There have been several solutions developed and implemented that address this problem. One approach is to predistort the modulation signal to compensate for the effects of the PLL. This has limited results and a finite frequency range. Another commonly used method is to encode the data stream with such techniques as Manchester coding or split phase coding. The basis of this coding is to send two complementary symbols for every data bit, thus a transition is guaranteed for every data bit. This is very effective in that it fixes the lowest frequency component of the modulation signal so that a loop bandwidth can be designed to have a minimal effect on the coded data. Unfortunately, this means that the effective data rate of the radio is doubled, increasing channel bandwidth. The encoding procedure is rather simple (exclusive OR the data and clock together). The decoding is much more complicated requiring some synchronization to guarantee the correct two symbols are used to decode the data bit. Other approaches avoid the problem altogether by modulating outside of the PLL loop. Modulating the reference crystal, for example, can do this. A varactor can be used with a crystal to pull the center frequency to the mark and space frequencies. Since the modulation occurs outside the loop, the PLL does not effect the modulation signal, it simply tracks the changes in the reference frequency. The pullability of the crystal will limit the achievable frequency deviation and thus the maximum data rate using this method. Given the variances in crystals and varactors, this would require tun-