SL871 Family Product User Guide
1VV0301170 Rev. 9
Page 57 of 89
2021-07-15
Not Subject to NDA
An antenna vendor should specify a nominal antenna gain (usually at zenith (directly
overhead) and antenna pattern curves specifying gain as a function of elevation and
azimuth. Pay careful attention to requirements 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.
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 Noise Floor
Equation 10-1 Carrier to Noise Ratio
Thermal noise is -174 dBm/Hz at 290 K.
We can estimate a typical 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.
