Digital Mixer

Yes you can do so in simulation.

No doubt this particular implementation would NOT work. Using a relay is just silly since its mechanical and probably has limited cycles before failing. I was curious if I could duplicate the sim from here: https://www.allaboutcircuits.com/technical-articles/how-to-multiply-RF-signals-without-a-multiplier-the-switching-mixer/ with EveryCircuit. I'm guessing theres a better way to handle the digital switch with transistors like the CMOS mentioned in the article. I probably should mention RF mixing is not something I work with much. Do you have a suggestion of a better way to approach the circuit?

This isn't mixing, it's sample and hold -- a useful part of an ADC. :) Surprisingly, it does seem to work. You can switch with FETs, but at RF the gate capacitance might be an issue. I think there's an NMOS switch in EC's examples titled Track and Hold. I'll post CMOS in a bit.

Hmm. It doesn't mix very well if the difference in frequencies is too great. Try 13MHz or 7MHz. Anyway, here are transistor-based switches: http://everycircuit.com/circuit/6103033003114496 . Also, now I remember, if input impedance and output amplitude aren't a problem, you can mix frequencies with just 2 resistors. I'm not going to bother reading that article. :)

@eekee, it does mixing even it looks like sample and hold. But if you think about sample and hold you will understand why it is like a mixer. Cause sample is multiply by 1 and hold is like ignore and multiply by zero. Its like a chopper modulator and gives you nicely the delta frequency and also the sum in this case here 21MHz which are just filtered out at the low pass RC filter.