
>
>
>
> tom@xxxxxxxxxxxxx
>
> www.teraspeed.com <http://www.teraspeed.com/>
>
>
>
>
> Teraspeed Consulting Group LLC
>
> 16 Stormy Brook Road
>
> Falmouth, ME 04105
>
> 401-284-1827
>
>
>
> From: bala [mailto:balaseven@xxxxxxxxx]
> Sent: Wednesday, October 16, 2013 12:08 PM
> To: tom@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] LVPECL termination and offset crossing
>
>
>
> Tom,
>
>
>
> Yes its true LVPECL driver.I am talking about LVPECL to LVPECL topology.I
> prefer thevenin termination as its gives good output.However,some vendor
> suggest 100 ohm parallel termination instead of thevenin(which is strongly
> recommended for PECL buffer).My question is,why i am getting crossing
> offset
> for single ended waveform while using 100ohm,whereas,no offset while using
> thevenin termination.
>
>
>
> Orin,
>
>
>
> These are 100 ohm differential LVPECL signals.I am aware of usual
> termination techniques,but LVPECL demands some specific termination
> schemes..
>
>
>
> Regards
>
> bala
>
>
>
> On Wed, Oct 16, 2013 at 11:51 PM, Tom Dagostino <tom@xxxxxxxxxxxxx> wrote:
>
> Bala
>
> You are leaving out several key pieces of information. You call this a
> LVPECL driver. Is it truly LVPECL or is it a driver that will produce
> LVPECL levels but using an LVDS or other driver topology set up for LVPECL
> levels?
>
> If it is truly an LVPECL driver you need to have enough standing current in
> the output stage to bias the transistors with enough current to operate at
> their designed current. Too little current will slow the output down. Too
> much current can also slow the output down and dissipate too much power.
> The recommended 50 Ohms to Vcc - 2V provides this current. But Thevenin
> equivalent circuits will work just as fine.
>
> If this is one of the pseudo LVPECL drivers using an LVDS like driver then
> treat it like a LVDS driver and terminate it with a 100 Ohm floating
> differential load or with two 50 Ohm loads tied to Vtt for this driver.
>
> If there is some other driver topology read the manual for the device,
> understand where the currents need to flow and design the appropriate
> termination. Some of these look like CMOS totem pole outputs and will need
> a termination to a Vtt voltage.
>
> Good luck
>
> Tom Dagostino
>
> Teraspeed Labs
> 9999 SW Wilshire St.
> Suite 102
> Portland, OR 97225
> USA
>
> 971-279-5325 Office
> 971-279-5326 FAX
> 503-430-1065 Cell
>
> tom@xxxxxxxxxxxxx
> www.teraspeed.com
>
> Teraspeed Consulting Group LLC
> 16 Stormy Brook Road
> Falmouth, ME 04105
> 401-284-1827
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@**freelists.org<si-list-bounce@xxxxxxxxxxxxx>]
> On
> Behalf Of bala
> Sent: Wednesday, October 16, 2013 7:00 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] LVPECL termination and offset crossing
>
> Hi all,
> I have some questions on the LVPECL termination techniques.DC path to
> ground
> is needed to enable my output transistor irrespective of my receiver logic.
> So I am adding 150ohm resistor to ground at driver side .Every vendor
> suggest different termination scheme for receiver side, some vendor
> recommend thevenin termination and some vendor suggest 50 ohm to Vcc-2.I
> understand that to avoid the addition of new voltage scheme for Vcc-2,we
> are
> using thevenin termination scheme. Some vendors suggest 100 ohm parallel
> termination. I did simulate 156Mhz differential LEPECL which has unbalanced
> differential choke on each signal, with two different termination scheme,
> one with 100ohm, another one with thevenin termination .Differential
> results
> looks good for both cases, whereas ,my single ended waveform shows
> 'crossing
> offset' when I use 100ohm termination. I did the simulation again with
> thevein termination and crossings are reasonable. Why we are getting
> 'offset
> crossing' while using 100ohm termination?
>
> --
> bala
>
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> --
> bala
>
>
>
>
>
>
>
> --
> bala
>
>
>
>
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--
bala
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