Going on hunting trips with my grandpa as a kid, we would carry a rather large GPS-navigator to help us keep from getting lost in the woods. It had a huge antenna sticking out the top and its battery never lasted longer than a few hours. Fast forward 20 years, and including GPS capabilities in your PCB design has become easier than ever.
Many new consumer devices include GPS modules in their design. If you don’t have GPS or RF-design experience, there are several design rules that will help you successfully integrate GPS capabilities into your product. The first step in the process is choosing a GPS module, but as with any choice, there are several factors to consider before selecting a module and beginning the design phase.
Active vs. Passive GPS Antennas
GPS PCB antennas come in two flavors: active and passive. Active antennas come with a low-noise amplifier (LNA) built into the module, while a passive antenna does not include an amplifier. Active antennas sit on their own board and connect to your PCB with a coax cable.
Some receivers come pre-packaged with either type of antenna. They can also contain a passive matching network that matches the output to 50 Ohms impedance. An active antenna has an advantage in that the LNA maintains the noise level in the output signal resulting in higher sensitivity.
Integrated GPS receiver
Passive antennas on PCBs should also use an LNA but the signal can degrade as it travels from the receiver to the LNA. While the LNA is designed to reduce noise in the output signal, any additional noise reduces the overall sensitivity. If you choose to use a receiver that requires an external LNA, the signal trace running to the LNA should be shielded or isolated from external EMI or crosstalk as much as possible.
GPS Antennas in Your PCB Design
Using a GPS antenna in your PCB brings it into the mixed signal regime. Any noise that is introduced at the antenna input due to EMI or crosstalk can degrade signal quality and even block the antenna signal entirely. The antenna signal is also susceptible to ground plane noise if not properly isolated from other components.
If the other components on your board are not properly isolated or shielded, the GPS antenna and receiver can degrade the signal in these other components. In some cases, the worst noise culprit is the receiver itself, especially when the receiver has an internal antenna. Crosstalk between the receiver and other components underscores the need to include the proper shielding.
Filtering is required to extract the GPS signal from the LNA. This is currently done by placing a surface acoustic wave (SAW) filter in between the LNA and the receiver input. SAW filters allowing for filtering of high frequencies above 1 GHz, such as those found in GPS applications. It would be impossible to extract the GPS frequency from the other noise found in the signal without using a SAW filter.
Shielding, Grounding, and Routing
The signal that is output from a GPS antenna/receiver will already be below the noise floor by up to 20 dB. Minor noise signals that would be acceptable in other applications can easily block the signal from your GPS receiver, and proper routing, shielding, and grounding are required for your GPS-enabled device to work properly.
Normally, when you divide up your PCB into functional blocks, you should also give each block its own ground plane. Your ground planes would then be routed back to the main ground lead in a star topology in order to prevent ground loops. The large size requirements of a ground plane surrounding a GPS receiver can make this difficult, especially on mobile devices.
Shielding works wonders in PCBs
If you properly shield your receiver, its matching network, and any external LNA in a shielding can, you can connect your RF and digital ground planes. Isolate the GPS receiver and matching network to its own RF ground plane and connect this to the digital ground plane at a single point. The RF ground plane will be the best place to ground the clock and data lines.
Antenna traces running to the receiver carry an analog signal and should always be placed as far away from digital traces and components as possible. Where possible, route your antenna traces inside a shielded enclosure. You can also bury your antenna traces on an internal PCB layer and place ground planes on all sides. The antenna should be placed just outside the shielding. All other electronics and the battery should be shielded from the antenna.
Design for Impedance Matching
If you are familiar with high-frequency design, you will know that attenuation and impedance matching are extremely important factors that contribute to signal degradation. Signals with higher carrier frequency have greater attenuation and longer trace results in a lower overall sensitivity. If you can help it, opt for a shorter trace between a passive antenna/receiver and your external LNA. This will help keep sensitivity high.
When running your antenna traces that carry your RF signal, it is best to avoid using vias as these increase the impedance of the trace. Each via creates an inductive discontinuity and adds about 10 Ohms of impedance to your trace at GPS RF frequencies. Vias with larger diameter will add larger impedance. If your receiver was already passively impedance matched to 50 Ohms, you will need to compensate for any vias that appear in the trace.
A great piece of PCB layout software like Altium Designer makes it easy to add GPS antennas and capabilities to your PCB design through its extensive range of layout tools. With smart design rule checking, auto-interactive routing potential, and analysis and simulation tools, your designs can come out as flawless as you need for a smooth transition to prototyping and manufacturing.
To learn more about Altium Designer and how it may help you create the GPS antenna you need, talk to an Altium expert today.
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