RF interference rhreats to GNSS signal clarity
The primary sources of interference are LTE and 5G mobile networks, plus Iridium, Globalstar, and to some extent, the Inmarsat uplink frequencies (1626.5 MHz to 1660.5 MHz).
The amplitude of potential received interferrors is a function of the frequency and distance between a potential transmitter and the GNSS antenna. For example, in North America, a 5G network is planned in a lower segment of the Inmarsat downlink (1525 MHz to 1536 MHz), and it is quite conceivable that a GNSS antenna could be physically close to an operating smartphone or handset. This new 5G network especially threatens L-band corrections services broadcast (1539 MHz to 1559 MHz).
GNSS RF Interference Sources
Most commonly, in-band interference results from distortion of high-amplitude signals due to non-linearity in the antenna low noise amplifier (LNA), or occasionally, from a jamming device used to disable tracking. In either case, once present, there is no filter, digital or analog, that can filter it out; prevention is the only cure.
Strong out-of-band signals can generate in-band interference by saturating the antenna LNA or by cross multiplication with other strong signals. For example, a single strong signal at 800 MHz could cause a “comb” of harmonics, each offset by the signal carrier frequency, including a harmonic signal at 1600 MHz, which falls close to the center of the GLONASS-G1 band.
Calian XF antennas will continue to provide pure GNSS reception in both low and high GNSS bands (1165 MHz to 1300 MHz; 1540 MHz to 1610 MHz) in the presence of interfering signals that are up to 90 dB stronger than the wanted -120 dBm GNSS signals, offset just a few tens of MHz from the band edge.
Mitigating radio frequency interference
In-band interference solutions
Preventing in-band interference in antennas involves addressing the non-linearity caused by high out-of-band signals. Achieving higher linearity in Low Noise Amplifiers (LNAs) often leads to increased bias currents and supply voltages. However, a more effective approach involves using RF filters to attenuate these high amplitude signals, ensuring LNAs operate as linearly as possible.
RF filter design challenges
Designing ideal bandpass RF filters is a complex task. These filters aim to be completely linear, with 0 dB insertion loss, sharply defined band edges, and steep skirts for deep rejection near the passband edges. However, in practical applications, filters often have insertion losses ranging from 0.8 dB to 3.0 dB, with less sharp corners at band edges and a finite roll-off, presenting unique challenges in filter design.
LNA evolution for interference
The design of antenna Low Noise Amplifiers (LNAs) has historically included one or more passband RF filters, typically after the first or second amplifier stages. This configuration was optimal for maximizing the signal-to-noise ratio without contributing to the noise figure. However, with the emergence of high-amplitude out-of-band signals, this design is now susceptible to front-end saturation, demanding new strategies to maintain antenna performance.
Calian XF products have high-linearity front-end amplifiers with additional pre-filters for each frequency band, right at the antenna element feed, integrated with the diplexer for dual/triple-band antennas and with additional in-line filters.
This has the advantage that the first and subsequent amplifier stages are protected, but at the cost of a somewhat higher noise figure because of the pre-filter insertion loss. There is no free lunch! Single-band antennas also feature a pre-filter, but the diplexer is not required.
Trade-offs in gnss amplification: pre-filters and antenna considerations
The necessity to use different filters to cover the various GNSS bands results in unequal group delays between the bands, introducing an effect known to the reference antenna community as Differential Code Bias (DCB).
The necessity to use different filters to cover the various GNSS bands results in unequal group delays between the bands, introducing an effect known to the reference antenna community as Differential Code Bias (DCB).
Calian uses SAW filters in the Accutenna® and Helical antenna lines and a combination of dielectric and SAWs RF filters in our high-end antennas (VeroStar™, VeraPhase®, and VeraChoke®). Multiple dielectric filters are also used for very high rejection triple-band members of the Accutenna line (TW3972XF, TW3967XF, etc.).
Calian's eXtended Filtering (XF) Line
XF filtering enables the antennas to produce clean and pure GNSS reception. The deep XF filter technology is currently available in the TW3000 family and soon available in all Calian’s product lines.
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