What is the difference between local oscillator and oscillator?

Answer 1

Oscillator is a generic word for any circuit that produces an alternative signal.

The shape of the signal, its stability and its output impedance are not considered in this case.

"Local oscillator" is one specific type of oscillator used in radio receiver for the purpose of frequency conversion. the input Radio Frequency( RF ) signal is "mixed" with the local oscillator signal and the subtraction of two frequencies is obtained at the output.

"Mixing" in this context means: "multiplication"\

In some other context in electronics (specially audio) mixing means "adding"

the mathematical operation for local oscillator is F out=Fin - F loc.osc.

The output of the mixing operation can also be Fin+F loc.Osc. But this higher frequency is usually filtered out.

The local oscillator needs to be a pure sine wave with a very good frequency stability and usually very stable output voltage.

Recently very good "synthesizers" are introduced in the market that permit generation "programmable local oscillation"

Answer 2

IF = Intermediate Frequency

A crystal oscillator is an oscillator circuit whose frequency of operation is primarily decided by the frequency for which the crystal was cut.

An intermediate frequency oscillator is the oscillator which operates at a specific frequency depending on it's purpose. For an FM receiver, the IF is 10.7MHz. For an AM receiver, the IF is 455kHz. For TVs, it's different for the luminance/chrominance/audio information.

When two frequencies are sent to a nonlinear mixer circuit, the frequencies output include both the original inputs, F2-F1, and F2+F1 where F2 is the higher frequency. So, if Fs (signal from antenna) and Fi (intermediate frequency, say 10.7MHz) are both sent to a nonlinear mixer, then Fi+Fs (along with others) will be output, which means that your desired signal (music from the radio, etc.) is NOW on a 10.7MHz carrier, where it can be singled out (via filter) and the signal is worked with at 10.7MHz instead of potentially 107+MHz. It's a lot easier to design a circuit at 10.7MHz then it is at 107+MHz.

If the crystal is cut to resonate at the IF, a crystal oscillator can be used for the IF oscillator. So, the IF oscillator COULD be a crystal oscillator. Hope that helps :-)