Enabling the prescaler on HLK-LD1115H 24 GHz radar module

Pin labels for SRK1101A.

The Hi-Sense HLK-LD1115H 24 GHz radar module uses the SGR SRK1101A as microwave active element. This nice little chip features a /16 or /8192 prescaler output that can be used to build a PLL control or, at least, calculate the frequency where it is transmitting.

But carefully poking around the 16QFN chip I couldn't find the DIVOUT signal. Also I noticed that on the board I received the VCCDIV input is floating but routed to two pads comfortably close to both Vcc (3V) and GND.

Thinking over these observations I concluded that the prescaler was not powered (floating) and therefore there was no DIVOUT. That is possible since there are other unpopulated pads, so the PCB could have been designed to accomodate several designs.

All that you need is a solder bridge!

I simply shorted the pad to 3V aaaaand magic happened! There was life on DIVOUT pin. Moreover, since VPTAT sits at 3V, both the internal temperature sensor is enabled AND the prescaler is set to /8192 (check the block diagram in the SGR SRK1101A datasheet).

So I got the /8192 output, 2.9373 MHz corresponding to 24062 MHz. After few minutes on the bench it moved to 2.9368 MHz, that is 24058 MHz. I don't care much of the drift, as long as I know where it is transmitting! At least in this phase.

The DIVOUT signal is routed to the uC on pin 12.

Technologies 50 years apart playing together.