androidfanboy:
Hi there,I'm designing a PCB with an LTE cellular module that has built-in GNSS and I have everything done except the antenna design and I'm no RF expert. I have attached a picture of the recommended schematics for active and passive GPS antenna as well as my current unfinished PCB layout. The GPS antenna is at the bottom and cellular antenna is at the top. Currently I am using this GPS chip antenna, which supports GPS, GNSS, and Beidou, which is what the cellular module supports per its datasheet.
Since I have quite a bit of extra space at the top I really want to design a PCB trace antenna for the cellular, but I have no idea how to do that and I couldn't find any reference designs online. Also, I am really not sure which off-the-shelf surface-mount antenna would suit me. The LTE module operates at LTE-FDD B2/B4/B12/B13 and LTE-M1 frequencies.
You need one that supports all the bands, it has to resonate on each.
Now to the design:
- The datasheet for the cellular module says "...the trace impedance must be controlled to 50 Ohms." Since the traces are so short in my layout (distance from the cellular module pin to the antenna pad is about 6mm), do I have to worry about this? For 1.6mm-thick double-layer FR4 substrate, traces need to be about 120mils wide (really, really fat) to be 50 Ohms, according to this online calculator.
Absolutely yes, 50 ohms it needs to be all the way from the antenna to the next device on the signal path, and you need enough clearance on either side to prevent coupling to nearby components. Every mm matters at
microwave frequencies, which this is.
- The cellular module datasheet says to use a low-noise amplifier (LNA) for a passive antenna, but I'm trying to skip this because RF LNA's like this one are insanely small (yea, like 0.7x1.1mm with 6 pads). Please tell me if I shouldn't do this.
Any 50 ohm MMIC LNA that covers the band(s) of interests will probably do, yes they are available in sensible
packages.
- The chip antenna gives a few mounting configurations, and it looks like mine is the first "End Mount" configuration with the trace going straight out to the antenna. The datasheet says to keep 1.5mm separation distance between the chip antenna's pad and the GND plane. I've done this. However, are the GND planes large enough on either side?
which datasheet is this? The GPS one?
- Will the capacitors (1210 packages) interfere with the GNSS antenna? I heard that big barrel objects aren't good near RF antennas. However, those large caps are needed to prevent large voltage drops on the input and there's really nowhere else to put them (I don't want those two sore thumbs on the bottom of the board, and they have to be close to the power pins on the module).
Antenna's best nowhere near the power components, that's a very noisy part of a PCB.
- I'm considering switching to this chip antenna instead, since it has higher gain (peak 3.7dBi vs 1dBi) and is much smaller and also requires a much smaller GND clearance area. Is there any real difference or should I go for this new chip?
I know I threw out a lot there, but any suggestions? Thanks!
Where's the image of the PCB?
I found an interesting variant GPS antenna which goes over ground plane, so allows stuff on the other
side of the PCB rather than forcing it to be empty: ONBOARD GPS Mini Antenna 2.4GHz