So Much Going On In So Few Components: Dissecting A Microwave Radar Module

In the days before integrated circuits became ubiquitous, providing advanced functionality in a single package, designers became adept at extracting the maximum use from discrete components. They’d use clever circuits in which a transistor or other active part would fulfill multiple roles at once, and often such circuits would need more than a little know-how to get working. It’s not often in 2024 that we encounter this style of circuit, but here’s [Maurycy] with a cheap microwave radar module doing just that.

On the board is an RF portion with a single transistor, some striplines, and an SOIC chip. Oddly this last part turns out to be an infra-red proximity sensor chip, so what’s going on? Careful analysis of the RF circuit reveals something clever. As expected, it’s a 3.18 GHz oscillator, but how is it functioning as both transmitter and receiver? The answer comes in the form of a resistor and capacitor in the emitter circuit, which causes the transistor to also oscillate at about 20 kHz. The result is that at different times in the 20 kHz period, the transistor is either off, fully oscillating at 3.18GHz and transmitting, or briefly in the not-quite-oscillating state between the two during which it functions as a super-regenerative receiver. This is enough for one device to effectively transmit and receive at the same time with the minimum of parts, there’s no need for a mixer diode as you might expect if it were it a direct conversion receiver. Perhaps in RF terms, it’s not particularly pretty, but we have to admit to being impressed by its simplicity. He goes on to perform a few experiments with the board as a transmitter or as a more conventional radar.

This isn’t the first such radar module we’ve looked at, here’s one designed from scratch. And we love regens, since they are so simple to build.

17 thoughts on “So Much Going On In So Few Components: Dissecting A Microwave Radar Module

  1. So much functionality pulled from a single BFS520 transistor.
    This writeup clarified a lot of thing to me about how these things could even function and be sensitive. Barely any workable dopplershift from humans walking in a room on 3GHz.

    So the “super-regenerative pulse radar” thing these have going on make it possible.

    ps. I think Jennys olderschool ham brain corrected the super-regen quench frequency to more human speech usable value of 20 kiloHerz, instead of the radar sensing distance related quench frequency of 20 MegaHerz.

  2. Could you link many of these together to form a much more powerful “phased array” radar if you could either “sync” the ococcilators with a single clock?

    1. I’ve got a pile of these things and I’ve always wanted to put several in each room and sort it out in software. I’m not a software guy though so they’ll never be used.

  3. This is super clever. But, a little worrying such a cheap transmitter can be had that doesn’t have FCC certification. 3.18ghz is a part 90 band, and since the carrier is getting modulated it’s going to splay all over. Just laziness on the oems part, since there are available bands for this kind of stuff.
    Probably this will fly under the radar, hah, but wonder when some alibaba express device will get semi popular and catch the attention of the FCC

    1. The 3.18 GHz frequency seems a bit weird, there are similar devices using 5.8 GHz, which is a nice and wide ISM band in most places, but I don’t think any are commonly sold as modules from the usual sources (Amazon, Alibaba, etc). Perhaps 3.18 GHz is set aside for these things in China and they didn’t bother to redo it when selling overseas?

      1. I have bought some “5.8GHz” doppler modules from aliexpress and they were on that band, but just barely and one of them was on weather radar band (5.4GHz).

        As for 3.18GHz, I guess they had to get away from other 2.4GHz users to avoid interference to and from them.
        And when you have a radar on S-band, the S-band radar band starts looking like the right place for it.

  4. These work fine with Arduinos, but as soon as I try to use them with an ESP the noise generated by the WiFi radio causes all sorts of false positives. Has anyone come across a design for a filter to keep this from happening?

    1. Perhaps the components take turns, just like the transmit/receive modes. ESP gets a presence signal from sensor, cuts power to the sensor until it can handshake the message across WiFI, then resumes monitoring.

  5. These exact boards are pretty good. Seem to sense you within about 3m in front or 1m behind. Used one of these to turn off a display when the user is away to save power. There are a bunch of similar ones around $5 on Ali which are pretty good too, but fine pitch headers.

  6. “In the days before integrated circuits became ubiquitous, providing advanced functionality in a single package”

    What’s the big black thing in the picture?

    1. “Oddly this last part turns out to be an infra-red proximity sensor chip”

      But that isn’t correct, it a IR prosimity sensor management chip, it has no IR functionality but is often used in old PIR devices with a seperste PIR sensor.
      These days the PIR sensors have it all built in it seems and dont’ need a management chip anymore.
      It’s nice though that people are siiting around finding new ways to use that old chip, the HaD spirit, and rather clever to realize its high sensitivity to changes can be used in alternative ways.

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