L-Band FM telemetry modulator (two-point modulation)

Conventional FM transmitters lack DC response to the modulation input. Two point modulation is a solution to get around this problem and achieves dc response. In this method, both the VCO in the PLL and the reference oscillator are modulated by the input modulation signal. Modulation of the reference oscillator provides the low frequency response and modulation of the VCO provides high frequency response. But low sensitivity of VCXO´s used in the reference channel is a problem for getting uniform response from dc to the required bandwidth. With the advent of inverted mesa crystals, high frequency VCXO´s generating in 100 MHz region (in fundamental mode) can be easily realized and this problem can be solved. Availability of Fractional-N Synthesizer, will reduce carrier phase noise, near the carrier. A block diagram of the proposed scheme is given below. In the main paper, loop design criteria to achieve constant frequency response of the deviation w.r.t modulation signal for all loop-bandwidths, will be presented.