Ultrasonic Anemometer Part 3 – Amplifier

Now that we’ve made some initial design choices about how the signals will be routed in the design, it’s time to start nailing down some of the finer details. But first, before getting into too much detail about the receiver design, let’s take a moment to survey the signals that we intend to receive using an oscilloscope:

Ultrasonic Anemometer Waveform 40kHz
Test waveform recorded using test setup

We can see both the transmitted and received signals using the simple test setup shown in the photograph below. In the final design the transducers will be much farther apart in order to minimize airflow disturbance.

40kHz ultrasonic transducers in a desktop vise.
Test Setup – Two transducers in bench vise

The received signal will be much weaker in the final design due to the increased spacing between transducers. Amplification will obviously be necessary. This leads us to the design of the first stage of the receiver.

The Amplifier

The incoming signal from the transducer will need to be amplified to account for the losses incurred during the waves journey through the air as well as the losses incurred in the transducers and multiplexer. Based on some rough oscilloscope measurements, we will need around 35dB of gain to amplify the 50mV signal that will reach the amplifier to get a 3V signal that we can then send on to the two processing stages. In order to make the circuit more immune to self-oscillation and to increase flexibility I will split the amplifier into two stages.

For the initial amplifier stage I have chosen to use a multi-feedback (MFB) band-pass topology. This topology will use an op-amp to both amplify and filter the incoming signal.  The gain of the first stage will be fixed at around 20dB. I’d like to have a Q of approximately 5. This is a trade-off between amplifier ringing and filtering ability. The transducers themselves have inherent filtering ability since they are tuned, thus, anything above 5 does not seem necessary at this point.

Amplifier Schematic
2-Stage Bandpass Amplifier Schematic

For the second stage I have chosen a basic inverting topology, whose gain could be easily adjustable by changing a resistor or using a potentiometer. I have drawn the second stage with about 20dB of gain. This will likely be adjusted later, but should be close enough for now. If the system ends up being too sensitive simply varying the output drive a few Hertz away from the resonant point will reduce the sensitivity without the need for any adjustments to the circuit. The output drive frequency is very easily modified in micro-controller firmware, and may end up as part of a calibration/initialization routine in the future.

Amplifier SPICE Simulation
Amplifier SPICE Simulation

It’s not yet clear to me weather or not separate bias circuits will be necessary, so I’ve included two at this point. The SPICE simulation doesn’t seems to show any adverse affects, but because there is a signal path to the bias circuit in the first stage, I’ve chosen to have another isolated bias circuit in the second stage just to be sure. I’ll find out later if this was over-kill (a colleague and I have a running joke that ‘over-kill’ is often ‘just-the-right-amount-of-kill’). The SPICE simulation shows results as expected, with just under 40dB of total system gain, and a nice peak at around 40kHz.

With a solid design in place for the amplifier I will move on to the envelope detector design process.