How does GNSS RF works. Exemple of application with the Maxim 2769 and HORUS 2000 Card from OpenGNSS.org

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OpenGNSS
www.OpenGNSS.org
proposes you this course
free of charge
OpenGNSS is a project cofinanced by HELILEO (www.
HELILEO.com), TECNALIA (www. TECNALIA.es), the
Regional Council of Aquitaine and the Basque government

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How GNSS receiver
radio frequency works ?

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GNSS Receiver
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RF (Hardware)
Reception and scanning of the I and Q signals
1) Antennas
a. The two types of antennas
b. Their characteristics
2) Preamp
3) RF filtering
4) Mixing
a. Frequency transposition
b. Local oscillator
c. Summary
5) IF filtering
a. Filter choice
b. Bandwitdth constraints
c. Filtering after the frequency transposition (RF→IF)
6) Scanning I and Q signals
4

5. www.OpenGNSS.org Functional scheme of a GPS
receiver
FPB : Filtre Passe Bande 5

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HORUS2000 card OpenGNSS
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HORUS2000 card OpenGNSS
Universal GPS Receiver : MAX2769
• Manufacturer : MAXIM
• Price : 3.79€
• Receiver covering the navigation
systems by satellite GPS, GALILEO
and GLONASS.
Applications :
- Navigation embedded systems
- GPS software
- Laptops
- Digital cameras
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HORUS2000 card OpenGNSS
HORUS2000 card allows to extract the
following signals :
Signals on L1 band
GPS L1, GALILEO E1
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HORUS2000 card OpenGNSS
MAX2769 component is programmable through the
following registers :
Register name Address Function Page
(A3:A0)
CONF1 0000 Configure RX and IF sections, set the pins for 17
individual blocks.
CONF2 0001 Configure AGC and output sections. 18
CONF3 0010 Configure the support and the test functions for 19
the IF filter and the AGC.
PLLCONF 0011 PLL , VCO and clock settings 20
DIV 0100 Division ratio and floor function, other… 21
FDIV 0101 Decimal division ratio, other… 21
STRM 0110 Number of frames to be chained through the DSP 21
interface.
CLK 0111 Fractional clock divider values. 21
TEST1 1000 Reserved test mode. 21
TEST2 1001 Reserved test mode. 21
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1. Antennas 2. Preamp 3. RF filtering 4. Mixing 5. IF filtering 6. Scanning
Passive Atenna Active Antenna
Frequencies
LP LNA Converter (IF)
Filter
RF Cable
LNA RF Filter IF Filter
Signal Inphase ADC
Signal en Quadrature
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1. Antennas 2. Preamp 3. RF Filtering 4. Mixing 5. IF Filtering 6. Scanning
a. The two types of Active antenna
antennas
- Passives
- Actives
• The electromagnetic ondulations of
the signal (in space) are changed
into electric current (cable at the
antenna output) Central Frequency : 1575.42Mhz
Bandwidth: ±3Mhz
Mitigation : Central frequency ±100MHZ>30db
Impedance: 50Ω
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3. RF
1. Antennas 2. Preamp
Filtering
4. Mixing 5. IF Filtering 6. Scanning
b. Their characteristics
• Passive antenna • Active antenna
It suits for GPS application where the It owns :
distance between the antenna and the - A LNA which balance the losses of
receiver is short (2.24→91.44 cm). the coaxial cable.
- A filter.
 Cheap
 Evoluted system
It requires :
- More expensive
- A Low Noise Amplifier (LNA)
- It requires an alimentation, generally
- A filter on the GPS
supplied through the coaxial cable of
the antenna.
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With HORUS2000 …
3. RF
1. Antennas 2. Preamp
Filtering
4. Mixing 5. IF Filtering 6. Scanning
• SMA connector : possibility of plugging a
RF antenna, passive or active.
• Configuration 1 : ANTEN=1
→ Active antenna alimentation.
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3. RF
1. Antennas 2. Preamp Filtering
4. Mixing 5. IF Filtering 6. Scanning
Passive Antenna Active Antenna
Frequencies
LP
LNA Converter (IF)
Filter
RF Cable
LNA RF IF Filter
Filtering
Signal Inphase ADC
Signal en Quadrature
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3. RF
1. Antennas 2. Preamp Filtering
4. Mixing 5. IF Filtering 6. Scanning
Goal : Amplify the received signal while
adding the less noise possible.
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With HORUS2000 …
3. RF
1. Antennas 2. Preamp Filtering
4. Mixing 5. IF Filtering 6. Scanning
• LNA1 for passive antenna: 19dB / 0.8dB (amplification / noise)
• LNA2 for active antenna : 13dB/1.2dB
• LNA-gated mode : ampli input linked.
→ Automatic detection of the antenna’s type
Configuration 1 : LNAMODE = 00.
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1. Antennas 2. Preamp
3. RF 4. Mixing 5. IF Filtering 6. Scanning
Filtering
Passive Antenna Active Antenna
Frequencies
LP
LNA Converter (IF)
Filter
RF Cable
LNA RF IF Filter
Filtering
Signal Inphase ADC
Signal en Quadrature
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1. Antennas 2. Preamp 3. RF Filtering 4. Mixing 5. IF Filtering 6. Scanning
Filter
• A filter allows to select a
frequency band while mitigate
characteristics
the others.
• 2 main characteristics :
• - Insertion losses or mitigation
of the components of the
desired frequencies.
- Bandwitdth at -3dB.
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With HORUS2000 …
1. Antennas 2. Preamp 3. RF Filtering 4. Mixing 5. IF Filtering 6. Scanning
• Filtering signal L1 - GPS • Filtering signal E5a -
GALILEO
• Central Frequency E5a
• Central Frequency
Fcen=1176.45MHz
F0=1575.42MHz
• Frequency F0=1166,22MHz
• BandPass Filter
• Low-Pass Filter
• Bandwidth: 2,5MHz
• Bandwidth : 18MHz
• Insertion losses < 1.5dB • Insertion losses < 1.0dB
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3. RF
1. Antennas 2. Preamp Filtering 4. Mixing 5. IF Filtering 6. Scanning
Passive Antenna Active Antenna
Frequencies
LP
LNA Converter (IF)
Filter
RF Cable
LNA RF IF Filter
Filtering
Signal Inphase ADC
Signal en Quadrature
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21. www.OpenGNSS.org
3. RF
1. Antennas 2. Preamp
Filtering 4. Mixing 5. IF Filtering 6. Scanning
A. Frequencies transposition
• Why?
• Received signals frequency : ≈1 billion of oscillations /s.
• The signal is modulated with the local oscillator (LO) in order to be be shifted
to an intermediary frequency (IF), low indeed null.
LO
L1=1575.42Mhz IF (<10Mhz)
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3. RF
1. Antennas 2. Preamp
Filtering 4. Mixing 5. IF Filtering 6. Scanning
b. The local oscillator
 A PLL frequency synthetizer :
- A quartz oscillator providing a reference frequency RF.
- A « ∑-∆ and N-fractionnal synthesizer » allowing to divide the
VCO frequency by N ( ).
- A Voltage Controlled Oscillator (VCO) which, according to the
input voltage provides a sine signal in a frequency band (for
the MAX2769 VCO, it’s 1550-1580MhZ for 0.4V < VTUNE < 2.4V).
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3. RF
1. Antennas 2. Preamp
Filtering 4. Mixing 5. IF Filtering 6. Scanning
b. The local oscillator
PLL frequencies synthetizer
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3. RF
1. Antennas 2. Preamp
Filtering 4. Mixing 5. IF Filtering 6. Scanning
b. The local oscillator
What is frenquency synthesis?
• Generate a signal (output) whose frequency is a programmable
multiple of a set input frequency .
• Feedback system (negative feedback) in order to control the
frequency : the PLL
• Obtaining an output signal with a very good spectral pureness.
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3. RF
1. Antennas 2. Preamp
Filtering 4. Mixing 5. IF Filtering 6. Scanning
c. Summary
PLL frequency
synthetizer
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3. RF
1. Antennas 2. Preamp
Filtering 4. Mixing 5. IF Filtering 6. Scanning
GPS Receipt
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With HORUS2000 …
3. RF
1. Antennas 2. Preamp
Filtering 4. Mixing 5. IF Filtering 6. Scanning
• PLL Division ratio integer :
Reference divider: RDIV=10Décimal.
• Frequency divider VCO :
fVCO/fCOMP=1 571.52Mhz/1Mhz=1 571.52
• PLL Division Ratio integer :
Floor divider : NDIV=1571Décimal
• PLL Division Ratio :
Decimal divider : FDIV=0.52Décimal
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1. Antennas 2. Preamp
3. RF
4. Mixing
5. IF 6. Scanning
Filtering Filtering
Passive Antenna Active Antenna
Frequencies
LP
LNA Converter (IF)
Filter
RF Cable
LNA RF IF Filter
Filtering
Signal Inphase ADC
Signal en Quadrature
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3. RF
1. Antennas 2. Preamp
Filtering
4. Mixing 5. IF Filtering 6. Scanning
a. Filter choice
• Low-pass if IF=0Hz • Bandpass if IF≠0Hz
The information being redundant, If IF is low (but≠0 ), we filter the
this part of the signal is sufficient to « symmetric » information of the
decrypt the information. signal.
IF=0Hz IF≠0Hz
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3. RF
1. Antennas 2. Preamp
Filtering
4. Mixing 5. IF Filtering 6. Scanning
b. Constraints on the bandwidth
o The largest the band is :
 More we get information about the signal.
 After regeneration, the filtered signal will be close to the input
signal.
- The signal is more sensible to noise and interferences.
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3. RF
1. Antennas 2. Preamp
Filtering
4. Mixing 5. IF Filtering 6. Scanning
b. Constraints on the bandwidth
A Filter:
- 60% of the signal
- 0 interference
B Filter:
- 100% of the signal
- 2 interferences
Useful signal
Bandwidth
Interferences
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3. RF
1. Antennas 2. Preamp
Filtering
4. Mixing 5. IF Filtering 6. Scanning
c. Filtering after the frequency transposition
1. Where does the frequency « substraction » come from during
mixing?
IF Filter(Low Frequency)
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3. RF
1. Antennas 2. Preamp
Filtering
4. Mixing 5. IF Filtering 6. Scanning
c. Filtering after the frequency transposition
2. Quality and cost of the device :
- Difficult to product narrow band filters in HF (1.5GhZ)
- Quality factor: Q = fcentral/BW
A filter able to select the main lobe of a GPS spectrum, whose
bandwidth would be 2x1.023=2.046Mhz, would have a quality
factor Q=1575.42/2.046≈770.
Typically, Q=50 for a filter sold in commerce.
3. It allows to limit the noise within the bandwidtch.
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With HORUS2000 …
3. RF
1. Antennas 2. Preamp
Filtering
4. Mixing 5. IF Filtering 6. Scanning
Bandpass Filter
Butterworth
•
• Configuration 1 :
- FCEN=001101↔ fCENTER=4.092Mhz
- FBW=10 ↔ Bandwidth of 4.2Mhz (±2.1Mhz)
- F3OR5=1 (3rd order) ll F3OR5=0 (5th order)
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3. RF
1. Antennas 2. Preamp Filtering
4. Mixing 5. IF Filtering 6. Scanning
Passive Antenna Active Antenna
Frequencies
LP
LNA Converter (IF)
Filter
RF Cable
LNA RF IF Filter
Filtering
Signal Inphase
ADC
Signal en Quadrature
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1. 3. RF
Antennas
2. Preamp
Filtering
4. Mixing 5. IF Filtering 6. Scanning
Scanning consists in two parallel activities :
 Sampling
 Quantification
The quality of the digital signal depends on
2 factors :
 Sampling frequency or ratio
 Resolution : number of bits used for
coding
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With HORUS2000 …
1. 3. RF
Antennas
2. Preamp
Filtering
4. Mixing 5. IF Filtering 6. Sampling
• 3 Displaying formats exist : • The ADC clock is a multiple (smaller) of
- Sign/amplitude the input reference clock.
- Two’s complement
- Unsigned binary
• Output on 2 bits:
- 2 bits for I output : I1 and I0
- 2 bits for Q output : Q1 and Q0
• Output on 3 bits :
- I only : I1, I0, Q1 (cf. example).
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With HORUS2000 …
1. 3. RF
Antennas
2. Preamp
Filtering
4. Mixing 5. IF Filtering 6. Scanning
011
010
001
000
100
101
110
111
Quantification by superior value and not centered
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Avec HORUS2000 …
1. Les 2. 3. Filtrage
antennes Préamplification RF
4. Le mixage 5. Filtrage IF 6. Numérisation
MSB 0 0 0 0 0 0 1 1 1 1 1 1 0 0 0 0
2ND 0 1 1 1 1 0 0 1 1 1 1 0 0 0 1 1
LSB 1 0 1 1 0 0 0 0 1 1 0 1 0 1 1 1
Output I 3bits Format Sign/Amplitude
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Thank you and let us meet on www.OpenGNSS.org
to participate in the development OpenSource
of a GNSS receiver
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