Every engineer has faced it: You chose a high-performance GNSS receiver and a certified antenna, yet your device is failing field tests. Why?
The problem doesn’t lie with a single component, but rather with how they all work together as a system — or don’t. A holistic, system-level design strategy is key for ensuring that the device as a whole is greater than the sum of its best-in-class parts.
Take the example of an asset tracker struggling to connect to the cellular network or whose Time-To-First-Fix (TTFF) takes forever. The first place to look for the weak link is the signal chain (Antenna -> Front-End Filter -> LNA -> Receiver) because it determines the device’s overall RF performance. If the asset tracker wasn’t designed holistically, then it’s at risk for problems such as:
- Band 13 Interference: The LTE modem’s noise desensitizes the GNSS receiver.
- Self-Jamming: Bluetooth, cellular, and Wi-Fi radios interfere with one another.
- Poor Efficiency: A great antenna is crippled by a lossy transmission line or poor integration such as antenna to metal clearance, ground plane size, antenna location etc.
Read on to learn how to avoid those and other performance pitfalls by:
- Understanding system sensitivity (a function of antenna gain, filter loss, and LNA noise figure) and dynamic range (the ability to handle strong interferers without desensitization).
- Using a system-level approach to ferret out the interferer.
- Designing out vulnerabilities rather than reacting to them.
A System-Level Investigation
Start with a system check using a spectrum analyzer. Don’t just look for the interference. Instead, look for the system component causing it. The power supply? Processor? Slew rate on Flash Memory Read/Write? Another radio?
Next, achieve localization through correlation. In the case of the asset tracker, force its LTE modem to transmit on Band 13. If your GNSS performance drops, you’ve proven a negative system-level interaction. This is a classic case of one subsystem (cellular) impacting another (positioning).
Now that the interferer has been identified, it’s time to make it a good neighbor. One option is filtering its output, such as with a filter on the LTE modem’s TX line or adding a SAW filter before the LNA (used with the GNSS antenna) to further attenuate the out of band signal (in this case at B13). Another option is implementing spatial and temporal separation between radios.
Careful antenna integration often goes a long way toward mitigating interference to maximize system performance. There are three key areas to consider:
- Antenna Placement and Ground Plane: A chip antenna works best when it’s integrated correctly, has sufficient metal clearance and is placed properly on a ground plane. When these conditions aren’t met, the antenna is inefficient, which forces the LNA to work harder and makes the entire system more vulnerable to interference. This highlights the importance of following the antenna integration guide to ensure it meets its datasheet specs.
- The Enclosure and Keep-Out: The closer that a plastic enclosure is to a patch antenna, the more likely that it will be detuned and lose gain. This attenuates the signal, exacerbating interference problems.
- Transmission Line Loss: A poorly matched trace also attenuates signal strength and exacerbates interference. A rule of thumb is that for every 1 dB reduction of transmission line loss, the system’s noise figure by reduces by 1 dB.
The Last Line of Defense
If interference persists even after integration is fully optimized, a filtered antenna is the next layer of protection. This strategy uses a single, purpose-built component to resolve a system-level problem.
In the case of the asset tracker, this means specifying a GNSS antenna with an integrated Band 13 notch filter. This protects the vulnerable GNSS receiver at the point of entry by blocking out-of-band signals before they can reach the LNA and desensitize it.
Conclusion: Building Robust Devices by Design, Not by Accident
When a new device flounders in labs or real-world tests, it’s a reminder that interference mitigation should never be a last-minute fix. Make it a system design principle: Instead of reacting to interference for several months or at the end of product development, design it out at the beginning. This avoids the expense and time-to-market delay that comes with finding and fixing problems.
Start with system-level planning, enforce rigorous antenna integration, and use targeted components like filtered antennas to solve predictable problems. Call in the experts, such as Taoglas’ GSA-30 GPS Sensitivity & Tracking Service. Taoglas application engineers can review your design to help you avoid common pitfalls and ensure your entire system delivers optimal performance.
