Ultrasonic Anemometer Part 2 – Signal Path

Since we’ve established the methodology behind how we will approach making these wind speed measurements, it’s time to begin the design. The design of this system was inspired by several other individuals who have also designed and built their own ultrasonic anemometers. Here you will find their own reports documenting their efforts:

The Signal Path

The first thing I chose to tackle was designing the overall flow of signals. Because the design has four transducers I needed either four transmitters and receivers, or, a way to select the appropriate signals and route them to one transmitter and receiver. In order to minimize cost and size I chose to build one receiver and transmitter and use a multiplexer to route the signals to the correct destination.

Ultrasonic Anemometer
Signal path for ultrasonic anemometer

The transmitter in this case is simple. I determined that the impedance of the transducers was approximately 680 Ohms at 40kHz. This allows me to drive them directly from a pin of the micro-controller at either 3.3 or 5 volts without cause for concern since most micro-controllers can source and sink around 20mA, or more. Additionally, the multiplexer will have some resistance, which should also protect the micro-controller. Because the transducers are tuned circuit driving them with a square wave directly from the micro-controller seems like it should be acceptable at this point. The transducer will filter out most of the harmonics in the square wave. I will re-evaluate this later in the design process.

Side note: In order to determine the impedance of the ultrasonic transducer I connected a variable resistor in series with the transducer. I then applied a voltage with a signal generator set to 40kHz. Next, I adjusted the variable resistor until the voltage amplitude was equal across the resistor and the transducer. At that point the impedance of the resistor and transducer were equal. Using a simple multi-meter I measured the DC resistance of the variable resistor which yielded a result of 680 Ohms. While this doesn’t provide the whole story regarding the impedance of the transducer, it does give enough information to move forward with the initial design. Plus, it’s very quick and easy to do with only basic equipment.

Designing the receiver in this system was the most difficult part of the hardware design process. I will start to cover that process in the next entry in this series.