I currently have two square wave signals at f1 and f2 (f1 >> f2) that serve as reference signals to two separate lock-in amplifiers that are subsequently connected in cascade to extract the component of the input signal at f1+f2 or f1-f2.
I am interested in attempting to eliminate one of the lock-in amplifiers, by somehow using the the square waves references at f1 and f2 to generate a phase stable reference at f1+f2. This could then be used as the reference for a single lock-in amplifier to directly detect at the intermodulation frequency.
Of particular importance is that the reference at f1+f2 must be phase locked to the precursor reference signals at f1 and f2. The precursor signals are generated by IR photodiodes that measure light passing through a rotating slit, and are thus subject to small drifts due to the slew rate of the motor, air currents, etc.
My problem is, then, how to generate this reference signal. As square waves, mixing the two reference signals (in a double balanced mixer) would yield all the odd harmonics, which for say f1=50~100kHz and f2=1~10kHz would be hard to filter to give just f1+f2 (particularly in a filter without some highly non-linear phase behavior).
Can anyone recommend a good starting point as to how this may be accomplished? Thank you in advance.