Re: minicircuits VCO
In general, the linear tuning oscillator is the ticket. FM modulation moves the frequency above and below a center frequency. If you analog modulate with a sine wave, you will get a uniform +/- deviation of the carrier. This will keep the baseband signal from getting distorted.
If you are digital modulating 2FSK it is less important, but there are some subtle variations. Lets say the VCO is NOT linear tuning. So when you put out a "1" the oscillator tunes up 5% in frequency from center, but when you put out a "0" the VCO tunes -7% in frequency. So where is the center frequency now? When being modulated, the center frequency appears to shift 1% down. When the modulation stops, the center frequency appears to shift back to the original "center". So what frequency do you tell the FCC your center is at? Hmmm.
A linear tuning VCO will keep the apparent center frequency the same if you are not modulating, or if you are modulating.