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From: keithw86 on 7 Jun 2010 15:44
On Jun 7, 2:34 pm, Paul Keinanen <keina...(a)sci.fi> wrote:
> On Mon, 7 Jun 2010 05:21:49 -0700 (PDT), "keith...(a)gmail.com"
>
>
>
> <keith...(a)gmail.com> wrote:
> >On Jun 6, 12:39 am, Paul Keinanen <keina...(a)sci.fi> wrote:
> >> On Sat, 05 Jun 2010 20:53:02 -0400, Phil Hobbs
>
> >> <pcdhSpamMeSensel...(a)electrooptical.net> wrote:
> >> >Paul Keinanen wrote:
> >> >> On Fri, 04 Jun 2010 23:48:27 -0400, Phil Hobbs
> >> >> <pcdhSpamMeSensel...(a)electrooptical.net> wrote:
>
> >> >>> JosephKK wrote:
>
> >> >>>> Naw, 80 GHz (U)LVPECL 8-bitters and maybe 12 or 16 bitters. Single 1.5 V
> >> >>>> supply.
> >> >>> In GaAs? Don't think so. Just driving the wires at that speed would
> >> >>> take insane amounts of power.
>
> >> >> Why would driving a 50 ohm transmission line require a huge amount of
> >> >> power ? On the receiver side, how many bits would be required to
> >> >> _reliably_ detect if 0 or 1 is sent ?
>
> >> >> Assuming -174 dBm/Hz thermal noise density at room temperature, at 80
> >> >> GHz bandwidth, the thermal noise power would be -65 dBm and assuming a
> >> >> few dB extra required for binary detection, we are still talking about
> >> >> a few nanowatts at the receiver end.
>
> >> >> Of course at these frequencies, the transmission line skin effect and
> >> >> dielectric losses on a PCB would be considerable, requiring a high
> >> >> transmitter power and hence limiting the transfer distance.
>
> >> >> At such high frequencies, a low loss waveguide would have nearly
> >> >> manageable dimensions for "long distance" communication across the
> >> >> PCB:-).
>
> >> >Lines on ICs aren't 50 ohms, they're all RC.
>
> >> When the speed goes up, the physical distances must be reduced, in
> >> which a single synchronous clock can be used and the logic considered
> >> by simple RC model.
>
> >> In the old days a complete 19" box might considered a single entity
> >> clocked by a central clock and the interconnections analyzed as RC
> >> circuits. The interconnection between the boxes was handled with
> >> serial or parallel transmission lines driven by proper line drivers
> >> and receivers.
>
> >> Later on a single card was a self contained unit with transmission
> >> line communication through the backplane.
>
> >> These days the interconnections between ICs on a PCB are often
> >> transmission lines.
>
> >> For even greater speeds, physically small sections within a single IC
> >> chip must be considered as independent entities, interconnected
> >> asynchronous transmission lines to transfer data between entities. The
> >> popularity of multicore processors is a clear indication of this
> >> trend.
>
> >> >There are millions of
> >> >them, so even with 200 mV swings you'd be talking about 400 watts per
> >> >million wires. Lava city.
>
> >> On an independent entity, much less than 1 mm in size, what forces
> >> using such huge voltage swing ?
>
> >> At lower speeds with unbalanced logic, the ground bounce will finally
> >> eat the noise margin. How about some ECL style gates with true and
> >> complement outputs, the ground potential fluctuations would not be
> >> significant, thus reducing the required voltage swing and hence power
> >> dissipation ?
>
> >> >Not to mention that the long lines all have
> >> >repeaters to preserve the bandwidth, which multiplies the power dissipation.
>
> >> How many decibels/mm are the losses on a transmission line on the
> >> chip?
>
> >Repeaters aren't used because of loss.  They're used because RC is too
> >high.  The delay of a line is ~ the square of its length.  At some
> >point a gate delay becomes less than the difference between (2l)^2 and
> >2l+gate.  I've seen lines with four repeaters.  Major work was done to
> >get the tools to just use inverters when there were an even number of
> >repeaters; even larger gain.
>
> I think that we disagree about what is intraunit and what is interunit
> communication.

What _are_ you talking about?

> In RF design, the old rule of thumb is that anything longer than about
> lambda/10 should be treated as a transmission line (in fact lambda/4
> is an impedance inverter).

We're not talking about RF design. We're not talking about lossless
transmission lines.

> In the old days, the lamda/10 limit was not an issue, as long as the
> equipment was in the same room. However, if you want to operate with
> 80 GHz clocks (as this thread started), you really have to keep the
> synchronous clock area about the size of the dot on paper at the end
> of this sentence.

Irrelevant. You're not going to get millions of transistors switching
in that area.

> At such frequencies, communication between the "dots" must be analyzed
> as transmission lines.

You confuse LC and RC transmission lines.
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