Understanding RF Front End for Better GPS Satellite Detection Sensitivity
The RF Front End contains five elements that can be manipulated to achieve better sensitivity: Low Noise Amplifier, Filters, Down Convertor, Clock, and the RF Circuit Layout.
The Low Noise Amplifier (LNA) is responsible for reducing the internal noise in the GPS receiver. LNAs available as standalone chips are more effective than the ones integrated into the RF Front End Circuit. Hence, the first step towards better GPS satellite detection sensitivity is to select an LNA based on its gain factor.
Filters are used to weed out unwanted noise in the RF Front End. As the signal passes through the Filters, the unwanted noise is cut off (based on the L1 signal frequency [pass band = 2 to 20 MHz] available to the general public against the L2 available only to military). For higher frequencies, the normal LC filters are okay. If the L1 used by GPS provider is lower, SAW filters are better. In any case, the manufacturers need to make sure that these filters do not create any loss of important information while trying to filter off the noise.
The Down Convertor has to divide the signals into lower intermediate frequencies before sending them to the DSP. It determines the signal processing parts of the DSP. Also, the Filters in the RF Front End are dependent on the Down Convertor. Hence, it should be selected carefully. The output from the Down Convertor is also an important factor that determines the GPS satellite detection sensitivity. A two bit output offers more GPS sensitivity than a single bit output.
The Clock inside the RF Front End also plays an important role in increasing the GPS satellite detection sensitivity. Other than clock errors, the GPS device manufacturer should also make sure that the clock offers fewer jitters and is highly stable. An unstable clock will contribute to Doppler thereby highly altering the GPS sensitivity.
Finally, the overall circuit of the RF Front End has to be considered. As the circuit deals with both analog and digital signals, ample care must be taken to ensure that none of the signal affects the other. Similarly, while creating parallel paths, there must be sufficient gap to avoid interference. Considering the size of GPS devices, it is slightly hard to find ample space, but a properly researched design can reduce interference of signals to protect and offer greater GPS sensitivity.