Designing a Coin-Sized Solar-Powered GPS Receiver
Contributed By Digi-Key's European Editors
GPS technology is becoming more and more integrated with low-power modes that mean tiny receivers can now be powered by a solar cell. One example is Retrievor, a collaboration of American, Australian, British, and Chinese companies that is raising funding via crowd sourcing to develop a coin-sized GPS tracking device. A small, self-powered GPS system can be used to track valuable items and even pets, using Android and Apple iOS apps to provide the location information.
The Retrievor unit measures 28 mm (1.10") in diameter and 10 mm (0.39") thick, integrating the antenna into the module to keep the size down. It uses the SiRFstarIV GPS processor that enables operation in challenging GPS environments, such as indoor tracking or when the end-user is on the move. This high level of GPS performance is achieved by using innovative GPS firmware, which can detect changes in context, temperature, and satellite signals, and updates its internal data whenever there is the opportunity so that it provides near-continuous navigation.
Power for the Retrievor comes from an integrated solar panel and motion charger feeding a 3.7 V lithium-ion battery, which can also be charged via micro USB. User-defined ping rates can be adjusted from every second through to once per day so the Retrievor may never need a recharge.
Building a GPS receiver
Highly-compact modules that combine the RF and antenna can be combined with energy-harvesting transducers and power management to provide the same kind of small system that is independent of power sources. With careful attention to the power budget of the system, solar cells such as those from Sanyo can provide all the power requirements. With these small form-factors, it is also vitally important to avoid problems with RF layout that can drain power and render the energy-harvesting source insufficient.
The M10478 from Antenova is a highly-integrated GPS RF Antenna Module suitable for L1-band GPS and A-GPS systems. The device is based on the same SiRFstarIV GPS architecture as used in the Retrievor, combined with Antenova’s high-efficiency antenna technology, and is designed to provide an optimal radiation pattern for GPS reception.
All front-end and receiver components are contained in a single package laminate base module, providing a complete GPS receiver for optimum performance. The M10478 operates on a single 1.8 V positive supply with low power consumption and several low-power modes for further power savings, allowing it to be powered by a 3.7 V lithium battery that is supplied by the solar cell. An accurate 0.5 ppm TCXO ensures a short time to first fix (TTFF) for mobile applications, and the module is supported by stand-alone software and is compatible with UART, SPI, and I²C host-processor interfaces.
Trickle Power (TP)
The module includes a ‘trickle power’ mode that reduces the power and makes energy-harvesting sources viable. The device enters a duty-cycle mode to reduce the average current consumption, but retains high accuracy and performance so that it can track weak signals.
Typically, under normal conditions, TP mode runs in full power for 100-900 ms and provides a fix, followed by a 1-10 second interval of a low-power standby state. Once in a while (typically every 1800 seconds) the module will return to full-power mode to update the ephemeris data.
When in TP, if the signal conditions are harsh (below 30 dB-Hz) the module will automatically switch to full-power mode to improve the navigation performance. When conditions return to normal, the module will return to TP mode. This results in variable power savings but, for a fixed output rate, much more reliable performance. Applications using TP Mode perform similarly to applications using full power, but with significant power savings in strong-signal conditions.
For designs where the antenna has to be added separately, the SG Series GPS receiver module from Linx Technologies is a self-contained high-performance GPS receiver with an on-board LNA and SAW filter. Based on the SiRFstar III chipset, it provides high sensitivity and the very-low power consumption helps maximize runtimes in energy-harvesting applications. With over 200,000 effective correlators, the SG series receiver can acquire and track up to twenty satellites simultaneously in just seconds, even at the lowest signal levels.
Housed in a compact reflow-compatible SMD package, the receiver requires no programming or additional RF components (except an antenna) to form a complete GPS solution. Five GPIOs are easily configured through simple serial commands, which, along with the module’s standard NMEA data output, make it easy to integrate, even by engineers without previous RF or GPS experience. The GPS core handles all of the necessary initialization, tracking, and calculations autonomously, so no programming is required. The RF section is optimized for low-level signals, and requires no production tuning of any type.
By default, the SG series will operate in full-power mode, but it also has a built-in power control mode called Adaptive Trickle Power mode when using an energy-harvesting source.
The SG series module is designed to use a wide variety of external antennas. The module has a regulated power output which simplifies the use of GPS antenna styles, which require external power. This allows the designer great flexibility, but care must be taken in antenna selection to ensure optimum performance.
A tiny portable device may be used in many varying orientations so an antenna element with a wide and uniform pattern may yield better overall performance than an antenna element with high gain and a correspondingly narrower beam. Conversely, an antenna mounted in a fixed and predictable manner may benefit from pattern and gain characteristics suited to that application. Evaluating multiple antenna solutions in real-world situations is a good way to rapidly assess which ones will best meet the needs of the application.
For GPS, the antenna should have good right-hand circular polarization characteristics (RHCP) to match the polarization of the GPS signals. Ceramic patches are the most commonly used style of antenna, but there are many different shapes, sizes and styles of antennas available. Passive antennas are simply an antenna tuned to the correct frequency while active antennas add a Low Noise Amplifier (LNA) after the antenna and before the module to amplify the weak GPS satellite signals, but take more power than may be available from the energy harvesting source, as the VOUT line provides 2.85 V at 30 mA to power the external LNA.
Maintaining a 50 Ω path between the module and antenna is critical as errors in layout can significantly impact the module’s performance. The module’s design makes integration straightforward; however, it is still critical to be careful in PCB layout. Failure to observe good layout techniques can result in a significant degradation of the module’s performance, driving up the power consumption as the chip compensates for the lower performance.
A primary layout goal is to maintain characteristic 50 Ω impedance throughout the path from the antenna to the module. The module should, as much as reasonably possible, be isolated from other components on the PCB, especially high-frequency circuitry such as crystal oscillators, switching power supplies, and high-speed bus lines and where possible, having separate RF and digital circuits in different PCB regions.
It is important not to route PCB traces directly under the module, which can be a challenge when designing such a small system. There should not be any copper or traces under the module on the same layer as the module, just bare PCB. The underside of the module has traces and vias that could short or couple to traces on the product’s circuit board.
Ideally, a large, uninterrupted ground plane should be placed on a lower layer opposite the module to create a low impedance return for ground and consistent stripline performance. Keeping the trace as short as possible and not passing under the module or any other component will help, as routing the antenna trace on multiple PCB layers using vias will add inductance. Instead, multiple vias should be used to tie together ground layers and component grounds.
For a small design, it is common to encapsulate the product, and there is a wide variety of potting compounds with varying dielectric properties. Since such compounds can considerably impact RF performance and the ability to rework or service the product, the designer has to take care in the choice and qualification of such material.
The module is designed to work with a backup battery that keeps the SRAM memory and the RTC powered when the RF section and the main GPS core are powered down. This enables the module to have a faster TTFF when it is powered back on. The memory and clock consume about 10 μA, making the solar cell a viable power source.
This also means that a small lithium battery is sufficient to power these sections. This significantly reduces the power consumption and extends the main battery life while allowing for fast position fixes when the module is powered back on.
One of the issues with energy harvesting is that the module requires a clean, well-regulated power source with noise less than 20 mV as the power supply noise can significantly affect the receiver’s sensitivity.
Devices such as the LTCR3108 from Linear Technology, are highly-integrated DC/DC converters optimized for harvesting and managing surplus energy from extremely-low input voltage sources. This allows it to take energy from small solar cells for a coin-sized GPS receiver as the step-up topology operates from input voltages as low as 20 mV.
The LTC3108 presents a minimum input resistance (load) in the range of 2 Ω to 10 Ω, depending on input voltage. As the input voltage drops, the input resistance increases and this allows the LTC3108 to optimize power transfer from sources with a few ohms of source resistance. A lower source resistance will always provide more output current capability by providing a higher input voltage under load.
It does this by adding a unique fixed VOUT option to a well-established architecture. The 2.2 V LDO powers an external microprocessor, while the main output is programmed to one of four fixed voltages to power the GPS receiver.
The LTC3108 also manages the charging and regulation of multiple outputs in a system in which the average power draw is very low, but there may be periodic pulses of higher load current required when the GPS receiver is polled for its location. The power manager is based around a MOSFET switch that forms a resonant step-up oscillator using an external step-up transformer and a small coupling capacitor. This allows it to boost input voltages as low as 20 mV high enough to provide multiple regulated output voltages for powering other circuits. The frequency of oscillation is determined by the inductance of the transformer secondary winding and is typically in the range of 10 kHz to 100 kHz.
The main output voltage on VOUT is charged from the VAUX supply and is user programmed to one of four regulated voltages using the voltage select pins VS1 and VS2. Although the logic threshold voltage for VS1 and VS2 is 0.85 V typical, it is recommended that they be tied to ground or VAUX.
When the output voltage drops slightly below the regulated value, the charging current will be enabled as long as VAUX is greater than 2.5 V. Once VOUT has reached the proper value, the charging current is turned off. The internal programmable resistor divider sets VOUT, eliminating the need for very-high-value external resistors that are susceptible to board leakage and consume power.
The power good comparator also monitors the VOUT voltage. This is an open-drain output with a weak pull-up (1 MΩ) to the LDO voltage and goes high once VOUT has charged to within 7.5% of its regulated voltage. If VOUT drops more than 9%, PGOOD will go low to signal the microprocessor, and it is designed to drive a chip I/O, not drive a higher current load such as an LED. The PGOOD signal can also be used to enable a sleeping microprocessor or other circuitry when VOUT reaches regulation.
The VOUT2 output can be turned on and off by the host, using the VOUT2_EN pin. When enabled, VOUT2 is connected to VOUT through a 1.3 Ω P-channel MOSFET switch. This output, controlled by a host processor, can be used to power external circuits such as sensors and amplifiers that do not have a low-power sleep or shutdown capability.
Minimizing the amount of decoupling capacitance on VOUT2 will allow it to be switched on and off faster, allowing shorter burst times and, therefore, smaller duty cycles in pulsed applications such as a wireless sensor/transmitter. A small VOUT2 capacitor will also minimize the energy that will be wasted in charging the capacitor every time VOUT2 is enabled. VOUT2 has a soft-start time of about 5 µs to limit capacitor-charging current and minimize glitching of the main output when VOUT2 is enabled. It also has a current-limiting circuit that limits the peak current to 0.3 A typical. The VOUT2 enable input has a typical threshold of 1 V with 100 mV of hysteresis, making it compatible with logic.
Dedicated GPS chip
Another option is to use a dedicated GPS chip rather than a module. The MAX2741 L1-band GPS receiver IC has a total voltage gain of 80 dB and a 4.7 dB cascaded noise figure for receiver sensitivity in applications requiring -185 dBW for indoor tracking solutions.
This dual-conversion receiver downconverts the 1575.42 MHz GPS signal to a 37.38 MHz first IF, and then a 3.78 MHz second IF. An integrated 2- or 3-bit ADC (1-bit SIGN, 1- or 2-bit MAG selectable) samples the second IF and outputs the digitized signals to the baseband processor. The integrated synthesizer offers the flexibility in frequency planning to allow a single board design to be employed for reference frequencies from 2 MHz to 26 MHz. The integrated reference oscillator allows either TCXO or crystal operation.
The receiver runs from a 2.7 V to 3.0 V supply, and draws only 30 mA when active, allowing an energy-harvesting source to be used. It is available in a 28-pin thin QFN package, and is specified for -40°C to +85°C at 3 V for such small, highly-integrated receiver designs.
With the latest GPS and power management ICs, modules and antenna designs, it is increasingly possible to develop ultra-small form-factor devices that are self-powered from the environment. Using solar cells to charge lithium batteries, interfaced by dedicated power management devices, allows GPS receivers to be used in many more places, opening up exciting new applications.
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