The first thing that needs to be done to get a synthesizer to change frequencies is that this command needs to be sent. Typically several register values need to be changed and the write time is the amount of time it takes to write these registers. For instance, if the SPI programming bus can be run at 75 MHz and one needs to program three 24-bit registers, then this can be done in slightly under 3 μs. The most intuitive way to reduce the write time is to increase the clock speed, which is definitely an effective method.

Another technique is to use shadow registers, also known as double buffering. For this approach, multiple registers can be written before they are needed and then the end of one write can trigger all the register to take place at the same time. This is assuming that one knows the next frequency that is required. In addition to eliminating the write time, shadow registers also can help prevent glitches. For instance, suppose it is required to write the feedback divider (N) in one register and then trigger the VCO calibration by writing to a different register. If the N divider is written first, the VCO will slam into the tuning rail because the frequency is invalid for that band. Then the VCO is programmed to calibrate and it will go to the correct frequency. However, having the VCO tuning voltage at the rail can increase the analog settling time after the VCO calibration is finished. So in summary, shadow registers can eliminate programming time and prevent unnecessarily long analog settling times.