two point modulation pdf

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Hope the following will be helpful:

The PLL frequency synthesizer presents a unique problem when designing an effective GMSK modulator. Data patterns consisting of long strings of ones or zeros have a spectral response that extends to DC. Most frequency synthesizers do not respond to this low-frequency signal because of their inherent high-pass filter characteristic when modulated at the VCO. The two-point modulation
technique circumvents this problem by splitting the Gaussian filter signal, sending one portion to the VCO modulation input, and the other to the PLL master oscillator. Since the master oscillator is not in the frequency control feedback loop, it can be modulated by the low-frequency portion of the signal. Modulation at this point, combined with modulation of the VCO, produces a composite signal with a
spectral response extending down to DC. This two-point modulation technique results in an essentially constant modulation sensitivity without regard to the loop bandwidth.

More at: http://www.eetasia.com/ARTICLES/2003..._NTEK_AN01.PDF