How FSK switching frequency is removed from IF output?

FSK modulation could be used to measure distance to moving objects, for example walking person. While testing a prototype FSK VCO with receiver I experienced one problem: if FSK switching frequency is near sampling frequency then there is a huge interference occurs, because FSK switching frequency is present in mixer IF output (or some form of it). If FSK switching frequency is increased, for example 5 times faster than sampling frequency, then it could be filtered out using low pass filter. But there is another problem: increased FSK switching frequency degrade VCO settling performance and adds more requirements to other circuit parts, such as need for lower time jitter of sampling time vs FSK switching time. I did not found any information regarding this problem. What is the best approach to remove FSK switching frequency from IF output?

maybe you should have a phase locked vco, and phase modulate it afterwards...there would be virtually no settling time. You could apply a phase modulation that looks like a frequency step, for instance.

Use FMCW for measuring distance and you will not have any problems.