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Frequency Shifting

  1. Aug 25, 2011 #1
    Hello. I am using a colpitt crystal oscillator of about 25Mhz. I want to use that as my base frequency and then have another circuit that can output multiple copies with slightly different frequencies like 26Mhz, 27Mhz etc. I have the book by Horowitz and I noticed something similar. It is a quadrature oscillator circuit. However, what I was able to gather is it only produces a 90 degree phase shifted copy of the signal that was used as the input.
    My question is suppose I modulate both of these signals with an audio wave, then how can a radio receiving operating at medium wave be able to distinguish between the two because both signals have the same frequency but are only out of phase. The answer might be obvious but I'd like to be sure.
    Second, how can I possibly create copies of the original colpitt output signal with slight frequency differences.
    I hope I can get an answer.
  2. jcsd
  3. Aug 26, 2011 #2


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    Staff: Mentor

    You are asking many questions... I'll address the multiple frequency part. Look at this intro page about Frequency Synthesizers:


    That is the traditional way to get multiple frequencies.
  4. Aug 26, 2011 #3
    Thanks, I'll have a look.
  5. Aug 26, 2011 #4
    Phase shifting a signal and using it to create signals of different frequencies are two entirely different problems. Phase shifting is easy, creating signal of different frequencies not so much.

    Several questions arise as to what you are trying to do: are you always using 25MHz and always want 26MHz, 27MHz etc? If so the easiest way might be to use several VCOs and give them different voltage inputs to get your desired frequencies.

    What is an audio wave? The way you frame it I suppose you are mean an analog signal, but give more detail about what you are trying to do. Are you amplitude modulation the carrier?

    But you are right. In general if all you do is phase shift a signal a receiver has a hard time distinguishing between the two signals it received. However, there is such a thing as phase modulation but typically this is used for digital signals when you want one of a few well defined levels and not analog. Again it goes back to what you are trying to do.

    The easiest way is to just use two oscillators of different frequencies. I am sure there are more complicated ways of doing it in analog, but off the top of my head I can't think of a way to take in a 25MHz sine wave and put out a 26MHz sine wave without using a VCO or some other oscillator also. You can do it digitally, but that probably adds a level of complexity you don't want to use in your design.
  6. Aug 26, 2011 #5
    Actually creating several oscillators was something I was trying to avoid because I'm just repeating the same thing. I was wondering if I can get a base signal of frequency of lets say 100mhz and build a filter circuit that can filter out whatever frequency I don't want. It doesn't matter when I said 25 26 MHz. I was just illustrating. Might a filter circuit of limited complexity be able to do the work.
  7. Aug 26, 2011 #6


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    Science Advisor

    A sine wave signal contains only the fundamental frequency.

    So, a 25 MHz sinewave contains only 25 MHz, with zero content of 24 MHz, 26 MHz or other frequencies.

    If the 25 MHz signal is a distorted sinewave, it will contain harmonics of 25 MHz. That is, it will contain components of 50 MHz, 75 MHz, 100 MHz in proportions depending on the type of distortion. These are whole-number multiples of the fundamental frequency.
    But even a distorted 25 MHz signal will not contain components of 24 MHz or 26 MHz unless it has already been mixed with these frequencies or modulated with a 1 MHz signal.

    So, you can't filter the signal to get components that are not in the signal already.
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