What is Two-Point Modulation?
A. In a typical voice radio architecture, the information signal (i.e. voice) is used to modulate a voltage-controlled oscillator (VCO) in a phase-locked loop (PLL) based synthesizer. This is referred to as single-point modulation. The VCO has an inherent high-pass filter characteristic, and this is adequate for voice applications where the majority of the information content is at relatively high frequencies. A wireless data transmission can contain considerable low frequency content (e.g. down to just a few Hertz), so the high-pass characteristic of the single-point modulation scheme is problematic. To achieve better low frequency response (and thus a better bit error rate), the low frequency content removed by the VCO must be replaced, and this can be accomplished by applying the information signal to a point outside of the feedback loop within the PLL. By applying the information signal to both the VCO and another point within the PLL (the reference oscillator is a common choice),
