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发表于 2011-9-17 00:40:50
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u can eliminate much of the requirement for complex RF design by using a GPS receiver module, but the signal
path from the antenna to the module RF-input is still a critical part of the board design that can have a big impact
on the system performance. A simple RF-signal path is not difficult to design provided you follow some basic
guidelines and use a software design tool for the geometry calculations. The following is intended to summarize
the basic RF design guidelines and give examples for proper layout and antenna-to-module RF signal path design.
Choosing an Antenna
As a general rule of thumb, when the antenna is located within 6” of the Module RF-input, you can use a
passive antenna. When the antenna is located beyond 6” from the module input, you should use an active
antenna to overcome the cable loss and maximize the signal-to-noise ratio. There are many choices of antenna
configurations for GPS; the best choice is often a balance between size, gain, bandwidth, noise, and cost. The
best advice is to test several antennas in the configuration of the final system to determine which provides the
best overall performance.
Antenna Configuration
• Passive Ceramic Patch – A ceramic patch antenna is a great low-cost choice that provides good sensitivity
and good omni-directionality. Its small size allows you to mount it within the same enclosure as the module.
It is also possible to mount the patch antenna on the same PCB as the module, but to reduce the possibility
of digital noise, we recommend that you mount the antenna on the opposite side of the board to the module.
In addition, to improve the performance of the patch, use the largest possible ground plane under the
antenna.
• External Active Antenna – An active antenna is essentially a passive antenna with a built-in LNA and a coaxial
cable to connect the antenna to the module. It may be located remotely from the module and requires antenna
power, normally provided via the coaxial cable. To attach the coaxial cable to the PCB, a coaxial connecter is
required. The active antenna usually costs more than a passive patch and consumes more power, but
performance in low signal environments is typically much better.
• Helix – The Helix antenna can also be passive or active. The advantage of a Helix antenna is that it is small
and easy to embed and it does not detune in proximity of people, which is why it is used in mobile devices.
Antenna Connections
The connection between the antenna and the RF-input of the module is the most critical part of the PCB design for
GPS. The goal is to provide a perfectly matched 50Ω transmission line environment between a 50Ω antenna and
the module RF-input to ensure maximum power transfer to the RF front-end. Any discontinuities due to unmatched
impedances in the signal trace, excessive vias, poor layout design, or energy coupling because of poor grounding
can reduce the GPS performance significantly or even render it non-functional. To reiterate, this signal path is the
most critical part of the GPS system design. Follow the guidelines below to maximize performance.
PCB Layout – 50Ω Connection
To move the RF signals to the modules, we suggest a 50Ω grounded co-planar waveguide. In general form, this
consists of the RF input signal with RF ground on either side and RF ground below. Theoretically, for a given RF
signal trace width, the surrounding RF grounds should be at least twice as wide. In addition, the gap between
the RF grounds and the RF signal is important. The coplanar waveguide is the lowest loss transmission line
configuration for connecting the antenna to the module. You can use other methods such as microstrip or stripline,
but we believe the coplanar waveguide is the most efficient. |
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