RE: [SI-LIST] : Interesting theoretical questions

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From: Jian Zheng ([email protected])
Date: Mon Nov 01 1999 - 17:29:08 PST

Following are my answers to the questions (check keyword ANSWER in the
following attached context):

Jian-X. Zheng, Ph.D
Zeland Software, Inc., 39676 Mission Blvd., Fremont, CA 94539, U.S.A.
Tel: 510-797-8109, Fax: 510-797-8241, Web:
Special Announcement: (1) We will be offering a training class on using the
IE3D/FIDELITY on December 2 and 3 of 1999. Please check our web site for
more information. (2) IE3D 6.0 is released. The IE3D 6.0 will be about
50-300% faster than the IE3D 5.X. If you have the ZELAND.EXE 5.2, you will
be able to use the IE3D 6.0. (3) IE3D 7.0 beta should be released before
December 1999. It is a big step from IE3D 6.0 according to its optimization
capability. It has the implementation of symbolic optimization goals. It
also features integrated IE3D+CURVIEW+PATTERNVIEW for automated pattern
calculation and pattern optimization. Interested IE3D users can request the
IE3D 7.0B for a try after it is released.
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of subas
> Sent: Monday, November 01, 1999 4:24 PM
> To: '[email protected]'
> Subject: [SI-LIST] : Interesting theoretical questions
> Hi all,
> I have some theoretical questions regarding high frequency signals:
> 1) What is the maximum frequency you can generate electronically by our
> current technology (GaAs???)? Can we have an oscillator that can oscillate
> in the frequency of the visible light, for example?

ANSWER: For planar circuits (PCB or millimeter wave IC), from what I know,
people can produce circuits as high as 100-400 GHz. It is not easy to go to
much higher. The reason for it is that the frequency is limited by the
substrate thickness for the GaAs circuits. Some people may be using
substrate as thin as 25 microns. For 25 micron thick substrate, the maximum
frequency you can go is about 300 GHz. When the substrate gets thin, the
GaAs circuits become fragile and it is very hard to process. The 2nd problem
involved is the high metallic loss.

To go to higher frequency, people have to use optical waveguide such as
optical fibers.

> 2) The way light gets reflected from a metal (silver in a mirror)
> is that it
> first induces current (as a receiving antenna) and again radiates (as a
> transmitting antenna) the EM wave instantly so that we get the impression
> that the light is being reflected by the metal surface. Then how come
> different metals can have different colors (gold - yellow) when we expect
> them all to reflect light of all colors?

ANSWER: At low frequency (compared to the frequency of light), plane waves
hits the metal and will get total reflection from the metal. At high
frequency, it is not true. How big a reflection is depends upon the complex
permittivity (EPS):

EPS = EPSr - j SIGMA / ( OMEGA * EPS0 )

EPSr is the relative permittivity of the material. SIGMA is the conductivity
of the material. OMEGA is the angular frequency. EPS0 is the permittivity in
free space: (8.86e-12).

Typical copper's EPSr is about 1 and SIGMA = 5.8e7 s/m. At 1 MHz, we have,

EPS = 1 - j 5.8e7 / ( 2.0 * 3.14159 * 1.0e6 * 8.86e-12 )=1-j1.042e+12

The refraction coefficient N is:

N = SQRT(EPS) = 7.2e+5 - j 7.2e+5

When light hit the copper, the reflection coefficient is:

(1-N)/(1+N)and it is almost -1.

At light frequency, the wavelength is about 7.0e-6 m. The frequency should
be 4.0e+5 GHz (or 4.0e+14 Hz). Assuming the EPSr and SIGMA do not change, we
still have,

EPS = 1 - j 5.8e+7 / ( 2.0 * 3.14159 * 4.0e+14 * 8.86e-12) = 1-j2600.

N = SQRT(EPS) = 36 - j 36.

The reflection coefficient is still high. However, it is quite different
from -1. With increased frequency, normally the SIGMA should decrease. The
reflection from metal should be significantly lower than the total
reflection, and it will differ with different frequencies. I think that is
the reason why we will have differnt colors, at least from the
electromagnetic point of view.

> 3) Has there been any computer made without any clocks (asynchronous)? How
> fast is this computer?

ANSWER: I have no idea about it. However, it seems to me it is impossible.
For a computer without a clock, it has to be analog. When I was in school, I
learnt that there were analog computers. However, you can not guarantee the
accuracy because analog signals are affected by noise. In order to guarantee
the accuracy, we have to digitize the signals. When we digitize it, we
should need the clocks for synchronization.

> 4) Why is optical computing getting no where? Is this because lasers are
> expensive? Or because of the lack of fast optical switch ( any non-linear
> optical device)?

ANSWER: I think it is more of the minimization and integration of optical

> Thank you very much.
> Subas
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