Avian’s Blog

The other day at the Institute, Klemen brought me this microwave motion sensor. Apparently, it was left over from an old municipal lighting project where street lights were to be only turned on when something was moving in the vicinity. I don't know what came out of this idea, but the sensor itself is quite fascinating.

There is no manufacturer name on the device. The bottom side of the PCB says GH1420 and IRP/07. It appears to be very similar to the AgilSense HB100 sensor module, but it's probably a cheap knock-off rather than the original. I haven't come across these yet, but it seems they are somewhat popular to use with Arduino (and as cheap DIY 10 GHz sources for amateur radio enthusiasts).

Image by AgilSense

The application note from AgilSense contains the block diagram above. The device transmits a continuous wave at around 10.5 GHz on transmit antenna. Any signal that gets reflected back to the receive antenna is mixed with the local oscillator. If the input signal is Doppler-shifted because it reflected off a moving object, you get some low frequency signal on the output. The application note says that a typical signal is below 100 Hz and in the μV-range.

After removing the metal can, the circuit appears fantastically minimalist. There are only two semiconductor elements, two passive elements and a dielectric resonator. The PCB substrate feels like plain old FR4. Copper traces are covered with solder mask and have what looks like immersion gold finish - unusual for a microwave RF circuit. If it weren't for the transistor in the high-frequency SMD package and the PCB microstrip wizardry, I wouldn't believe this runs at 10 GHz. They didn't even bother to solder the RF shield can onto the ground plane.

I'm not very familiar with extremely high-frequency design, but this does look more or less like what the block diagram above promises. The X shaped element is most likely a high-frequency NPN transistor used as an oscillator. Base is upper-right, collector is lower-left and the two remaining pins are the emitter (with vias to the ground plane on the other side). The +5 V power lead provides collector voltage through a resistor on the lower-right. The quarter-circle things on the PCB are butterfly stubs in low-pass filters.

The collector of the transistor is capacitively coupled with the base through the white cylindrical resonator. This provides the feedback that drives the oscillator. What's interesting is that there is no bias on the base of the transistor. Either it is working as a class C amplifier or there's something more fancy than a plain bipolar transistor in that SMD package.

The output of the oscillator is coupled to the transmit antenna on the bottom and to the mixer in the center. The little rectangular stubs on the way probably help with impedance matching or some filtering of oscillator harmonics. The trace from the receive antenna comes in on the top of the picture. The mixer is probably some kind of a diode arrangement, although I don't recognize this setup. The low-frequency output from the mixer then exits through another low-pass filter to the lower-left.

Apparently that's all you need for a simple Doppler radar. I was surprised at the extreme minimalism of this device and the apparent huge divide between design effort and manufacturing costs. I'm sure a lot of knowledge and work went into figuring out this PCB, but once that was done, it was probably very simple to copy. I wonder if this specific setup used to be covered by any patents.