Eric,
Good luck with your set-up, it sounds interesting. Thanks for pointing out Zurich Instruments, did not know they existed, will need to save some money. :)
Anyway, if the signal you want to mix originates from other components, it will be hard/difficult to input those signals into a DAQ, digitize them, mix the digitized versions, then filter, and output the filtered reference signal. I have no idea what the lag would be between input and output, that is, your filtered output will always be lagging behind your two master frequencies. If your frequencies are ultra-stable, this should not be a problem, but if they drift on the order of the time lag, it's a problem.
My idea was use a set of NI function generators that are phased locked together to control everything. If your other instruments could take an external input then it may be feasible.
For example,
1.) Function generator 1 output Sin[ 2 pi f1 t + q1 ]
2.) Function generator 2 output Sin[ 2 pi f2 t + q2 ]
Since you generated these signals in LabVIEW, that is, you did the math, then take signal 1 and square it, you get
Sin[2 pi (2f1) t + 2 q1] + DC, Filter out DC signal - simple subtraction in LabVIEW
Then take that squared signal and multiply by signal 2, you get a side band of
Sin[2 pi (2f1 + f2) t + (2 q1+ q2)] + other harmonics (You would need to filter these out)
That is your Lock-in Reference, all of your other frequencies are phased synchronous with this reference. All your math would be done before your outputted these points to the function generator, thus you have no lag, and phase synchronous frequencies. Your third function generator would output the mixed signal to the lock-in. That is why I was wondering whether NI had devices with four outputs, that could be phased locked. (Outputs greater than 1 seem to come in powers of two.)
Have a good weekend.
Regards,
mcduff
