Do I need a GPS simulator to design a device that contains a GPS receiver and a pre-designed antenna?

Question

I have designed a device using an STM32 Nucleo board, a GPS module, a GSM module, some discrete components, and a bread board. It is a sort of a car tracker.

I have decided to design my own custom board which includes all the modules that I have bought separately. I have designed some boards before but I haven't gotten anything fabricated yet. There isn't much info on the topic and what little is written on the internet does nothing but confuse me more.

I came across an article on this website, which says that I need to either buy a GPS simulator which costs around 10 to 15 thousand dollars or add an entire pre-built module to my PCB. What confuses me is:

1. How can I add an entire module to my PCB? Should I add round pin holes to my PCB and connect them to my MCU using traces and then solder the entire module to those pin holes? That sounds a bit silly to me but that's what I understand when they say "use a module instead of a chipset" because I can sort of "reverse engineer" the module that I have and discover which components are present but then I still don't know the stack up.
2. Given that I am going to buy a GPS antenna as well as a GPS receiver chip, I thought all I needed to design were the transmission lines. Why would I need a simulator? (Isn't that used to test your GPS antenna? I'm buying a pre designed one.) I am planning to use a SIM808 chip and there is a hardware design guide for it on the internet but it doesn't say anything about a simulator there, only that some traces must have their impedance limited to 50 ohms.
3. When they say a GPS module which one of these do they mean?

I guess I could just wing it and design something and get it manufactured to see if it works or not but making sure that I understand everything correctly before I do so could save some time and money.

Answer 1

RF design is not trivial. "It looks exactly the same" doesn't mean it will work exactly the same.

The very material of the PCB makes a difference to how the signals behave at the frequencies used for GPS signals. The traces form inductors, and the traces together with the PCB material form capacitors. You can normally ignore that kind of thing when working with microcontrollers or audio signals. When you get to gigahertz signals, that kind of stuff matters. The thickness and composition of the PCB (fiberglass layers and epoxy) can change the capacitances enough to cause problems - and you have no control over those variations. People who do that kind of thing work together with the PCB manufacturer to be sure that variations don't cause problems. A hobbyist doesn't have the equipment to measure the effects - or much ability to get the PCB manufacturer to do anything to fix problems.

Your mention that for the SIM808 "some traces must have their impedance limited to 50 ohms" says you don't understand impedance enough to design a PCB that will work at the required frequencies.

GPS signals are extremely weak. Receiving them requires getting a lot of things right.

If I were doing your project, I'd use complete modules with the PCB taking the place of the bread board. Yes, pads on the PCB to solder the modules to, or pin headers or sockets to connect the modules.

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The SIM808 isn't just a GPS receiver. It is a GSM cellphone module with a GPS receiver. Besides the difficulties in getting the GPS part to work correctly, you'd also have to deal with the GSM parts.

The GSM parts are subject to FCC regulations (or whatever your country's equivalent is.) That means that if you get it wrong and cause problems for other people, the FCC can come and take your toys away.

For that reason alone you should stick to the modules.

Answer 2

There are plenty of traps in designing RF circuitry, and (in the case of GPS) it is quite easy to end up with something that doesn't work at all, and (in the case of GSM) doesn't conform to the necessary standards.

So before rushing to design a PCB, I'd prototype up the proposed circuitry, connecting the Nucleo board to GPS and GSM modules. I'd recommend keeping these two functions as separate modules; in the case of GPS, you can get a module with a built-in patch antenna from various vendors, e.g. Quectel L80-M39, Antenova M20048-1, and many others. Having an integrated antenna means no RF design problems; the GPS signal is really weak, and it doesn't take much of a problem to blank it out entirely.

With regard to the GSM module, that will normally have an SMA or U.FL connector for an external antenna, and to meet emission standards you have to strictly follow the manufacturers guidance.