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).

26 September 2023

Emission of HLK-LD1115H as seen with a CDM324

In the picture below we can witness two radar modules looking at each other.

A CDM324 basic radar module was connected to an RTL-SDR to check the emission of a HLK-LD1115H. The greatest fear was that the HLK carried too much noise requiring an extremely wide modulation, but the picture shows a situation that is not worse than 2x CDM324 talking to each other. And we should consider that with this setup it is impossible to tell which radar is noisier.

The short-term frequency instability is probably caused by CDM-324 sensitivity to temperature variations, as it was fully exposed to ambient air.

The transmitting radar shows about -15 MHz drift at warm up in the first couple of minutes. I could also confirm that the transmission is continuous and not modulated.

On the second HLK module I own I pulled to ground the TXON control pin. Curiously on the serial port output it would report detection of movement and presence, alas with a lower intensity than the stock counterpart. With the receiving setup I could confirm that disabling TX introduces about 40 dB attenuation in the RF output.

I like the fact that the HLK-LD1115H can detect a movement 40 dB down, as it gives some headroom for completing a QSO at "some" distance. My current "record" with 2x CDM324 is 70 meters, without parabolic dish(es). 

What's next? Either trying to inject a modulation or pull the frequency closer to the other HLK and repeat the receiving experiment.

24 September 2023

Prescaler output to VO pin (HLK-LD1115H)

Now that the prescaler has been enabled in the Hi-Sense HLK-LD1115H 24 GHz radar module, I would like to have that signal easily accessible.

Let's dive into another hardware mod.

As already observed, the DIVOUT signal goes to pin 12 of the microprocessor. Additionally there is a 100 ohm resistor in series, close to the uP. The firmware on the microprocessor seems to ignore this new input, so the 100 ohm resistor is a good point to intercept the prescaler output.

Next to it there's another 100 ohm resistor (a tiny black rectangle), that protects the VO movement/presence detection signal coming out of pin 11 of the microprocessor (tnx Mauro for the info!). Cool, VO goes all the way to the pin header!

Here is the plan: remove both 100 ohm resistors and create a jumper as shown in the picture below. If you are brave enough you can use a 100 ohm resistor, like rotating 90 degrees one of the originals. Click on the picture to get a larger version and see the small details.

(Not reversible) Mod to bring DIVOUT to VO pin.

Guess what? It works. Now it's easier to see frequency drift with temperature and experiment with thermal insulation.

Next step will be to disable the transmission in one module so that it becomes "receive only" (act on TXON signal) and try to receive the other module. This might require also some action on the frequency control voltage.

21 September 2023

Enabling the prescaler on HLK-LD1115H 24 GHz radar module

A picture of SRK1101A with added in post-production textual labels of each pin.
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 tiny solder bridge!
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.
Technologies 50 years apart playing together.

18 September 2023

[Crazy idea] HLK-LD1115H for digital transmission?

This is just an idea that needs to be verified once I receive the second HLK-LD1115H 24 GHz radar.

The radar module outputs a serial stream with the status of "mov"evement, "occ"upancy or null. "mov" and "occ" strings are followed by two numbers, the first being the "spectral line" and the "signal strength".

Now, if two of these guys face each other and their frequencies are not too far apart (TBD, but I'd say < 1 MHz), a receiving end should detect an on-off modulation of the counterpart as if it was a movement.

The module outputs 10 status lines per second on the serial port at 115200 baud. If each status line represents a bit, we can theoretically achieve 10 bps, or about 1 baud, that is one 8-bit symbol per second. We could speed up things a little bit using the Baudot code that uses 5-bit symbols; then you need a start and stop bit for a total of 7 bits or 1.4 baud.

Sounds slow? Yes, sure. But don't forget that WSPR runs at 1.5 baud. And that you would be operating on 24 GHz with the simple help of a microcontroller!

Another way to exploit this system would be to encode very slow Morse code, where the decoding brain (the grey matter one) can make up for decoding errors reducing the need of a robust(ish) data encoding or CRC or FEC or ...

17 September 2023

Back on 24 GHz with Hi-Link HLK-LD1115H

General info: I am discussing the (ab)use of these radar modules as amateur radio transceiver on 24 GHz, not their motion/presence sensing application.
After a relatively successful contact with CDM-324 free running modules over a 100 metres distance, my friend and "wideband FM gigahertz partner" Mauro started looking for better alternatives, but still cheap.
RF side of HLK-LD1115H 24 GHz radar module. Note how small the chip is as compared to the standard 0.1" pin header visible on top.
RF side of HLK-LD1115H

There's a nice writeup of the current (2022-2023) offering of these modules by Bertnik at https://revspace.nl/FMCWRadar .

Mauro chose the Hi-Link HLK (probably www.hlktech.net) HLK-LD1115H module on aliXss that costs about 5€/USD each. Apparently those use a clone/copy/emulation of an Infineon BGT24 chip, with the same features but less output power: the SGR SRK1101A. They have on-chip temperature sensor for compensation, they are tunable (300-600 MHz/V), they include a prescaler with /16 or /8192 output so you can even build a PLL control around them.

Cool, aren't they. Well, they are as cool as tiny: there is almost no way to solder directly on their pins as the spacing for the 16QFN is 0.5 mm, so we needed a module that routes on the PCB as many pins (= functions) as possible.

I bought a couple of HLK-LD1115H to play with. Why two? To have a ready backup if I break one module and, if everything goes well, to have a receiving or transmitting counterpart as Mauro doesn't live exactly next door.

I did the usual overimposed front+back picture to try to reverse parts of the schematic diagram, but being a 4-layer board some traces are not visible and vias just "disappear".

Plan B, then! Under the most powerful lens I own and with the help of a continuity tester, search where each pin goes by poking pads around the PCB. The process is not complete and the result will be documented in a future post.

Logic side of HLK-LD1115H.

You can click on both pictures to get a larger version. The larger TSSOP chip is an STM32F030F4P6 microcontroller while the smaller is a dual op-amp RS622 with 7 MHz GBW. One top-right corner is a 3.0V regulator LN30.

11 September 2023

SN75188 / MC1488

I have some MC1488/SN75188 ICs laying around. They are "line drivers designed to interface data terminal equipment with data communications equipment" (souce: TI datasheet); the first edition of the datasheet was in 1983. Today in 2023 they are practically useless from my current view of upcoming projects.

I checked the datasheet in search for ideas for some side-use-case application, like an oscillator, LED blinker, whatever, just to have an excuse to melt some solder.

While they contain 5 inverting logic gates, this xx88 chip requires dual power supply which is uncommon today. But scrolling down the datasheet, almost at the end, an "application information" turns a useless part into a possible lifesaver:

logic level converter sample application for SN75188 IC
Logic level translator applications. Courtesy of TI datasheet.


With a TTL or DTL input, these chips can be used to drive old digital technology like RTL, DTL, HNK or (negative) MOS. What for? Well, 1970's technology I like to play with is often built around non-modern TTL digital logic signals and in case of a chip failure these xx88 might help to obtain a properly formatted drive.