I want to use the HB100 to make an FSK modulation on a microwave signal. The reason why I use HB100 is that in this way the RF circuit is ready. I thought about doing an FSK modulator like the scheme attached in this question and I would ask you if it is feasible. I make the modulation on a low frequency VCO (for example 1 kHz) controlled by a microcontroller, the I put HB100's transmitter signal and this low frequency signal to an rf mixer like the mixer implemented in the HB100. In this way I get a signal centered at 10.525 GHz that varies its frequency depending by the low frequency VCO (fHB100 + f0, fHB100 - f0). Can I use this approach to make an FSK modulation?
If you put a fixed carrier frequency (10.525 GHz) into a mixer/multiplier and feed the other input of the mixer with a baseband signal you will get double sideband suppressed carrier amplitude modulation and not FSK. To get FSK you need to frequency modulate the carrier such as by using varactor diodes built around an oscillator's tank circuit.