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Subject: Re: Some basic questions

Date: 10/17/01 at 12:52 PM
Posted by: John Dunn - Consultant
E-mail: ambertec@ieee.org
Message Posted:

In Reply to: Some basic questions posted by Abbas on 10/16/01 at 1:14 AM:

Hi, Abbas.

That is some list! Well, let me try a little bit here.

Noise figure is the degradation of signal to noise ratio (SNR), expressed in decibels, between what goes into an amplifier and what comes out of it. Since any amplifier always adds at least some small amount of noise to whatever input it receives, the SNR is always degraded to at least some degree.

Noise figure plays the most important role in the first amplifying stage of an RF chain. You want as much gain and as low a noise figure in that first stage as is possible. Secondary stages can be somewhat forgiven for their noise figures if the first stage is really good in that regard.

I see that the terms dBd and dBi appear in antenna specifications. They refer to how many decibels an antenna's emitted signal will be compared to a hypothetical reference antenna. With dBd, the reference is (I think) with respect to a standardized half-wave dipole antenna and with dBi, the reference is with respect an isotropic antenna.

Can someone else add to this?

John Dunn - Consultant
Ambertec, Inc.

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