Connecting an automobile to the constellations that comprise GNSS requires an antenna and some very sensitive and specialized receivers. Each vehicular system has its own GNSS receiver, and those receivers supply GNSS coordinates to their respective systems, data which is utilized throughout the cabin. Keeping each receiver free from potential noise floor issues, spurious emissions or LO leakage from an adjacent receiver is paramount to the success of the system. Mini-Circuits offers a solution that not only isolates the receivers from one another, but is low-loss, tiny, rugged and reliable. Read on to find out how Mini-Circuits isolates RF front ends when connecting GNSS signals to multiple vehicular systems.
GNSS in Modern Day Vehicular Systems
GNSS (Global Navigation Satellite System) is really a catch-all acronym for the entire set of satellite constellations that provide global positioning, navigation and timing. As GNSS-derived positional accuracy has improved over the years from meters to centimeters (see Figure 1), the range of possible applications in automotive has greatly expanded and continues to do so.
When the vehicle is operational and in motion, GNSS data is concurrently utilized for navigation, vehicle tracking, a myriad of ADAS systems (e.g. lane position, adaptive cruise, collision avoidance), and V2X, telematics, eCall, and more. If the vehicle is involved in a crash, it is often turned off yet must continue to utilize GNSS to provide data (coordinates) automatically to emergency services. Additionally, when turned off and theft is detected, the vehicle must engage its tracking protocols to assist with recovery. It is important that these emergency scenarios remain isolated from the many operational applications that already use GNSS data when the vehicle is turned on and in motion.
The objective is to determine how multiple vehicular systems can operate from a single GNSS antenna while remaining isolated from one another.
GNSS Use Cases –Feeding Multiple Systems Separately and Simultaneously
To operate systems that depend upon GNSS simultaneously, two or more GNSS RF inputs must be derived from a single feed/antenna so as not to increase the complexity and the size, weight, power and cost (SWaP-C) of the cabling in the process. In implementing the solution itself, SWaP-C is of paramount importance.
The most straightforward way of deriving a pair of nearly identical outputs from a single RF input is by using a power divider. Since the critical constellations (GPS, Galileo, etc.) fall approximately within the 1.1-1.6 GHz frequency band (L1 is 1.559-1.61 GHz, L2 is 1.215-1.254 GHz, and L5 is 1.164 to 1.214 GHz), the design shown in Figure 2 utilizes the AECQ-200-qualified SCG-2-242AT+ in-phase power divider with an external 0603 100Ω resistor. This RF power splitter operates from 1-2.4 GHz, a bandwidth that easily covers all the required L1/L2/L5 GNSS signals.
Figure 2 illustrates how the SCG-2-242AT+ in-phase power divider furnishes isolated signals to those functions necessary with the ignition turned ON (orange) or those necessary when the ignition is ON or OFF (light blue). In the GNSS frequency range of 1.1-1.6 GHz, the typical loss is approximately 3.5 dB or less as shown in Figure 3. The amplitude unbalance, illustrated by the difference between the two insertion loss curves, is typically less than 0.1 dB. Also in Figure 3, the isolation between the two GNSS RF outputs ranges from 13 to 26 dB (from a factor of 20 to 400), and the return loss of each of the respective ports is excellent.
Figure 3: Plots of (a) insertion loss from Sum-to-Port 1 and Sum-to-Port 2, (b) isolation from Port 1 to Port 2, and (c) return loss of all ports
The SCG-2-242AT+ splits the GNSS antenna input and achieves excellent electrical performance. GNSS signal levels are often very weak, so there is little room in the link budget for resistive splitting (-6 dB theoretical) or other exotic solutions when a traditional 3 dB in-phase power divider can be utilized. Additionally, since GNSS signal levels are very weak, the signal-to-noise ratio at the vehicle is quite low, and additional noise cannot be tolerated at the inputs to the sensitive receivers. Having sufficient isolation prevents LO leakage and spurs from one receiver from contaminating another. The low loss, low amplitude unbalance and high isolation of the SCG-2-242AT+ each plays a role in ensuring that the disparate systems onboard the vehicle that utilize GNSS remain as clean and as free from desensitization of one another as possible.
Tiny and Tough RF Power Divider – No Temperature Too Extreme
While RF/electrical performance has proven to be excellent, the mechanical properties are also too impressive to overlook. The power divider is a Low Temperature Co-fired Ceramic (LTCC) component that comes in the GE0805C-1 package, measuring only 0.08 x 0.05”. The 100Ω resistor is external and is a mere 0603 chip resistor. This volumetrically efficient design can fit virtually anywhere on a crowded circuit board and has very low mass, satisfying most tight SWaP-C requirements. Power handling is 2W although that is rarely needed for a receive side application. As far as the cost end of SWaP-C, rest assured that the SCG-2-242AT+ is competitively priced for high-volume automotive applications.
As if the RF and mechanical performance weren’t compelling enough, the SCG-2-242AT+ is an LTCC part, monolithic, rugged and reliable, perfectly suited for harsh operating environments and wide temperature extremes (-40 to +105⁰C).
A Treasure Trove of Components – AEC-Q200-Ready and Available for Qualification
Mini-Circuits found the power splitter application in this article to be right in our wheelhouse, and we’re confident that across the spectrum of our portfolio, many additional components are capable of satisfying even more demanding automotive requirements.
Nearly 80 components are already AEC-Q200 qualified, which includes filters, diplexers, 90° hybrid splitters, baluns and custom solutions, and more than 10 times that many are available for qualification. Furthermore, the ways in which we can furnish custom designs for automotive are limitless. This level of component readiness along with our global presence (offices in the US, EU, UK, Taiwan, South Korea, China and Japan) makes Mini-Circuits an ideal partner for the automotive market space of today and well into the future.
