Here is how you do it at higher frequencies:
h**p://www.polyphasemicrowave.com/pages/single-sideband_modulator.html
I imagine you can do a similar thing at 40 KHz with op amps and analog multipliers. The box with the "X" in it in the center is a 0/180 degree hybrid, which can be done with 2 op amps of gain 1 and -1. The 0/90 degree splitter can be done with a resistive splitter and a 90 degree LC phase lag circuit (or an RC op amp circuit). Mixers (circles with the "X"s in them) can be analog multipliers. Power combiner at output can be one summing op amp.