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
    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_


    Mar 22, 2007
    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_


    Mar 22, 2007
    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
    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.
  5. Gadget

    Distinguished Member

    Jan 10, 2006
    For fixed frequency a low frequency xtal oscillator multiplied up as mentioned above, also makes it possible to modulate the frequency enough to get usable wide band Frequency Modulation FM.
    The more common way of doing things these days however, is with PLL (Phase Lock Loop) circuits, using an oscillator working at close to the desired end frequency, the output of which (as well as going to the PA stages) is divided down and then constantly compared to an xtal reference. Any error is then corrected, making for a stable output that is easily Frequency Modulated to the 75Khz bandwidth normally used. Another advantage of this system is that by changing the division ratio, the frequency of the output can easily be changed with no loss of stabily and no xtal change.
    There are some nice cheap IC's, that along with a couple of cheap Xtals, do all this for you, along with Multiplexing for stereo audio.
    Check out the BH1417 chip.