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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?

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  • \$\begingroup\$ seriously, a proper 10 GHz-capable mixer (not spurs all over the place) is way harder to come by than a 10 GHz oscillator, so you're "solving" the wrong problem (aside from you not solving the problem, see Andy's answer) \$\endgroup\$ – Marcus Müller May 16 '18 at 9:54
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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.

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  • \$\begingroup\$ Yes, I will modulate the amplitude in this way. But the output signal of the mixer will be composed by two signals, one with the sum and one with difference of the input signals, right? So if I change the frequency of the baseband signal, would it changes also the frequency of the output signal? For example, for '1' logic I set the baseband signal at 1 kHz, for '0' logic I set the baseband signal at 1.5 kHz and I put on the mixer a baseband signal that varies in the time. Then I can use one component (sum or difference) like a freq. modulated signal. It's possible to make something like that? \$\endgroup\$ – Fabio May 16 '18 at 10:05
  • \$\begingroup\$ This will be amplitude modulated - you will simultaneously have sum and difference frequencies and this cannot be extracted as being FSK - FSK is when the carrier is first shifted up and then shifted down in frequency i.e. the shift follows the digital data. However, if you filter and extract the upper (or lower sideband) and kill-off the other sideband you will have a form of FSK but it won't be centred at 10.525 GHZ but slightly offset. If you can make this filter and live with the offset then yes, it becomes FSK. \$\endgroup\$ – Andy aka May 16 '18 at 10:45
  • \$\begingroup\$ Sorry, at the first message I made a mistake with "centered", it's not necessary to be centered at this frequency. The important thing is that this new transmitting signal varies discretly between two different values of the frequency. So, an other HB100 used as receiver will receive my modulated signal, and with its integrated mixer I take the translated version in baseband of the transmitting signal. Finally, I measure the period with an mcu which I can know the binary data. \$\endgroup\$ – Fabio May 16 '18 at 11:06
  • \$\begingroup\$ @Fabio OK sounds interesting but removing one of the sidebands to make it an offset FSK is quite tricky considering that you will need a filter that has to reasonably cut-off anything below 10.524999 GHz whilst letting pass anything above 10.525001 GHz. That will be a big hurdle. \$\endgroup\$ – Andy aka May 16 '18 at 11:25
  • \$\begingroup\$ Why is needed the filter? HB100 has the mixer input connected one at its transmitter antenna and the other at the receiver antenna. For example, if I set the low frequency VCO of transmitter station at 1.5 kHz, I will send a microwave signal of 10.525 GHz - 1.5kHz (supposed I consider only the difference). The receiver will mix this signal with its own transmitter signal, that is again fixed at 10.525 GHz. Then, at receiving station I keep the difference frequency component that would be 10.5250015 (RX) - 10.525 GHz (TX) = 1.5 kHz... the initial baseband signal. \$\endgroup\$ – Fabio May 16 '18 at 11:49

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