A 3-V delay-modulated PLL synthesizer for analog FM transmitters

An analog FM modulator operating within the loop bandwidth of a frequency synthesizer is presented. Its operation is based on delay-modulating the reference frequency of the synthesizer PLL, thus causing phase modulation in the synthesized carrier. To convert delay-control to FM, modulating input is first integrated. The modulator circuit is implemented in a 1.2µm BiCMOS process, has 2.8 V supply voltage and 1.5 mA supply current. In in-system tests, the modulator showed 7 kHz/V gain, 20 Hz(rms) residual FM and -31..-37 dB THD on audio band.