KiCad TO-92L footprint for hand soldering

Today I received a small batch of PCBs - THT for hand soldering - and halfway building I had to troubleshoot a short on the input Vcc line.

I had a suspect: the 78L05. Well, not "him", but how I soldered it. While I had plenty of space on the board, I used the TO-92L footprint where pins are very close to each other. If you add a bit of rush, sub-optimal lighting and worsening eyesight (with age), a bridge is almost granted.

One of the TO-92 footprints available in KiCad.

Next time I will use a wider footprint, nevermind if I have to bend pins. No wonder KiCad has TO-92L_Inline_Wide!

 

 

Polarization of LD1115H vs CDM324

Before popping the HLK LD1115H radars I noticed that I could get a stronger signal when the orientation of the antenna arrays between TX and RX (CDM-324) was 90 degrees apart.

I found that the signal was stronger when the modules were positioned - relative to each other - as shown in the picture below.

This relative position provided strongest signal in the lab environment.

Before conducting any "long" distance test it is worth checking the effect of "swapping" polarization on one side.

 

 

S 042 P and UAA170 from the cellar

At a local flea market I bought a plastic drawer parts storage cabinet with 25 compartments. Drawers had unsorted components inside, mainly carbon resistors with 1W or more rating. At home I emptied everything so that I could wash it and I found some interesting parts, like:

• "S 042 P" mixer IC
• "UAA170" LED driver
• "TDA1200" FM-IF
• SGS BFT 56 with extra long legs

Considering the ICs I found in the lot I think the previous owner wanted to build a stereo FM receiver kit from the Italian magazine "Nuova Elettronica" (https://www.adrirobot.it/lx-193-sintonizzatore-fm-con-decoder-stereo/).

When I was a teen I was fascinated by the project of a shortwave receiver using an "S 042 P" IC, that I've never been able to meet in the wild for the following 35 years! I think it was already discontinued or obsolete when the article was printed. I wonder if I could finally build that project now!

In the meantime I might try building a running light with the UAA170 and a 555. It could even be made into a freeform sculpture .

Overvoltage on HLK LD-1115H - don't

In case you wonder how these radar modules handle overvoltage, the answer is: very bad. I gave 12V to both of my specimen while trying to understand where the 5 MHz modulation was coming from (TX or RX CDM-324 side) and they let out the magic smoke.

I removed both 3V regulators but the 3V line is still shorted to ground. There is a little chance that something else blew before the small 24 GHz chip but I doubt so.

I will check with Mauro if it is worth continuing with these modules since the output power is much lower than CDM-324 or the original Infineon part. We do need the tamed frequency drift these radars offer, but RF power is useful too to get decent "DX" results.

24 GHz QRX.

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!).

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).

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.

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.

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.