Doppler Velocity Sensor puts FFT and AGC to work
Some people hate going back to finished, dusted-off projects. We understand that; it's a bit like reading a book you've already read when there are so many more to choose from. But rereading a book sometimes reveals subtle nuances you missed the first time around, and revisiting projects can be much the same, like with that new and improved Doppler radar speed sensor.
It looks like we were negligent in writing [Limpkin's] last go-around with the CM324 microwave module, a 24 GHz transceiver that you can get cheaply from the usual sources , but we've featured this handy little module in many other projects. [Limpkin's] current project uses the same module to create a Doppler velocity sensor, but with a bit more sophistication. While the original used a simple comparator to produce a square wave proportional to the Doppler shift and displayed the speed over a simple terminal session, version two takes a different approach.
First, [Limpkin] chose to implement the entire sensor in hardware. The front end is quite different: an op amp with 84dB of gain followed by an automatic gain control (AGC) stage built from a MAX9814 microphone preamp. Extracting the velocity from the module's output is left to an STM32F301 running an FFT algorithm on the signal coming out of the analog circuit, which essentially selects the largest peak in the spectrum and calculates the Doppler shift from it, displaying the results on a screen. LCD display.
Of course, as a [Limpkin] project, there is much more than that. The article is very detailed, navigating some nice dens like characterizing the amplifier chain and diving into the details of the Johnson-Nyquist noise to track down spurious oscillations. There's some interesting stuff here, and it's worth reading in depth; there's also the video below that lets you see (and hear) what's going on.
Some people hate going back to finished, dusted-off projects. We understand that; it's a bit like reading a book you've already read when there are so many more to choose from. But rereading a book sometimes reveals subtle nuances you missed the first time around, and revisiting projects can be much the same, like with that new and improved Doppler radar speed sensor.
It looks like we were negligent in writing [Limpkin's] last go-around with the CM324 microwave module, a 24 GHz transceiver that you can get cheaply from the usual sources , but we've featured this handy little module in many other projects. [Limpkin's] current project uses the same module to create a Doppler velocity sensor, but with a bit more sophistication. While the original used a simple comparator to produce a square wave proportional to the Doppler shift and displayed the speed over a simple terminal session, version two takes a different approach.
First, [Limpkin] chose to implement the entire sensor in hardware. The front end is quite different: an op amp with 84dB of gain followed by an automatic gain control (AGC) stage built from a MAX9814 microphone preamp. Extracting the velocity from the module's output is left to an STM32F301 running an FFT algorithm on the signal coming out of the analog circuit, which essentially selects the largest peak in the spectrum and calculates the Doppler shift from it, displaying the results on a screen. LCD display.
Of course, as a [Limpkin] project, there is much more than that. The article is very detailed, navigating some nice dens like characterizing the amplifier chain and diving into the details of the Johnson-Nyquist noise to track down spurious oscillations. There's some interesting stuff here, and it's worth reading in depth; there's also the video below that lets you see (and hear) what's going on.
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