GNSS Receiver Front-ends II: Components

Transcription

1 GNSS Receiver Front-ends II: Components GPS Receiver Technology MM8 Darius Plaušinaitis Based on original slides from Ragnar V. Reynisson

2 Agenda Receiver Component Overview Filters Low Noise Amplifiers (LNAs) Mixers Frequency Synthesizers ADCs 2008 Danish GPS Center 2

3 A Simple Receiver Architecture An RF frontend of a GNSS receiver RF FRQ Conv IF Det I Q In GNSS this is usually a part of the digital signal processing I LO 90 Q Filter unwanted Filter LNA output Filter mixer output Convert IF signal signals and amplify Convert to IF freq Amplify signal to baseband I/Q 2008 Danish GPS Center 3

4 Receiver Components Filters

5 Filters s (t) in s (t) out s (t) in s (t) out s (t) in s (t) out Low-Pass High-Pass Bandpass Filters are used to extract the desired frequency band from the input while rejecting the remainder: Low pass: Reject high frequencies High pass: Reject low frequencies Band pass: Reject frequencies higher and lower than passband 2008 Danish GPS Center 5

6 Filter Properties 0dB -1dB -3dB Ideal filter Insertion loss Less frequency selective More frequency selective f center 2008 Danish GPS Center 6

7 Filter Key Parameters Passband attenuation/gain Stopband attenuation Bandwidth 3 db bandwidth Cutoff frequencies Filter order Filter type (Chebyshev, Butterworth, ) Passband/Stopband gain ripple Phase shift Active/Passive filters 2008 Danish GPS Center 7

8 Filter Parameters (Example) Sample lowpass filter (Chebyshev I, 2nd order) 0 Passband Transitional band H(ω) [db] Stopband ω [rad/s] 2008 Danish GPS Center 8

9 Filters (Example 2) Phase (deg) Magnitude (db) Bode Diagram Frequency (rad/sec) Same filter converted to bandpass Center frequency 1 rad/s Bandwidth 1.33e-2 rad/s Corresponds to GHz (GPS) 2008 Danish GPS Center 9

10 Filters For physical filter construction, there is a tradeoff between the filter specifications and the complexity of the filter At RF, active filters are possible, but even state-of-theart active filters are noisy 2008 Danish GPS Center 10

11 Filters Lumped element (Capacitor/Inductor) filters are limited by component quality factor (loss) Lossy components mean less sharp filters. Possible to obtain some selectivity, but never enough For sharp filtering at RF, Surface Acoustic Wave filters are used Standard filters available for GPS Impossible to integrate on IC Tradeoff between selectivity and filter loss Filter noise figure = filter loss! High attenuation also detrimental to selectivity, specially at input of receiver 2008 Danish GPS Center 11

12 Receiver Components Low Noise Amplifiers

13 Low Noise Amplifiers Placed early in the receiver chain (typically: immediately after antenna filter) Purpose: Amplify the signal while adding a minimum of noise Later stages (mixers, baseband amplifiers) often are very noisy High gain/low noise figure early in the receive chain improves overall receiver noise figure Design problems: Gain/Noise/Distortion tradeoff Power consumption 2008 Danish GPS Center 13

14 LNA Key Parameters Gain (Power/Voltage) Noise Figure Linearity (Intercept points/compression) Power Consumption (Noise/Gain/Linearity tradeoff) Input/Output impedance matching LNA between antenna filter/image rejection filter Passive filters depend on some nominal impedance at output Important to present correct impedance to input/output LNA source/load impedance imperative to noise/gain/linearity performance tradeoff 2008 Danish GPS Center 14

15 Receiver Components Mixers

16 Mixers Used for frequency conversion of information Mixers used at different places in the receiver chain Mixer functional description Multiplication/Non-linearity Switching/Clipping Image problem Single Balanced / Double Balanced Image Rejection Mixer 2008 Danish GPS Center 16

17 Mixers: Mixer function Functional description of a mixer: Analog multiplier Multiplies input signal and local oscillator (LO) signal Multiplication in time domain Convolution in frequency domain (Fourier) Information in the input signal converted both up and down in frequency sum and difference of carrier/lo frequencies 2008 Danish GPS Center 17

18 Mixers: Frequency Domain View * 2008 Danish GPS Center 18

19 Mixers: Frequency Upconversion Modulation Carrier 2008 Danish GPS Center 19

20 Mixers: Frequency Downconversion Danish GPS Center 20

21 Mixers: Key Parameters Conversion gain: Ratio of output signal power (at IF) to input signal power (at RF) Isolation: The ratio between LO input power and the fraction of the LO that appears at input/output ports Noise figure Balanced/Unbalanced Intercept points Power consumption 2008 Danish GPS Center 21

22 Mixers: Balanced / Single Ended Simple mixers: large unwanted frequency content at mixer output Local oscillator signal (+ harmonics) RF signal (+harmonics) Intermodulation products Using more than one mixer, this can be alleviated 2008 Danish GPS Center 22

23 Mixers: Image Signal Consider the down conversion alone: The distance from the RF signal and the LO signal corresponds to the Intermediate Frequency (IF) A signal that has the same distance from the LO signal (at opposite side) will also be converted to the IF Image signal 2008 Danish GPS Center 23

24 Mixers: Image Rejection Steps must be taken to ensure that any signal in the image band is eliminated before down-conversion Image reject filter Image rejection mixer Image rejection filter must pass the wanted (RF) frequency band while reject the image frequency choice of IF must be sufficiently high to aid in filtering Image rejection mixers use phase properties of wanted/image signals to reject the image signal Uses 4 mixers along with phase shifts Increased power consumption Matching of components important 2008 Danish GPS Center 24

25 Receiver Components Oscillators & Synthesizers

26 Oscillators & Synthesizers Oscillators are sine-wave (or square-wave) generators (local oscillators) Simple oscillators provide signal at a single frequency Oscillator consists of amplifier and a resonating feedback circuit (tank) Oscillator frequency can be tuned by varying feedback circuit Old days: Mechanical tuning of LC circuits Now: Electrical tuning of tank circuit 2008 Danish GPS Center 26

27 Oscillators & Synthesizers: Key Parameters Output amplitude/power Harmonic distortion Phase noise Power consumption Clock (frequency) stability For Frequency Synthesizers VCO frequency range Reference frequency/feedback division 2008 Danish GPS Center 27

28 Oscillators An ideal oscillator supplies a single sine-wave An actual oscillator includes phase noise, thermal noise and signal harmonics (signal is sine-wave like but not quite) RF oscillators are either fixed or voltage controlled Ideal Oscillator Actual Oscillator Phase noise Harmonics 2008 Danish GPS Center 28

29 Crystal Oscillators A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency Crystal oscillators have good short term stability, low phase noise React to temperature changes and vibrations Various types exist, but the most popular are: TCXO temperature-compensated crystal oscillator OCXO oven-controlled crystal oscillator 2008 Danish GPS Center 29

30 Frequency Synthesizer A frequency synthesizer is basically a Phase Locked Loop with a fixed reference frequency at the input and frequency division in the feedback Usually used for generating channel frequencies with a certain spacing (no need for channels in GNSS) PLL Has noise shaping properties compared φ to free-running OSC 2008 Danish GPS Center 30

31 Oscillators & Synthesizers Synthesizers shape the noise function Close to center frequency: less noise than freerunning oscillator Outside loop bandwidth: more noise 2008 Danish GPS Center 31

32 Receiver Components Demodulators

33 Demodulators I/Q demodulator Correlates the input RF signal with the Inphase and Quadrature Resulting signal is baseband information signal 2008 Danish GPS Center 33

34 Receiver components Analog to Digital Conversion

35 Analog to Digital Conversion After the I/Q demodulator and baseband filter/amplifier, signal is converted to digital domain (traditional receiver) Already the IF signal is converted to digital domain in a GNSS receiver Typically IF signal sampling requires a higher sampling rate then baseband signal sampling 2008 Danish GPS Center 35

36 Analog to Digital Conversion Quantization of signal adds noise to the signal Noise power density assumed flat Important that the signal exploits the full range of ADC Quantization noise stays the same reduction in SNR Oversampling improves SNR but costs more in terms of power 2008 Danish GPS Center 36

37 Analog to Digital Conversion Sampling frequency selection depends on number of factors Sampling also can be used for frequency translation (like mixers) Sampling frequency must be bigger than 2*f of the highest frequency component of the sampled baseband signal Sampling frequency must be bigger than 2*B (B two-sided signal bandwidth) of the sampled IF (bandpass) signal Signals must be filtered prior sampling to avoid signal aliasing 2008 Danish GPS Center 37

38 Analog to Digital Conversion Figures show steps (in frequency domain) of a signal sampling 2008 Danish GPS Center 38

39 Aliasing Due To Incorrect Sampling Frequency 2008 Danish GPS Center 39

40 Frequency Translation 2008 Danish GPS Center 40

41 GPS Receiver Technology Receivers III: Receiver Architectures

42 Agenda GPS Receiver requirements The Superheterodyne Receiver Direct Conversion Receivers A GPS receiver IC 2008 Danish GPS Center 42

43 GPS Receiver Requirements Input signal Typical input signal around 130dBm (0.1fW) Signal bandwidth of interest: 2MHz Noise floor of 110dBm (Signal power < Noise power!) Large gain through receiver -130dBm corresponds to an RMS amplitude of 70nV at the antenna To exploit the ADC to the fullest, need p-p voltage of 1-2 volts at the input Difference: 137 db (voltage) Noise floor 20 db over signal level around 117 db difference Typical voltage gain for GPS receiver: db Code correlation provides 43 db of processing gain 2008 Danish GPS Center 43

44 Superheterodyne Receiver Invented by Armstrong (1918) Good sensitivity performance Good dynamic range Gain/Selectivity spread along the chain Gradual attenuation of interfering signals Not very flexible (i.e. usably for more than one system) RF processing IF processing Baseband processing I 0 90 f IF Q LO1 ( f - f = ± f ) LO RF IF 2008 Danish GPS Center 44

45 Superheterodyne Receiver: RF Processing Antenna filter removes unwanted input frequencies Low loss: High loss at the start of the receiver chain kills sensitivity LNA amplifies input signal while adding as little noise as possible Goal: Amplify signal above noise floor of subsequent circuits, most notably mixer Image rejection filter (IRF) attenuates signal at mixer image frequency Higher selectivity than Ant.Filter possible as higher loss allowed LNA Antenna filter Image rej. filter 2008 Danish GPS Center 45

46 Superheterodyne Receiver: IF Processing Mixer converts the signal to the IF frequency using the LO signal as a reference IF filter removes unwanted mixer output (harmonics/im). IF amplifier amplifies resulting signal again Design dilemma: Choice of IF frequency Mixer Local Oscillator IF amplifier IF filter 2008 Danish GPS Center 46

47 Superheterodyne Receiver: Baseband Processing IQ demodulator converts IF signal into I and Q baseband components IFLO runs at the IF frequency BB filter and amplifier take care of the last gain and selectivity Design dilemma: BBF/BBA noise performance Baseband circuits are influenced by flicker (1/f) noise Gain is cheap at baseband compared to IF (power) but placing too much gain at BB can hurt selectivity I/Q demodulator IF LO 0 90 I Q BB filter BB amplifier 2008 Danish GPS Center 47

48 Superheterodyne Receiver: Choice of IF The choice of IF presents a tradeoff situation High IF: Relaxed requirements to image rejection filter attenuation More harsh requirement to IF filtering Lower selectivity Low IF: Mixer post filtering and gain easier Harsh requirements to IRF attenuation more loss in IRF Lower sensitivity 2008 Danish GPS Center 48

49 Superheterodyne Receiver: Dual Conversion Solution to IF frequency selection: Choose both First IF is high to ease requirements to Image Reject Filter Second IF is low to provide cheap gain/selectivity Cost: Higher power consumption/higher complexity/less flexibility RF processing First IF Second IF Baseband processing I 0 90 f IF Q LO1 LO Danish GPS Center 49

50 Superheterodyne Receiver Advantages/Drawbacks Advantages High Dynamic Range (Requirements spread over many blocks, many possibilities for variable gain) Low Noise Disadvantages Harsh requirements to filters, especially at RF Not ICfriendly! Complex Low flexibility Very hard to re-use components for other systems Important for GPS GPS used in more and more peripheral applications cost must be as low as possible to promote proliferation 2008 Danish GPS Center 50

51 Direct Conversion (Homodyne) One way of addressing the Image Rejection/IF problem is to choose an IF of zero Removes image problem completely Very flexible: Possible to re-use components for other systems IC-friendly Not good for a traditional GNSS receiver (due Doppler of all signals), but could be used for an SDR RF processing Baseband processing 0 90 f RF I Q 2008 Danish GPS Center 51

52 Direct Conversion Receivers - Problems Unfortunately, there is no such thing as a free lunch Gain placed in two places: RF and Baseband Stability problems More harsh requirements to each block Local oscillator at same frequency as signal LO signal impervious to filtering Leakage problems LNA gain vs. Mixer noise performance LNA gain limited Low noise for Mixer (1/f + thermal) 2008 Danish GPS Center 52

53 Direct Conversion Receiver - Problems Second order distortion produces a DC (constant) term at the mixer output Strong Interfering signals LO leakage signal mixes with itself The high gain in the BB amplifier can cause a significant DC offset at the ADC Extremely high iip2 requirement for the receiver Depending on the signal, some of the offset may be high-pass filtered before digitization 2008 Danish GPS Center 53

54 Direct Conversion Receiver - Problems 2008 Danish GPS Center 54

55 Direct Conversion Receiver Advantages/Drawbacks Advantages: Highly integrateable (no sharp RF filters) Simple Disadvantages Strict requirements for Gain/Linearity/Noise Not very sensitive Problems with LO leakage 2008 Danish GPS Center 55

56 An Example Of a Front-end Active Antenna Gain 30 db Noise Figure 2.5 db Rooftop Cable Loss 8.0 db Amplifier(s) Gain 50 db Noise Figure 4.0 db Amplifier(s) Gain: 50 db Noise Figure 4.0 db Analog-to-Digital Converter F SAMPLING MHz 4 Bit Samples BPF BPF BPF ADC Bandpass Filter F CENTER = MHz 3db BW 50 MHz TCXO F = 10.00MHz PLL Output dbm PLL Bandpass Filter F CENTER : MHz 3db BW 18 MHz 40 Bandpass Filter F C : MHz 3db BW 6.0 MHz Sampling Clock Digital Samples To Signal Processing Algorithms IF = MHz 2008 Danish GPS Center 56

57 Integrated GPS Front-end 2008 Danish GPS Center 57

58 Summary Receivers can be implemented in many ways Superheterodyne has good performance regarding noise/dynamic range Needs sharp filters Not very flexible/adapts poorly to integrated circuits Direct conversion bypasses the image problem New problems arise: flicker noise, gain spread over few blocks 2008 Danish GPS Center 58

59 Questions and Exercises Danish GPS Center 59