Previously I was using a timer generated square wave (f=200 kHz, Vpp=10 V) to drive the ultrasound transmitter. However, noise (perhaps timing jitter) in this oscillator was broadening the interference fringes, so I was looking for a better drive. In my first attempt to replace the oscillator I picked up a Si5351 chip on a breakout board from Adafruit. This chip outputs up to 3 square waves with Vpp=3.3 V, at frequencies between 2.5 kHz and 200 MHz. Here is the first data which look promising but, as you can see, I now need an amplifier on either the source or the detector side, since the reduced drive amplitude is only giving a 10 mV signal at the detector.