Confusion about GPS RF switches in nRF9160 DK

[GNotman]

After reading the support forums a bit, and browsing the documentation, I'm not sure exactly what the purpose is of the RF switch U34 in the GPS circuit. From what I've read, the switch exists to protect or isolate the LNA and the GPS front-end during LTE transmission (by using C73 as a RF ground), but I fail to understand why this matters exactly, since this switch is only active when GPS_EN is off.

For example, when we're actively transmitting with LTE, we don't really care about reading a GPS signal, so when the LNA is off, does it matter if some of the LTE signal couples into our GPS antenna and LNA stage?

When we're actively acquiring a GPS fix, we obviously won't be using LTE, so we have just a straight path to the GPS antenna. I'm not quite sure what the internal architecture looks like of the nRF9160, so perhaps I'm overlooking something, any information is welcome.

Right now my idea is to put a GPS SAW filter between the LNA and the GPS antenna, I guess this would be sufficient for improving jamming resistance right?

The schematic:

https://www.nordicsemi.com/-/media/Software-and-other-downloads/Dev-Kits/nRF9160-DK/pca10090_schematic_and_pcb_rev085.pdf

[Øyvind]

Hello, 

Yes, according to our HW experts you are correct:

When the LTE radio is operating, the RF signal can intermodulate in the GPS LNA input stage and create unwanted harmonics. I do not know if the LNA you are using is prone to this problem, but you should be safe either if using an LNA with an integrated filter, by adding a filter on the input of the LNA or use an RF switch to disconnect the GPS antenna when the GPS_EN signal is off. You can also try to measure the input power on the LNA when in modem TX, if you see less than -30dBc input you should also be fine.

Right now my idea is to put a GPS SAW filter between the LNA and the GPS antenna, I guess this would be sufficient for improving jamming resistance right?

Yes, this is right. A filter on the input of the LNA is sufficient. U34 is extra isolation insurance, but not necessary. However, U33 is needed if you do not have the filter on the input of the LNA. 

Hope this answers your question. 

Kind regards,
Øyvind

[GNotman]

Thank you for the quick reply! Just to verify, is this because the GPS and ANT pin are internally connected? And this is why you want a 'quiet' GPS pin even if you're not trying to receive GPS?

[Øyvind]

GNotman said:

is this because the GPS and ANT pin are internally connected?

 No, these are not connected internally.

GNotman said:

nd this is why you want a 'quiet' GPS pin even if you're not trying to receive GPS?

 Depending on the design, and where in the world the equipment is based, the value of unwanted harmonics may exceed the stated limits in that part of the world.