SAW Filters for GPS Antennas:

SAW filters are a critical component in the RF front end of GPS and GNSS devices. They enable receivers to maintain high sensitivity while preserving selectivity by rejecting unwanted signals and interference. This article explains what SAW filters are and how they improve GPS performance in real-world deployments.

Data Alliance GPS antennas—including active GPS antennas and GPS combo / combination antennas (GPS + LTE and/or WiFi)—integrate SAW filters to ensure reliable operation in high-interference environments typical of IoT, fleet, and industrial applications.

What is a SAW filter?

Surface Acoustic Wave (SAW) filters are specialized RF devices designed to selectively pass or reject signals at specific frequencies. In GPS applications, they are tuned to the L1 band (1575.42 MHz).

SAW Filter

SAW filters operate by converting electrical RF energy into mechanical (acoustic) waves using piezoelectric materials such as quartz, lithium niobate (LiNbO₃), or lithium tantalate (LiTaO₃). This conversion occurs through an Interdigital Transducer (IDT), which consists of precisely spaced electrodes.

The generated surface acoustic waves propagate across the substrate and are then converted back into electrical signals. This process inherently filters frequencies based on the physical design of the device.

SAW filters are compact, cost-effective, and widely used in RF systems up to ~3 GHz. However, filter selectivity and center frequency can shift with temperature. Non-temperature-compensated designs may drift slightly, which is an important consideration in outdoor or industrial deployments.

SAW filters for GPS devices and applications:

In GPS systems, SAW filters are used to isolate the desired satellite signal from interference sources such as cellular transmissions, harmonics, and broadband RF noise.

Typical GPS SAW filters:

• Center frequency: 1575.42 MHz (L1)
• Bandwidth: ~20 MHz (1565–1585 MHz)
• Rejection: 
  • ~10 dB at 1525 MHz
  • ~22 dB at 1625 MHz
• Insertion loss: ~0.9 dB
• Return loss: ~14 dB

These filters are typically implemented as compact SMT components and are integrated into active antenna designs or receiver front ends.

Why filtering is essential in GPS systems:

GNSS receivers already use digital signal processing to extract satellite signals. However, RF interference directly impacts performance before signal processing begins.

Interference sources include:

• LTE/4G/5G transmitters near GPS band edges
• Co-located radios in combo devices (WiFi, cellular)
• Harmonics and spurious emissions
• Urban RF noise environments

This interference can:

• Increase Time To First Fix (TTFF)
• Reduce positional accuracy
• Cause intermittent signal loss

SAW filters improve signal integrity by removing out-of-band energy before it reaches sensitive RF stages.

GPS receivers require specificity as well as sensitivity.

GPS receivers have to be sensitive because the GPS signal suffers significant attenuation as it travels to earth. Low Noise Amplifiers (LNAs) are routinely used with GPS receivers to increase signal reception.

However, this makes the receivers vulnerable to the effects of electromagnetic interference from a variety of sources which can impair the device’s ability to lock onto a clear signal. Multimode devices which have numerous wireless technologies operating simultaneously may experience jamming and coupling from cellular signals can also be disruptive.

A SAW filter, placed in front of the LNA helps protect against the effects of such noise as it will be efficiently filtered and rejected. GPS SAW filters make a GPS system more reliable, especially where interference is regularly encountered. In urban areas, or where a GPS antenna is located close to transmitting antennas they are especially advantageous. They require competent installation to ensure that only the center frequency is allowed through.

Managing insertion losses from SAW filters.

The weakness of GPS signals means that insertion loss from the inclusion of a SAW filter on the receiver’s RF line can be significant. The decision to include a SAW filter must balance these receiver chain insertion losses against the rejection of out-of-band signals that can be achieved.

The type of SAW filter used may affect insertion losses.

• SAW filters with Interdigital Transducers (IDTs) have electrodes that project perpendicular to the direction of propagation of the surface acoustic wave. This geometric arrangement is known to limit the performance of the filter, particularly by limiting its maximum bandwidth. The number of finger electrode pairs in an IDT has an inverse relationship with the fractional bandwidth of the filters. Larger bandwidth filters will have few IDT fingers but this leads to the generation of bulk waves and a significant rise in filter insertion loss.
• Slanted Finger Interdigital Transducer (SFIT)  filters are a type of SAW filter used in GPS applications that have lower insertion loss compare to the convention IDT design. They have a greater density of electrodes, due to their slanted orientation, meaning that they can also achieve a small shape factor.

The type of piezoelectric material and its preparation can also affect the insertion losses of a SAW filter. 

The position of a SAW filter in the receiver line also affects performance.

SAW filters are typically used in series with an LNA. Positioning the filter before the LNA input can prevent the LNA from becoming saturated with high-power jamming signals and enhance its selectivity for the target frequency. GPS filters can also be positioned after the LNA to suppress EMI and unwanted signals that may couple to the line.

Superior protection for mission-critical GPS applications

SAW filters provide robust protection in demanding environments where GPS reliability is essential, including:

• Fleet tracking systems
• Industrial IoT devices
• Emergency response equipment
• Co-located multi-radio systems

Compared to ceramic or LC filters, SAW filters offer:

• Better selectivity
• Smaller size
• Improved consistency

Installation and deployment considerations:

For optimal performance:

• Use quality coaxial cables with low loss
• Maintain proper impedance matching (50 ohm)
• Avoid placing GPS antennas near high-power transmit antennas
• Ensure correct grounding in vehicle or industrial installs

Improper installation can negate the benefits of SAW filtering.

Conclusion:

SAW filters are a fundamental component in modern GPS and GNSS antenna systems. By rejecting interference while preserving weak satellite signals, they significantly improve reliability, accuracy, and time to first fix.

Although they introduce some insertion loss, their role in protecting sensitive RF front ends—especially in active and multi-band antenna systems—makes them essential for real-world deployments.

Data Alliance integrates SAW filtering into its GPS antenna solutions to ensure dependable performance in challenging RF environments.