The ZED-F9T, and presumably other u-blox GNSS receivers, can power an active antenna with a VCC_RF pin. The VCC_RF voltage is specified as being essentially the same as the module voltage (VCC) (typical is VCC-0.1V). What's the point of this? The same voltage is already supplied to the module, so why not tap off the main supply? Does the ZED-F9T provide additional noise filtering for this supply? Something else? I checked the datasheet and the integration manual and couldn't find any actual explanation of when this feature might be useful.
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\$\begingroup\$ You might want to switch this voltage, filter it and fuse/current limit it. \$\endgroup\$– KartmanCommented May 18, 2023 at 13:53
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\$\begingroup\$ What do you mean switch it? I didn't see anything that indicated VCC_RF was configurable. If I want to use a different voltage than VCC, I need to supply that externally anyway. As for filtering and fuse/current limiting it, I can still do that from the main supply. \$\endgroup\$– MattHuszCommented May 18, 2023 at 14:23
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I posted this question on the u-blox forum and received an answer there. It seems that VCC_RF is VCC after some filtering. There are some hints of this in the integration manual, which refers to it as the "filtered VCC_RF supply" and later says "In the case where the module supplies the voltage via VCC_RF, a higher value resistor will be needed to ensure the module supply inductor is protected." So there does seem to be at least an inductor in there to somewhat isolate it from VCC supply noise.
Obviously, you need to be very careful not to exceed the 150 mA current rating of that inductor.
It does seem like it would be pretty easy to achieve the same result with an external ferrite bead and possibly a pair of capacitors (pi filter) if VCC_RF doesn't meet your needs for some reason or you don't want to risk damaging something internal in case you fail to protect against a short.
Unless I'm still missing something, this pin seems fairly optional and they could have gotten away with omitting it and recommending that users filter the supply in the integration manual.