DANISH GPS CENTER
GNSS Receiver Front-ends II: ComponentsGPS Receiver Technology MM8Darius Plauš[email protected]
Based on original slides from Ragnar V. Reynisson
DANISH GPS CENTERAgenda
• Receiver Component Overview• Filters• Low Noise Amplifiers (LNAs)• Mixers• Frequency Synthesizers• ADCs
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DANISH GPS CENTERA Simple Receiver Architecture
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I
QRF DetIFFRQ
Conv
Filter unwantedsignals and amplify
LO
Filter LNA outputConvert to IF freq
Filter mixer outputAmplify signal
90°
I
Q
Convert IF signalto baseband I/Q
An RF front-end of a GNSS
receiver
In GNSS this is usually a part of the
digital signal processing
DANISH GPS CENTER
Receiver ComponentsFilters
DANISH GPS CENTERFilters
• 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
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s (t)in s (t)in s (t)ins (t)out s (t)out s (t)out
Low-Pass High-Pass Bandpass
DANISH GPS CENTERFilter Properties
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-3dB-1dB0dB
Insertion loss
Ideal filter
Less frequency selective
More frequencyselective
fcenter
DANISH GPS CENTERFilter 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
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DANISH GPS CENTERFilter Parameters (Example)
• Sample lowpass filter (Chebyshev I, 2nd order)
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10-1
100
101
-70
-60
-50
-40
-30
-20
-10
0
ω [rad/s]
H( ω
) [dB
]
Passband
Transitionalband
Stopband
DANISH GPS CENTERFilters (Example 2)
Bode Diagram
Frequency (rad/sec)
Pha
se (d
eg)
Mag
nitu
de (d
B)
-70
-60
-50
-40
-30
-20
-10
0
10-0.09
10-0.07
10-0.05
10-0.03
10-0.01
100.01
100.03
100.05
100.07
-180
-135
-90
-45
0
45
90
135
180
• Same filter converted to bandpass– Center frequency 1 rad/s– Bandwidth 1.33e-2 rad/s Corresponds to 20 MHz @ 1.5 GHz (GPS)
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DANISH GPS CENTERFilters
• 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-the-art active filters are noisy
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DANISH GPS CENTERFilters
• 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
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DANISH GPS CENTER
Receiver ComponentsLow Noise Amplifiers
DANISH GPS CENTERLow 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
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DANISH GPS CENTERLNA 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
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DANISH GPS CENTER
Receiver ComponentsMixers
DANISH GPS CENTERMixers
• 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
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DANISH GPS CENTERMixers: 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
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Mixers: Frequency Domain View
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*
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Mixers: Frequency Up-conversion
0 1 2 3 4 5 6 7 8 9 10
-1.5
-1
-0.5
0
0.5
1
1.5
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0 1 2 3 4 5 6 7 8 9 10-1.5
-1
-0.5
0
0.5
1
1.5
Modulation Carrier
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Mixers: Frequency Down-conversion
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0 1 2 3 4 5 6 7 8 9 100
0.5
1
1.5
0 1 2 3 4 5 6 7 8 9 100
0.5
1
1.5
0 1 2 3 4 5 6 7 8 9 10-1.5
-1
-0.5
0
0.5
1
1.5
0 1 2 3 4 5 6 7 8 9 100
0.5
1
1.5
DANISH GPS CENTERMixers: 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
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DANISH GPS CENTER
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
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DANISH GPS CENTERMixers: 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
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DANISH GPS CENTERMixers: 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
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DANISH GPS CENTER
Receiver ComponentsOscillators & Synthesizers
DANISH GPS CENTEROscillators & 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
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DANISH GPS CENTER
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
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DANISH GPS CENTEROscillators
• 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
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Ideal Oscillator Actual Oscillator
Phase noiseHarmonics
DANISH GPS CENTERCrystal 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
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DANISH GPS CENTERFrequency 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)
• Has noise shaping properties comparedto free-running OSC
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φ
PLL
DANISH GPS CENTEROscillators & Synthesizers• Synthesizers shape the noise function• Close to center frequency: less noise than free-
running oscillator• Outside loop bandwidth: more noise
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DANISH GPS CENTER
Receiver ComponentsDemodulators
DANISH GPS CENTERDemodulators
• I/Q demodulator • Correlates the input RF signal with the In-
phase and Quadrature• Resulting signal is baseband information
signal
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DANISH GPS CENTER
Receiver componentsAnalog to Digital Conversion
DANISH GPS CENTERAnalog 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
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DANISH GPS CENTERAnalog 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
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DANISH GPS CENTERAnalog 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
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DANISH GPS CENTERAnalog to Digital Conversion
• Figures show steps (in frequency domain) of a signal sampling
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DANISH GPS CENTER
Aliasing Due To Incorrect Sampling Frequency
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DANISH GPS CENTERFrequency Translation
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DANISH GPS CENTER
GPS Receiver TechnologyReceivers III: Receiver Architectures
DANISH GPS CENTERAgenda
• GPS Receiver requirements• The Superheterodyne Receiver• Direct Conversion Receivers• A GPS receiver IC
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DANISH GPS CENTERGPS 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: 106-107 dB
• Code correlation provides 43 dB of processing gain
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DANISH GPS CENTERSuperheterodyne 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)
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0°
90°
I
Q
RF processing IF processing Baseband processing
LO1 ( )f - f = fLO RF IF±
fIF
DANISH GPS CENTER
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
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Antenna filter Image rej. filter
LNA
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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
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Local Oscillator IF filterIF amplifierMixer
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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
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0°
90°
I
Q
BB filter
BB amplifier
I/Q demodulator
IF LO
DANISH GPS CENTER
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
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DANISH GPS CENTER
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
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0°
90°
I
Q
RF processing Second IF Baseband processing
LO2
fIF
First IF
LO1
DANISH GPS CENTER
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 IC-friendly!
– 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
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DANISH GPS CENTERDirect 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
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0°
90°
RF processing Baseband processing
fRF
I
Q
I
Q
DANISH GPS CENTER
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)
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DANISH GPS CENTER
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
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DANISH GPS CENTER
Direct Conversion Receiver -Problems
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DANISH GPS CENTER
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
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DANISH GPS CENTERAn Example Of a Front-end
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Digital SamplesTo Signal
ProcessingAlgorithms
IF = 9.548 MHz
ADCBPF
Active AntennaGain ≈ 30 dB
Noise Figure ≈ 2.5 dB
BPF BPFBandpass Filter
FCENTER = 1575.42 MHz3db BW ≈ 50 MHz
Bandpass FilterFCENTER : 47.74 MHz3db BW ≈ 18 MHz
Amplifier(s)Gain ≈ 50 dB
Noise Figure ≈ 4.0 dB
PLL
Bandpass FilterFC : 47.74 MHz
3db BW ≈ 6.0 MHz
Amplifier(s)Gain: ≈ 50 dB
Noise Figure ≈ 4.0 dB
PLL Output1527.68 MHz
@ 7 dBm
Analog-to-DigitalConverter
FSAMPLING ≈ 38.192 MHz4 Bit Samples
Rooftop CableLoss ≈ 8.0 dB
TCXOF = 10.00MHz
40Sampling
Clock
DANISH GPS CENTERIntegrated GPS Front-end
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DANISH GPS CENTERSummary
• 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
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Questions and Exercises
http://gps.aau.dk
DANISH GPS CENTER
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