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Subject: Re: voltage control oscillator

Date: 03/15/03 at 8:22 PM
Posted by: John Dunn - Consultant
E-mail: ambertec@ieee.org
Message Posted:

In Reply to: Re: voltage control oscillator posted by Jerry on 03/15/03 at 5:58 PM:

Hi, Jerry.

Well, this isn't my specialty by any means, but let's take a crack at them this way.

A voltage controlled oscillator, a VCO, is used where an oscillator is required to put out a frequency that can be varied in accorance with a controling electronic signal instead of just being set to some one particular frequency.

The term "copler" is, I think, a typo for the word "coupler". The most common use of the word coupler is in reference to a directional coupler which is used to take a sample of an RF signal. If we speak of a "20 dB coupler", the reference is to a directional coupler in which a sample of some signal is obtained at a level 20 dB smaller than the sampled signal itself.

A mixer is a device for hetrodyning two input signals to obtain their sum and difference frequencies. The IDEAL coupler, and I do stress the word "ideal", would operate on two input signals by multiplying them together according to the following equations:

Let one input signal be cos(2 pi f1 t) and a second signal be cos(2 pi f2 t).

There is a trigonometry identity that cos a * cos b = ½ cos(a+b)+ ½ cos(a-b). When this identity is applied to our two signals above, the ideal mixer output product would be:

½ cos (2 pi (f1 + f2) t) + ½ cos (2 pi (f1 - f2) t) where the first term is at a frequency equal to the sum of the two input frequencies and the second term is at a frequency equal to the difference of the two input frequencies.

Departing from the ideal, there would also be other outputs from the mixer, but those would be considered spurious.

John D.
ambertec@ieee.org

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