It means your carrier oscillator must be rigidly reliable so that it deviates very little from 100 MHz. Howsoever it derives initially, it should not be affected by other circuitry. One thing that helps is rock-solid regulation of the power supply. If you use a crystal then it would help if you make a constant temperature for it (crystal oven).
Your frequency counter must detect change within 30Hz. Just maybe you can include pre-scaling or lissajous figures to take readings. Your frequency is within the commercial FM band however amplitude modulation might interact in unexpected ways with frequency modulated airwaves.