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Altera_Forum's avatar
Altera_Forum
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18 years ago

Phase modulation

The attached picture shows phase modulation of a clear clock by a sinusoidal signal.

Would you plz help me figure out what kind of circuit does this?

I know there r generators that can generate this kind of signal but I want to know whats the basic -easiest- way to achieve such modulation.

I also have some questions:

1. UI is the time of one period of the clock. Right? so 2/3 is chosen such that the maximum modulation doesnt result in losing a clock period or adding another one?

2. isnt the period supposed to be 1/fc (for the ideal clock)? why is it considered 2pi in this case?

Thank you.

2 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    18 years ago

    The diagram text is wrong. It says sin() but shows a square wave. The term jitter is used normally for an unwanted phase modulation.

    I can't see, if the diagram is intended as an example to explain phase modulation or clock jitter analysis or if it shows a generated signal with specific parameters. Thus I can't comment the 2/3 modulation index, apart from confirming that it has to be below unity.

    The question, how to generate a similar signal may be answered different, depending on the required application parameters.
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    18 years ago

    Hi,

    The sin(2*pi*fc*t) indicates the fundamental sinusoid component of the clock signal. This is a standard equation and the 2*pi allows the sine values to cycle around...

    The picture explains a clean clock jitter on the basis of two sinusoid phase modulation. You can see there is a clear relationship between clock jitter and the amplitude of the modulating sinusoid.

    In the digital domain you can reproduce that in a DDS circuit(Direct Digital Synthesiser) by generating the clock "sinusoid" component according to word value decided by a lut containing one cycle of the modulating sinusoid amplitude values).

    edit: alternatively, use altera NCO and set to phase modulation then pass the phase modulation value as variable input from the LUT. (altera NCO is an implementation type of DDS)

    Kaz