SE873 Family Product User Guide
RF Signals
1VV0301216
Rev.4
Page 44 of 69
2018-08-24
An antenna vendor should specify a nominal antenna gain (usually at zenith, or directly overhead)
and antenna pattern curves specifying gain as a function of elevation, and gain at a fixed elevation
as a function of azimuth. Pay careful attention to the requirement to meet the required design,
such as ground plane size and any external matching components. Failure to follow these
requirements could result in very poor antenna performance.
It is important to note that GNSS antenna gain is not the same as external LNA gain. Most antenna
vendors will specify these numbers separately, but some combine them into a single number.
Both numbers are significant when designing the front end of a GNSS receiver.
For example, antenna X has an antenna gain of 5 dBic at azimuth and an LNA gain of
20 dB for a combined total of 25 dB. Antenna Y has an antenna gain of -5 dBic at azimuth and an
LNA gain of 30 dB for a combined total of 25 dB. However, in the system, antenna X will
outperform antenna Y by about 10 dB (Refer to
for more details on
external LNA gain).
An antenna with higher gain will generally outperform an antenna with lower gain. However, once
the signals are above about -130 dBm for a particular satellite, no improvement in performance
would be realized. But for those satellites with a signal level below about -135 dBm, a higher gain
antenna would amplify the signal and improve the performance of the GNSS receiver. In the case
of really weak signals, a good antenna could mean the difference between being able to use a
particular satellite signal or not.
System Noise Floor
The receiver will display a reported C/No of 40 dB-Hz for an input signal level of -130 dBm. The
C/No number means the carrier (or signal) is 40 dB greater than the noise floor measured in a
one Hz bandwidth. This is a standard method of measuring GNSS receiver performance.
The simplified formula is:
C/No = GNSS Signal level – Thermal Noise – System NF
Equation 9-1Carrier to Noise Ratio
Thermal noise is -174 dBm/Hz at 290K.
We can estimate a system noise figure of 4 dB for the module, consisting of the pre-select SAW
filter loss, the LNA noise figure, and implementation losses within the digital signal processing
unit. The DSP noise is typically 1.0 to 1.5 dB.
However, if a good quality external LNA is used, the noise figure of that LNA (typically better than
1dB) could reduce the overall system noise figure from 4 dB to approximately 2 dB.
PCB stack and Trace Impedance
It is important to maintain a 50
Ω
impedance on the RF path trace. Design software for calculating
trace impedance can be found from multiple sources on the internet. The best method is to contact
your PCB supplier and request a stackup for a 50
Ω
controlled impedance board. They will give
you a suggested trace width along with PCB stackup needed to create the 50
Ω
impedance.
It is also important to consider the effects of component pads that are in the path of the 50
Ω
trace.
If the traces are shorter than a 1/16th wavelength, transmission line effects will be minimized, but
stray capacitance from large component pads can induce additional RF losses. It may be
necessary to ask the PCB vendor to generate a new PCB stackup and suggested trace width that
is closer to the component pads, or modify the component pads themselves.
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