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HCM / LCM signalling

 
 
Geronimo Stempovski
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      02-07-2007
I just read an interesting paper about high-speed I/O's power dissipation.
Unfortunately there is an equation I don't quite understand. Maybe someone
is in the mood for discussing and explaining the correctness of the equation
to me.
The formula I am talking about is (1) in the paper [
http://www.ee.ucla.edu/faculty/paper...s2_nov2006.pdf ]

For high-common mode signaling (which standard would that be, anyway? TTL?
CMOS? SSTL?) it is assumed
P = V*Vswing/Z0 = V*Vrx/Z0*H(f)

For low-common mode signaling (LVDS? CML? LVPECL?) it states
P = Vswing^2/2*Z0 = Vrx^2/2*Z0*H(f)^2

What I don't understand is the factor 2 (2*Z0) in the calculation of the
low-common mode signaling. Furthermore I'm not sure if the H(f)^2 is
correct.

Any help is highly appreciated! Thanks a lot in advance!

Regards, Gero


 
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Jon Slaughter
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      02-07-2007

"Geronimo Stempovski" <(E-Mail Removed)> wrote in message
news:45c9973a$0$30327$(E-Mail Removed)-online.net...
>I just read an interesting paper about high-speed I/O's power dissipation.
> Unfortunately there is an equation I don't quite understand. Maybe someone
> is in the mood for discussing and explaining the correctness of the
> equation to me.
> The formula I am talking about is (1) in the paper [
> http://www.ee.ucla.edu/faculty/paper...s2_nov2006.pdf ]
>
> For high-common mode signaling (which standard would that be, anyway? TTL?
> CMOS? SSTL?) it is assumed
> P = V*Vswing/Z0 = V*Vrx/Z0*H(f)
>



If you look at fig. 3 they show cmos.

> For low-common mode signaling (LVDS? CML? LVPECL?) it states
> P = Vswing^2/2*Z0 = Vrx^2/2*Z0*H(f)^2
>
> What I don't understand is the factor 2 (2*Z0) in the calculation of the
> low-common mode signaling. Furthermore I'm not sure if the H(f)^2 is
> correct.
>


look at fig 3.

Notice that they say that for LCM the voltage is dependent on V_swing^2.
Then right above equations 1 they say that H(f) is the attenutation. So they
basicaly substitute the ration V_RX/H(f) = V_swing



> Any help is highly appreciated! Thanks a lot in advance!
>


Hope that helps,
Jon


 
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Geronimo Stempovski
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      02-07-2007

"Jon Slaughter" <(E-Mail Removed)> schrieb im Newsbeitrag
news:5pkyh.25331$(E-Mail Removed). net...
>
>
>> For low-common mode signaling (LVDS? CML? LVPECL?) it states
>> P = Vswing^2/2*Z0 = Vrx^2/2*Z0*H(f)^2
>>
>> What I don't understand is the factor 2 (2*Z0) in the calculation of the
>> low-common mode signaling. Furthermore I'm not sure if the H(f)^2 is
>> correct.
>>

>
> look at fig 3.
>
> Notice that they say that for LCM the voltage is dependent on V_swing^2.
> Then right above equations 1 they say that H(f) is the attenutation. So
> they basicaly substitute the ration V_RX/H(f) = V_swing
>


Thank you, Jon! That sure helps me in understanding where the H(f)^2 comes
from. But what about the 2*Z0 ? Why is it 2*Z0 and not just Z0 ? Maybe I
just can't see the wood for the trees...

Regards, Gero


 
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Jon Slaughter
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Posts: n/a
 
      02-07-2007

"Geronimo Stempovski" <(E-Mail Removed)> wrote in message
news:45c9e547$0$18836$(E-Mail Removed)-online.net...
>
> "Jon Slaughter" <(E-Mail Removed)> schrieb im Newsbeitrag
> news:5pkyh.25331$(E-Mail Removed). net...
>>
>>
>>> For low-common mode signaling (LVDS? CML? LVPECL?) it states
>>> P = Vswing^2/2*Z0 = Vrx^2/2*Z0*H(f)^2
>>>
>>> What I don't understand is the factor 2 (2*Z0) in the calculation of the
>>> low-common mode signaling. Furthermore I'm not sure if the H(f)^2 is
>>> correct.
>>>

>>
>> look at fig 3.
>>
>> Notice that they say that for LCM the voltage is dependent on V_swing^2.
>> Then right above equations 1 they say that H(f) is the attenutation. So
>> they basicaly substitute the ration V_RX/H(f) = V_swing
>>

>
> Thank you, Jon! That sure helps me in understanding where the H(f)^2 comes
> from. But what about the 2*Z0 ? Why is it 2*Z0 and not just Z0 ? Maybe I
> just can't see the wood for the trees...
>
>


Because there are twice as many transistors instead of one? To be honest I
really don't know ;/ He doesn't define what Z0 is so I'm only guessing. It
could also be due to the (V_swing/sqrt(2))^2 = V_swing^2/2 where it was some
rms like value. This seems to be the likely reason as he says the voltage
used is proportional to V_swing but its definately not clear IMO and not
written well also(IMO Ofcourse, it might be completely obvious to someone
else). He doesn't seem to define the terms well and seems to take it for
granted that you already know what he's talking about.

For example, he uses V_S in the fig but I never see him mention what V_S is.
I can only conclude that its V_swing.

I would expect that V_swing is peak to peak and so he uses the average in
the power equation but I could be entirely wrong here.

IMO it is a poorly written paper. It could just be me though.

Jon


 
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