MAX-M10M - Integration manual
Parameter
Immunity level (dBm)
-28
-45
-28
-30
-25
-25
-28
-15
-10
Table 31: MAX-M10M out-of-band immunity for the low-gain mode at selected frequencies.
4.3.3 Out-of-band rejection
RF interference is typically first coupled into the antenna and subsequently conducted into the
receiver input. Typical out-of-band interference sources include transmitting antennas of other
radio systems.
Estimation of the RF interference level coupled into the receiver antenna is a starting point for
RF front-end design. For designs with other radio systems, the maximum power coupled into the
antenna can be estimated from the maximum transmission power and the isolation between the
antennas. Practical values for antenna isolation can range from 15 - 20 dB down to 6 - 10 dB for very
small devices. RF interference may also couple from external sources such as nearby mobile devices
or base stations.
A simplified test board can be used to estimate the isolation between two antennas. The size
of the board and the placement of the antennas must match the final design. Connect the RF
cables to the antenna inputs and measure S21 over the frequency band of interest with a vector
network analyzer (VNA).
The required out-of-band rejection or isolation is the difference of the maximum power coupled into
the antenna input terminal and the immunity level of the receiver RF input. The required isolation
is realized with appropriate filtering, typically with one or two SAW filters. Amplification on the RF
path reduces the out-of-band rejection and needs to be considered in filter selection. The type and
number of filters are selected based on the estimated interference level and the immunity of the
receiver.
RF interference from other parts of the design is more difficult to estimate. One option is to measure
the interference level at the receiver input using a spectrum analyzer. Interference within the design
is primarily a problem at the receiver in-band, where it cannot be addressed by filtering on the RF
path. Outside the GNSS band, the required filtering is determined by the estimated interference
level and the immunity of the receiver.
4.3.4 Antenna power supply
An active antenna supply network to connect antenna supply to the RF signal line is shown in
. The inductance L3 connects the antenna power supply to the RF signal line. The capacitance
C14 filters out high-frequency interference from the power supply and the resistor R8 limits the
short-circuit current.
The type and value of L3 is selected to have a resonance peak at GNSS frequencies. This provides a
high series impedance above 500 Ω at GNSS L1 frequencies, creating an impedance mismatch with
respect to the 50 Ω RF signal line. This minimizes the effect of the feed point on the RF signal line,
and isolates the antenna supply from the RF signal line at GNSS frequencies. Both R8 and L3 must
have sufficient current and power rating to withstand the short-circuit current. Example component
values for the antenna supply network are given in
, and
.
The
can be used to detect open and short circuits on the antenna supply
network and disconnect the antenna supply if a short circuit is detected.
UBX-22038241 - R02
4 Hardware integration
Page 70 of 92
C1-Public
