Making the Electronics for CDM324 – 24GHz Doppler Motion Sensor

The Doppler Effect

I’m sure you’re quite familiar with the Doppler effect: you send an RF signal at a given frequency to a target, and if this object/person is moving the reflected signal’s frequency will be shifted. This is the reason why a fire truck’s siren has a higher pitch when the truck is going towards you than when it is going away.

Making the Electronics for CDM324 – 24GHz Doppler Motion Sensor

The CDM324 / IPM165 Motion Sensor

You may recall the article I wrote a couple of years ago about a nearly identical Doppler sensor, the HB100.
While the HB100 is using a 10.525GHz frequency, this new module uses 24.125GHz! This has the main advantage of being compatible with European regulations (ETSI #300 400) and having good penetration in dry materials. Moreover, as the main frequency is higher the patch antennas are smaller, hence the tiny 25x25x6mm module.
This motion sensor can easily be purchased on eBay under the name CDM324. Oddly enough, looking for “cdm324” on your favorite search engine won’t bring any interesting results.
I therefore spent several hours tracing the origins of this tiny sensor. I finally arrived to the conclusion that it likely is a clone of the InnoSenT IPM 165, which is itself very similar to the AP96 from Agilsense.
Specification-wise, the CDM324 antenna pattern is slightly narrower than the HB100’s: 80° azimuth and 32° elevation versus 80° azimuth and 40° elevation. The Radiated Power (EIRP) is more or less the same: 16dBm vs 15dBm. Finally, the advertised power consumption is identical to the HB100’s: up to 40mA @ 5V.

Finding the Right Amplification Circuit

Now that I had traced the origins of this CDM324, I could find its recommended amplifier schematics.
The one you see above comes from InnoSenT. It consists in 2 cascaded inverting band pass filter circuits:

You’ll however notice that the op-amp positive input in our suggested schematics is tied to vcc/2, allowing us to have a centered amplification output as well.
From the formulas above, we can compute the circuit total gain and cutoff frequencies. I’ll spare you the maths, the suggested amplification is 1018 (60dB) and a band pass filter from 3.4Hz to 1.06kHz.
The InnoSenT application note allows us to put these frequencies in context:

The high cutoff frequency is therefore set for a 24km/h or 15.4m/h speed. However, this isn’t exactly true as the filter cutoff slope isn’t vertical. Here’s the simulated amplification output generated using LTspice:

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