RE: [SI-LIST] : Interesting theoretical questions

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From: Jian Zheng (jian@zeland.com)
Date: Mon Nov 01 1999 - 17:29:08 PST


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

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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: http://www.zeland.com
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> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of subas
> Sent: Monday, November 01, 1999 4:24 PM
> To: 'si-list@silab.eng.sun.com'
> 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
switch.

>
> Thank you very much.
>
> Subas
>
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