Antenna cable
The Yocto-GPS is sold with a ceramic active antenna at the end of a ten centimeter coaxial cable
ending with a U.FL connector. This type of cable is rather fragile: avoid bending it at a right angle,
you could damage it. Be particularly careful about the antenna-cable juncture.
Coaxial antenna cables are fragile: avoid bending them
U.FL connectors
Theoretically, U.FL connectors are not designed to be connected more than about 10 times. Make
sure that the U.FL plug offers a good contact: when connected, it must not be able to rotate freely
around the connector. If you plan to build a system with a removable external antenna, use a female
U.FL to SMA adaptor cable and a corresponding antenna. Almost all external antennas have an SMA
connector.
External GPS antenna usually have an SMA connector
Choosing an antenna
You do not need to use the GPS antenna delivered with the Yocto-GPS. There are many GPS
antennas on the market. You must select an active antenna: active antennas include a small
electronic amplifier limiting the loses due to the antenna cable. They are usually required when the
distance between the antenna and the GPS receiver goes above 10 centimeters or so. An active
GPS antenna is most often powered directly through the antenna cable. You can use any active GPS
antenna directly with the Yocto-GPS as long as
• it can work when powered in 3.3V
• its gain does not go above 35db.
4.3. USB power distribution
Although USB means
Universal Serial BUS
, USB devices are not physically organized as a flat bus
but as a tree, using point-to-point connections. This has consequences on power distribution: to
make it simple, every USB port must supply power to all devices directly or indirectly connected to it.
And USB puts some limits.
In theory, a USB port provides 100mA, and may provide up to 500mA if available and requested by
the device. In the case of a hub without external power supply, 100mA are available for the hub itself,
and the hub should distribute no more than 100mA to each of its ports. This is it, and this is not
much. In particular, it means that in theory, it is not possible to connect USB devices through two
cascaded hubs without external power supply. In order to cascade hubs, it is necessary to use self-
powered USB hubs, that provide a full 500mA to each subport.
In practice, USB would not have been as successful if it was really so picky about power distribution.
As it happens, most USB hub manufacturers have been doing savings by not implementing current
limitation on ports: they simply connect the computer power supply to every port, and declare
themselves as
self-powered hub
even when they are taking all their power from the USB bus (in
4. Assembly and connections
14
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Summary of Contents for Yocto-GPS
Page 1: ...Yocto GPS User s guide...
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Page 6: ...22 Characteristics 557 Blueprint 559 Index 561...
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