30 September 2023

Let's modulate the HLK-LD1115H (WBFM)

Since the aim of all the activities on the HLK-LD1115H radar module is to come up with a simple 24 GHz transceiver, it was time to have it send some information. Unless you opt for an on-off modulation like Morse code, the easiest way is to fiddle with the VCO control voltage to obtain frequency modulation. An easy "entry point" is shown in the picture. That node is in the middle of a resistive voltage divide and it's also quite easy to solder to (I do hand soldering!).

Detail picture of accessible VCO control pads to change the frequency of the radar module.
Accessible VCO control pads.

How much voltage is needed? Looking at the chip specs, the VCO moves 650 MHz per Volt. Since we need to be able to receive the FM with available tools, the resulting RF signal should have a maximum bandwidth of 250 kHz. Also, considering the phase noise observed on the non-modulated signal (remember: we don't know if the source is in the TX or RX side, or both), the wider the modulation, the better.

A rough calculation tells us:

250 [kHz] / 650000 [kHz/V] = 385 [microV]

That's not a big deal: just divide whatever voltage you have with large enough resistors and start from "all the way down". I used a signal generator set at about 800 Hz in high-Z output, fully attenuated and fed to the radar through a 10x oscilloscope probe and a 100 nF capacitor.

With the same receiving setup as the previous experiment/post I achieved easy modulation of this 24 GHz radar module. I also compared the SDR signal with a real wideband FM radio and the latter gave much better audio signal to my ears (less noise).

Next step will be to pull radar frequencies close together and into the HAM allocation (24.000 to 24.050 GHz, they are a bit high on 24.100 GHz now) and use one radar as receiving end without bypassing the onboard "low frequency" amplifier (the op-amp has a 10 MHz GBW).

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