# Type of Oscillator used in FM stations

Discussion in 'General Electronics Chat' started by sagarin, Mar 30, 2007.

1. ### sagarin Thread Starter New Member

Mar 27, 2007
6
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Greetings to all.......

As you know there are many types of oscillators like crystal oscillators, R-C, R-L, which dont produce a freuency in the FM range(88-108MHz), What is the kind of oscillator used in FM stations? Can you explain?

2. ### Dave_ Member

Mar 22, 2007
28
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In the Armstrong FM System for example, the crystal oscillator, at for example 10.8MHz, is mixed with the carrier frequency (12.8MHz). The difference in frequencys (2MHz) is then fed into a frequency multiplier (x48)
Which is how you get the higher frequency range.

3. ### Dave_ Member

Mar 22, 2007
28
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We would generally use combinations of x2 and x3 to boost the signal up to its required frequency.

Here's some blab about the frequency multiplier from my course notes:

A frequency multiplier is a combination of a nonlinear element and a bandpass filter. A transistor operating in class c mode (less than 50% conduction) and a tuned LC circuit to an nth harmonic of the input modulated signal are commonly used for multiplication factors n from 2 to 5. In principle, we may tune the LC circuit to any integer number of the fundamental frequency, but efficiency drops off significantly for n>3. Where higher orders of multiplication are required, multipliers may be cascaded.

4. ### antseezee Active Member

Sep 16, 2006
45
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Typically a crystal oscillator is used, in conjunction with Mixers & Frequency Multipliers to get the same effect. 2's & 3's are stacked to get the multiplied frequency. For example, if you want a higher FM radio station, you need a VERY high oscillator amount which can be a smaller value crystal, sent through multipliers (a 3, 3, 3, 3 = 81). Then, you would need a slightly smaller mixer tuned to a portion of the Oscillator's frequency, since the difference of the Oscillator & Mixer is the resulting frequency that gets aired.

Armstrong is a great example of a FM transmission system. It's better if you saw a block diagram showing the process. Sometimes they throw in 90 degree phase shifters depending upon the system.