DC Accurate FSK Modulator

A typical, low cost, FSK modulator is implemented by injecting the modulation voltage into the phase lock loop of the carrier synthesizer. This is done in two ways, by summing the modulation voltage and the loop error voltage together and applying that to a VCO tuning port. Or, by using a separate tuning port usually having lower sensitivity to voltage changes. The modulating data would change the carrier frequency by a predetermined amount (deviation). This, however, causes a frequency error within the PLL that the loop begins to correct for. The effect of the loop correcting for the frequency errors caused by the modulation is analogous to passing the modulating signal through a high pass filter. Lower frequency components of the modulation signal get filtered off (or even eliminated). This results in unreliable communications for random data streams, especially for long strings of non-changing bits.