How GNSS receiver radio frequency works ?

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

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How GNSS receiver radio frequency works ?

  1. 1. www.OpenGNSS.org 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
  2. 2. www.OpenGNSS.org How GNSS receiver radio frequency works ?
  3. 3. www.OpenGNSS.org GNSS Receiver 3
  4. 4. www.OpenGNSS.org 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. 5. www.OpenGNSS.org Functional scheme of a GPS receiver FPB : Filtre Passe Bande 5
  6. 6. www.OpenGNSS.org HORUS2000 card OpenGNSS 6
  7. 7. www.OpenGNSS.org 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 7
  8. 8. www.OpenGNSS.org HORUS2000 card OpenGNSS HORUS2000 card allows to extract the following signals : Signals on L1 band GPS L1, GALILEO E1 8
  9. 9. www.OpenGNSS.org 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 9
  10. 10. www.OpenGNSS.org 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 10
  11. 11. www.OpenGNSS.org 1. Antennas 2. Preamp 3. RF Filtering 4. Mixing 5. IF Filtering 6. Scanninga. 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 11
  12. 12. www.OpenGNSS.org 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. 12
  13. 13. www.OpenGNSS.org 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. 13
  14. 14. www.OpenGNSS.org 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 14
  15. 15. www.OpenGNSS.org 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. 15
  16. 16. www.OpenGNSS.org 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. 16
  17. 17. www.OpenGNSS.org 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 17
  18. 18. www.OpenGNSS.org 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. 18
  19. 19. www.OpenGNSS.org 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 19
  20. 20. www.OpenGNSS.org 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 20
  21. 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) 21
  22. 22. www.OpenGNSS.org 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). 22
  23. 23. www.OpenGNSS.org 3. RF 1. Antennas 2. Preamp Filtering 4. Mixing 5. IF Filtering 6. Scanning b. The local oscillator PLL frequencies synthetizer 23
  24. 24. www.OpenGNSS.org 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. 24
  25. 25. www.OpenGNSS.org 3. RF 1. Antennas 2. Preamp Filtering 4. Mixing 5. IF Filtering 6. Scanning c. Summary PLL frequency synthetizer 25
  26. 26. www.OpenGNSS.org 3. RF 1. Antennas 2. Preamp Filtering 4. Mixing 5. IF Filtering 6. Scanning GPS Receipt 26
  27. 27. www.OpenGNSS.org 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 27
  28. 28. www.OpenGNSS.org 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 28
  29. 29. www.OpenGNSS.org 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 29
  30. 30. www.OpenGNSS.org 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. 30
  31. 31. www.OpenGNSS.org 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 31
  32. 32. www.OpenGNSS.org 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) 32
  33. 33. www.OpenGNSS.org 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. 33
  34. 34. www.OpenGNSS.org 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) 34
  35. 35. www.OpenGNSS.org 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 35
  36. 36. www.OpenGNSS.org 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 36
  37. 37. www.OpenGNSS.org 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). 37
  38. 38. www.OpenGNSS.org 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 38
  39. 39. www.OpenGNSS.org 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 39
  40. 40. www.OpenGNSS.org Conclusion Thank you and let us meet on www.OpenGNSS.org to participate in the development OpenSource of a GNSS receiver 40

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