Frequency Jammers
Cell phone users today can choose between several cell phone service providers who use a range of networks, such as GSM, EDGE and UMTS, to provide services. Table 1 illustrates the frequencies used in current cell phone networks to provide services. Transmit frequencies range from 380 MHz to 2570 MHz, and receive frequencies range from 390 MHz to 2690 MHz, while the GPS signal is at 1575.42 MHz. As seen in Table 1, there are currently gaps in the spectrum used by cell phones -- between 1465 MHz and 1710 MHz in the transmit band and between 1513 MHz and 1805 MHz in the receive band. The GPS signal falls into this gap; however, there is still a lot of noise in this area of the spectrum, and the noise bandwidth at the GPS receiver's input needs to be reduced to prevent the sensitivity of the receiver from being degraded.

To decrease the noise bandwidth, filtering is added to the input of the GPS receiver. Most GPS modules ready to be integrated into electronic devices contain a filter, such as a surface acoustic wave (SAW) filter, and an accompanying low noise amplifier within the module. Due to the physical proximity of the cell phone and GPS antennas, the noise level at the GPS input is much higher than it would be for a standalone GPS unit, so an additional filter may be required. The addition of the SAW filter decreases the effects of the noise on the receiver's sensitivity. The filter's insertion loss will increase the noise figure of the GPS receiver, but, if implemented properly, it should have the overall desired affect.

SAW filters are typically the filter of choice, as they are small, perform well and are readily available. As shown in Figure 1, typical filter dimensions are 1.2 X 1.4 X.46 mm[3].

SAW filters designed for GPS signals can be realized with insertion loss performance as low as 1.2 dB, thus minimizing the degradation to noise figure and receiver sensitivity. Other filters that show promise in the future are film bulk acoustic resonator (FBAR) and bulk acoustic wave (BAW) filters. FBAR quintplexers are currently available for the GSM network. The quintplexer has separate ports for the PCS receive band, the PCS transmit band, the cellular receive band, the cellular transmit band, the GPS and a common antenna[4]. With careful circuit board layout, this device could be used in a highly-compact design for a GSM phone with an integrated GPS module.

Layout considerations
Proper layout is critical for the successful design of a system integrating a GPS and a cell phone. The GPS receiver is highly sensitive and may need to decode very low power signals in the order of -150 dBm[2]. This sensitivity could make it susceptible to noise generated within the integrated system. Such noise should be minimized to the extent that it does not interfere with the GPS. Compartmentalizing sections with shields can help eliminate some noise. For this to be successful, it is important that the shields have intimate contact to ground. EM modeling should be performed with commercially available 2.5D and 3D simulators to verify that the ground ring to which the shield is connected is actually a ground for the desired frequencies. Careful simulation of the ground ring, choice of via size and via location ground layer choice, and interconnect to the main power source ground will lead to a robust shield scheme.

The choice of how to layout ground is also an important consideration. Often, top-side grounds are used along with one or multiple internal ground layers. Top-side grounds can be very helpful when trying to isolate signals that are routed on the board surface. The same thorough approach that was applied to the shield grounding ring would be helpful in ensuring the top-side ground is performing the desired task. Figure 2 shows a printed circuit board (PCB) stack-up with multiple internal grounds.

Typically, the ground on a circuit board is a fairly good current return path. To expect this return path to have no voltage potential differences over its area is a bit optimistic. Even on small PCBs, voltage potential differences can be observed many times. For this reason, PCBs with RF transceiver and digital processors are often fabricated with separate grounds for each. If only one type of circuit is on the PCB there is a possibility that one ground plane is enough

Multiple internal grounds are typically used in two fashions. One implementation is to have more than one layer in the PCB stack-up, to pick up a common ground. The other is to have ground planes on one layer of the PCB stack-up separated from each other and joined only at the main power source ground terminal. If multiple layers are used as a common ground, there cannot be any voltage potential between the grounds or a noise path will be created.

As mentioned above, the separation can take the form of separate layers in the PCB stack-up or a ground layer that has been separated into a digital and an RF section. In this case, the object is not to try to achieve a perfect common ground but rather to isolate the noise generated by a particular type of part and keep it from interfering with another type of part. The grounds then connect to the main power source ground terminal. The main power source ground terminal is the only point where the separated grounds contact each other. This technique could be helpful with isolating power supply noise and keeping it away from the GPS unit's power supply. For example, the power supply noise could easily be generated by a GSM phone when it transmits. Some GSM phones may draw 2.2A during the 577μS that they are transmitting [1]. Lithium-Ion batteries perform well as a source for the system, but a fairly large capacitor may be needed to act as a local current well. However, given the cell phone's small form factor it can be difficult to get a capacitor of a large enough value while also maintaining a small foot print. As a result, there will be a small drop of a few tenths of a volt on the power rail over the transmit duration. If there is resistance between the ground plane used by the phone and the ground terminal of the main power source, the ground plane used by the phone can become noisy. Keeping the ground planes separate would be very helpful to isolate and reduce noise interference.

The dip in voltage in the power plane being fed from the battery when a TDD phone transmits is a significant amount of noise on the power plane and could be detrimental to the performance of GPS. Utilizing a dedicated GPS power plane, either as a dedicated layer in the board stack-up or as an isolated section of a layer that is fed from a dedicated regulator, will help keep the power rail of the GPS clean.

Bypassing
Issues can also arise if the power supply lines to the GPS are not properly bypassed. Prudent circuit board layout may help simplify the bypassing scheme, but it is not always possible to route all the noisy signals away from sensitive components. Researching and determining self resonant frequency (SRF) and ESR values is important when selecting bypass capacitors. There are several online resources available to aid in this type of investigation, such as the free downloadable program available on Murata's Web site [5], as shown in Figure 3. Generated by Murata's online program, the chart shows a S21 plot of capacitors that are configured to shunt signals to ground.

Choosing the capacitors that are most effective at attenuating undesired signals can help reduce or eliminate the unwanted signal on the power rail.

Conclusion
Integrating devices into a small form factor system that can produce relatively large RF outputs signals, with devices that are designed to receive relatively low RF signals, can be a challenging task. This is the situation facing designers working to integrate GPS receivers integrated into cell phones. Filter selection, shielding and bypassing all help to minimize or resolve these issues. A comprehensive understanding of the potential interference signals can help with filter selection, which decreases the noise bandwidth. A careful design of the grounding layout on the PCB enables effective shield design to help reduce noise. The grounding layout is also important for bypass implementation. To bypass effectively, the frequency limitations of the capacitors must be understood. To do this, designers must understand the possible frequencies that may propagate through a PCB and create noise on the power rails, so that values for bypass capacitors can be identified. Starting with clean power rails is another effective way to reduce noise interference. Separating the GPS power plane from other power planes that are inherently noisy is a good start. By implementing these approaches in the design phase, designers can reduce the amount of time resources spent on identifying noise sources and reducing noise to acceptable levels once the PCB has been built.

References
[1] SAGEM, SCT TMO MOD SPEC 553 E, Date 23/11/04 Section 1.6 Product Features Page 8/61
[2] Locosys Data Sheet of Locoys GPS Module, Ver 1.1 Section 4 GPS receiver page 3/28
[3] SAWTEK , Data Sheet of 856561, 1575.42MHZ SAW Filter Electrical Specifications Rev A 06-Mar-2006, page 2 of 6

[4] AVAGO, Data Sheet ACFM7101, PCS\Cellular\GPS Quintplexer
[5] http://www.murata.com/designlib/mccdl/index.html

About the Author
Bob Bernert is the lead wireless engineer at Tessera. He received a Bachelor of Technology in Electrical Engineering degree from the Rochester Institute of Technology and an MSEE degree from California State University Northridge. bbernert@tessersa.com

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