SAW filters are a key addition to the radio frequency front end of GPS devices and help ensure that while receivers are sensitive they can retain specificity by separating and rejecting unwanted signals and interference. This short article explains what SAW filters are and the use in GPS devices.
What is a SAW filter?
Surface Acoustic Wave (SAW) Filters are specialist electronic devices that are capable of filtering radio frequency signals. This means that they are either permeable to or capable of blocking, RF signals of a specific frequency.
SAW filters typically perform their filtering action by absorbing radio frequency energy and converting it into either acoustic or mechanical energy by using piezoelectric materials such as quartz, lithium niobate (LiNbO3), or lithium tantalite (LiTaO3). The exchange of energy takes place in a component called an Interdigital Transducer (IDT) that contains an array of electrodes that convert the electricity, generating Surface Acoustic Waves. These Surface Acoustic Waves then travel across the piezoelectric material which converts the energy back into electrical energy for dispersal.
SAW filters are compact and cost-effective units that are manufactured for a range of applications. SAW filters are typically used for microwave frequencies up to a maximum of 3 GHz, with the selectivity of the filter gradually declining at frequencies above 1.5 GHz. A SAW filter will have a center frequency but its efficacy and specificity can be affected by environmental conditions such as temperature. Non-Temperature Compensated (TC) filters often experience an increase in center frequency at lower temperatures and a decrease in center frequency at higher temperatures.
SAW filters for GPS devices and applications:
A GPS filter is a type of SAW filter that is used to filter out signals that interfere with GPS, enhancing the reception and overall performance of a receiver. They are usually Surface-Mounted Technology units and designed to pass only a specific frequency (e.g. 1575 MHz L1).
A typical SAW GPS filter will deliver a 20 MHz bandwidth around its center frequency, meaning that an L1 Filter that has a center frequency of 1575 MHz will have a passband of 1565 to 1585 MHz. The filter will reject interference and unwanted signals at 10 dB at 1525 MHz and at 1625 MHz the rejection will be 22 dB. Depending on the manufacturer, example insertion losses and return losses for a SMT GPS filter can be around 0.9 dB and 14 -dB respectively.
GNSS receivers are already capable of processing received signals to remove unwanted elements and amplify the satellite-based signals. Part of the process of acquiring a GPS “fix” is separating the GPS signal with a code that is present in the GPS satellite broadcast.
However, the presence of radio frequency interference causes the Time To First Fix (TTFF) to be delayed. The interaction of the GPS broadcast signal with the ionosphere and atmosphere it passes through on the way to the receiver can produce distortions and delays. A GPS filter separates out these unwanted frequencies and makes the true signal clearer.
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 are a critical enhancement to GPS receivers used in emergency settings where the performance of GPS devices needs to be reliable and robust. SAW filters have been demonstrated to out-perform ceramic filters and LC filters, making them a routine addition to GPS receivers deployed in challenging settings.
Posted by George Hardesty on 23rd Mar 2021