Ultrasonic Anemometer Part 18: Analog Signal Processing

20160320_AnemometerAnalog_001Recently, I’ve sucessfully tested the new driver ciruit for my ultrasonic anemometer. It performed even better than I expected and I will be happy to use it pretty much as it is.

By the way: If you want to get an overview over how this project has developed over time check out the overview page. If you’re more interested in my latest design, this link will take to my new attempt.

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So we have a circuit that can send powerful ultrasonic pulses from the right transducer, receive the signal from the opposite transducer and route it through an amplifier. The next task is to tell the time-of-flight from the received signal. A contemporary  approach would probably involve some sort of DSP and software. My last approach used some analog circuitery to detect the zero crossings as well as the envelope of the received signal. Since most of the heavy lifting was done in hardware, a simple 8-bit microcontroller like the one on the Arduino UNO could be used to do the measurements.

For my new approach I haven’t quite decided which route to take.  To me they both have a certain appeal. And over the last year or so I’ve had quite some ideas on how to process the signal in hardware so I’ll give it a try and see how it works out.  In this post I’ll go through this new circuit and explain how it works (or is supposed to work) step-by-step.

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The zero crossing detector (ZCD) is almost identical to to my last design. The amplifier output is biased to half the positive supply voltage and fed into a fast comparator (Microchip MCP6561R). On the comparator’s output we get a precise digital signal indicating if we are currently looking at the positive or negative half-wave. Right at the logic edge we observe a zero crossing which can then be used to very precisely determine the phase shift relative to the transmitted singal.

The more challenging part is to tell the absolute phase – this is where my last project was only partly successful. It used an active low-pass filter to get the envelope of the received singal. This envelope was then compared to some threshold and the time from the transmition of the singal to when the envelope exceeded the threshold was measured. With plenty of averaging this gave a usable but far from perfect indication of absolute phase. So this time I’ll try something entirely new.

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The amplified singal is first run trough an active full-wave rectifier as found on page 257 of Horowitz and Hill’s 3rd edition of Art of Electronics. It uses two op amps as well as some resistors and diodes to produce a singal that corresponds to the absolute value of its input. The two op amps come in a single package. It’s the same type as for the amplification stage of the driver circuit – a Texas LMC6482.

Now the rest of the circuit is a bit more adventurous. It attempts to produce a signal that corresponds to the peak of the previous half-wave. So it is steady during each halv-wave period and should give a (hopefully precise) indication of the amplitude of the received signal. This singal can then sampled by an ADC at 80kHz triggered by the zero crossing detector. 80kHz is not that fast for a (say) 10-bit ADC and definitely much slower than what we’d need if we sampled the amplified signal directly.

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The advantage of measuring the amplitude is the following: We can find the peak of the amplitude in simple software and use the time when the peak occured to find the absolute phase. So we are no longer dependant on the absolute amplitude (as we were with the envelope detector approach) but only care about when the peak in amplitude occured. I think (hope) this is a much more reliable approach.

In order to find the peak of each half-wave I use a pair of simple diode-plus-capacitor peak detectors. One is held stable (“hold”) and fed through an op-amp buffer to minimize droop while the other is looking for the next peak (“sample”). At the beginning of the sample period the capacitor is discharged through a n-channel mosfet that is turned on for just an instance.

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The whole mechanism is controlled by the output of the zero-crossing detector so absolutely no software intervention is needed to produce it. The microcontroller can just wait for the ZCD to trigger an interrupt (as before) and take a sample of the output.

The circuit is not that complex and used an inexpensive and readily available 74HC4053 multiplexer at its center. I don’t have any idea yet how this thing will perform but I must say this little circuit was a lot of fun to design.

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Until my next post I will have built and tested this cuircuit and will let you know how it performs. Until then I leave you with the eagle files as well as PDFs of the schematic and layout as a zip file.

Here’s how the final circuit looks and how it performs: Part 19.

2 thoughts on “Ultrasonic Anemometer Part 18: Analog Signal Processing”

  1. Sorry…how do you get that beautiful prototype pcb’s….are made by milling machine…cnc?..congrats for your work

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