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