Telit Wireless Solutions SL869T3-I, Руководство пользователя

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SL869T3-I Product User Guide
1VV0301546 Rev. 3
Page 42 of 68 2021-03-31
The most common source of interference is digital noise, often created by the fast rise
and fall times and high clock speeds of modern digital circuitry. For example, a popular
netbook computer uses an Atom processor clocked at 1.6 GHz. This is only 25 MHz away
from the GNSS signal, and depending upon temperature of the SAW filter, can be within
its passband. Because of the nature of the address and data lines, this would be
broadband digital noise at a relatively high level.
Such devices are required to adhere to a regulatory standard for emissions such as FCC
Part 15 Subpart J Class B or CISPR 22. However, these regulatory emission levels are far
higher than the GNSS signal.
9.9. Shielding
Shielding the RF circuitry generally is ineffective because the interference is received by
the GNSS antenna itself (which is the most sensitive portion of the RF path). The antenna
cannot be shielded because it could not then receive the GNSS signals.
There are two solutions, one is to move the antenna away from the source of interference,
and the other is to shield the digital interference source to prevent it from getting to the
antenna.
9.10. Cellular Coexistence
Depending upon which cellular standard is being used, the cellular module can output up
to +33 dBm of power at the transmit frequency. Placing both the GNSS and cellular
antennae on the same board will typically yield approximately 30 dB of isolation between
the two antennae, which results in the SL869T3-I having no more than +5 dBm applied to
its RF input pin.
9.11. Powering an External LNA (or Active Antenna)
An external LNA requires a source of power. Many active antennas accept a 3 V or 5 V DC
voltage that is impressed upon the RF signal line.
Two approaches can be used:
1. Use an inductor to tie directly to the RF trace. This inductor should be at self-
resonant at L1 (1.57542 GHz) and should have good Q for low loss. The higher the
inductor Q, the lower the loss will be. The side of the inductor connecting to the
antenna supply voltage should be bypassed to ground with a good quality RF
capacitor, again with self-resonance at the L1 frequency.